An update on the nutrition of dairy sheep grazing

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Small Ruminant Research 77 (2008) 93–112

An update on the nutrition of dairy sheep grazing Mediterranean pastures夽 G. Molle a,∗ , M. Decandia a , A. Cabiddu a , S.Y. Landau b , A. Cannas c a

b

AGRIS Sardegna, Dipartimento per la Ricerca nelle Produzioni Animali, 07040 Olmedo, Italy A.R.O., Department of Natural Resources and Agronomy, Institute of Plant Sciences, Bet Dagan 50250, Israel c Dipartimento di Scienze Zootecniche, University of Sassari, 07100 Sassari, Sardinia, Italy Available online 7 May 2008

Abstract In the light of recent findings in sheep nutrition and feeding behaviour, the diets of grazing dairy sheep should be based on forages encompassing a variety of complementary nutritional values and containing moderate levels of diverse plant secondary metabolites, until recently regarded as “anti-nutritional”. In lactating sheep, pastures of tannin-containing legumes like sulla (Hedysarum coronarium) and chicory (Cichorium intybus) can be integrated with annual grasses for establishing artificial pastures under rainfed conditions. Diets based on these forages, while ensuring high milking performance, can mitigate the unbalance of CP to energy ratio of grazing sheep. By grazing sulla and Italian ryegrass (50:50 by area) as spatially adjacent monocultures or in timely sequence (complementary grazing) sheep eat more and perform better than by grazing the ryegrass pasture only. Concentrate supplementation of lactating sheep should be preferably based on sources rich in digestible plant fiber (soyhulls or beet pulps), particularly from mid-lactation onwards and when supplementation levels are high. Milk urea concentration is confirmed as a useful monitoring tool to balance protein nutrition and curb the waste of N at animal and system level. Finally, challenging tasks for future research on dairy sheep grazing management and nutrition are on-farm application of recent technological advances, such as image-based estimation of pasture biomass and quality, evaluation of sheep dietary quality by faecal Near Infrared Reflectance Spectrometry, and establishment of remote control systems. © 2008 Elsevier B.V. All rights reserved. Keywords: Grazing; Dairy sheep; Milk; Nutrition; Behaviour; Mediterranean pastures

1. Introduction Dairy sheep production is mainly located in the EU Mediterranean countries, with Italy and Greece being leading countries for sheep stock, milk and cheese production (Table 1). Other Mediterranean countries of the east and southern shores and Eastern European countries, 夽 This paper is part of the special issue entitled “Sheep and Goat Farming: grazing systems of production and development” guest edited by P. Morand-Fehr. ∗ Corresponding author. Tel.: +39 079 387233; fax: +39 079 389450. E-mail address: [email protected] (G. Molle).

0921-4488/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2008.03.003

such as Romania (Table 1), also play a role in the sheep milk production sector. In most of these regions, dairy sheep feeding is based on pasture grazing, although production systems range from highly extensive (based on natural grassland or rangeland) to very intensive, based on forage crops, agriculture by-products and concentrates, as in the case of complete-diet based production systems (Landau and Molle, 2004). This review focuses primarily on dairy sheep nutrition in Mediterranean grazing systems. Basic principles will be updated and feeding and management strategies will be derived keeping the following as basic target: production of quality foods modulated by agronomic,

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G. Molle et al. / Small Ruminant Research 77 (2008) 93–112

Table 1 Dairy sheep census and production in Europe (FAO, 2005) Heads

Milk production

Cheese production

N × 1000 (%)

T × 1000 (%)

T

Portugal 530 Spain 2,050 France 1,345 Italy 7,000 Albania 1,470 Greece 7,000 Bulgaria 1,351 Cyprus 175 Romania 5,446 Hungary 750 Europe 27,117

2.0 98 7.6 400 5.0 264 25.8 820 2.4 76 25.8 700 5.0 116 0.6 22 20.1 344 2.8 32 100.0 2,872

3.4 13.9 9.2 28.6 2.6 24.4 4.0 0.8 12.0 1.1 100.0

(%)

16,400 4.5 47,500 13.0 50,000 13.6 94,350 25.7 1,200 0.3 130,000 35.5 13,000 3.5 3,500 1.0 9,900 2.7 780 0.2 366,630 100.0

N = number of heads; T = metric tons.

economic, ethical (e.g. animal well-being) and environmental considerations. The ultimate objective is to allow the farmers to make a living from locally adapted dairy sheep production systems, which represent the core of Mediterranean dairy sheep industry (Morand-Fehr et al., 2007). The reader should refer to other sources for detailed information on the effect of grazed forages on sheep milk and cheese quality (Cabiddu et al., 2005; Pulina et al., 2006). Great efforts have been made in the last decade to clarify the functioning of Mediterranean grazing systems, with particular focus on the interactions between their two main components, i.e. the pasture – consisting of forage crops or improved semi-natural pastures (semi-extensive farming systems) – and the grazing animal. In this review the body of knowledge in this area is updated on the basis of: (i) recent applied research focused especially on dairy sheep; and (ii) evidence based on more fundamental research on sheep and cattle nutrition. Aspects such as agronomic performance and environmental impact of grazing management will be also briefly discussed. Pastures are nowadays regarded as multi-use areas, and not as a simple feeding resource. Areas where dairy sheep are extensively managed are often considered important habitats for many wild species, plants and animals, whose biodiversity should be adequately preserved. This is particularly the case for natural parks, wildlife protected areas or any areas subjected to nature conservation schemes (e.g. Natura 2000 Network, Directive 92/43 EEC). When adequate management is applied, grazing these areas represents a valuable economic way to achieve the conservation targets (Scimone et al., 2007; Wallis De Vries et al., 2007). On the other hand, pastoral systems using forage crops intensively cropped and

managed with high stocking rates may result in environmental hazards such as soil erosion, nutrients’ runoff and leaching as well as emission of green-house gases. These environmental aspects are to be taken into account for optimal management of grazing resources for dairy sheep as well as for other herbivore species (e.g. Peyraud and Delaby, 2006). Although this review is mainly devoted to nutrition, part of it will address behavioural aspects such as preference, which is the unconstrained expression of feeding ‘selectivity’. An animal-friendly approach is in fact envisaged, entailing a ‘dialogue’ between man and animal, which keeps in mind the grazing system as a whole in a multi-disciplinary perspective. The animal component of this ‘dialogue’ is based on animal behaviour, i.e. the ‘body language’. The importance of taking preference into consideration while managing the nutrition of grazing animals stands on the widely accepted concept that feeding selectivity originates in long-term animal adaptation to its environment, which is a pre-requisite for optimal life-long performance (Prache and Peyraud, 2001). Dairy sheep grazing systems are usually based on artificial and natural or semi-natural pastures. It must be recognized that both play an important role although their relative weights on annual production of milk and meat are usually quite different: forage crops usually support most of the productive stock during lactation (winter–spring), while the natural and semi-natural pastures, based on annual self-regenerating forage species, play a major role for the grazing nutrition of replacement lambs as well as for the maintenance of non-lactating, pregnant ewes (summer–autumn). From an agronomic point of view, forage crops are established, cultivated and rotated from year-to-year across the arable proportion of the farm, whereas the natural pasture is only fertilized and re-established on occasions, usually when sward composition deteriorates. This mosaic of cropped plots and natural grassland is typical of Mediterranean dairy sheep farm landscape. The following sections will primarily address: the nutritive value of Mediterranean pastures and forage crops (Section 2); the choice of forage species to be established (Section 3); their spatial distribution (as mixtures or monocultures); and their grazing management (Section 4). Then the focus will be directed on the supplementation of grazing sheep (Section 5). Afterwards, methods for monitoring the nutrition of grazing ruminants, with particular focus on sheep, will be envisaged (Section 6). A final section (Section 7) is aimed at both resuming the main concepts from a practical standpoint, and envisaging future research needs.


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Table 2 Dry matter percentage, chemical composition (%DM) and nutritive value of some Mediterranean forages as selected by lactating sheep (hand-plucked samples) Forage

N ryegrassa

Annual Sullaa Burr medica Subclovera Sullab Burr medicb Chicoryb Safflowerb

72 24 24 24 3 3 3 3

DM

CP

NDF

NSC

Tannic phenols

NEL (Mcal/kg DM)

19.4 (3.2) 16.4 (1.8) 19.1 (2.8) 19.6 (1.9) 15.7 (2.4) 18.1 (3.9) 12.0 (2.9) 15.0 (1.5)

20.1 (1.8) 23.2 (4.0) 28.1 (1.2) 21.6 (2.0) 23.4 (2.7) 24.1 (2.8) 14.5 (0.9) 18.6 (7.6)

39.3 (5.0) 29.8 (3.7) 29.6 (3.9) 31.7 (5.3) 37.7 (2.5) 35.9 (6.1) 35.0 (0.7) 37.9 (8.3)

22.7 (8.8) 33.4 (10.2) 30.1 (5.1) 31.5 (6.3) 25.4 (5.7) 27.3 (3.5) 33.7 (1.6) 30.2 (4.2)

0.3 (0.1) 2.1 (0.6) 0.2 (0.1) 1.0 (0.3) 2.6 (0.1) 0.6 (0.1) 1.0 (0.4) 2.4 (1.0)

1.85 (0.1) 1.78 (0.2) 1.86 (0.1) 1.70 (0.1) 1.55 (0.1) 1.74 (0.1) 1.73 (0.0) 1.75 (0.0)

CP/NEL (g/Mcal) 119 (24) 133 (33) 152 (9) 128 (16) 151 (23) 141 (6) 86 (5) 100 (6)

Means and (S.D.) N = number of samples; DM = dry matter; CP = crude protein; NDF = neutral detergent fiber; NSC = calculated non-structural carbohydrates; NEL = net energy for lactation. a Forages at growing phase (January–April) (unpublished data). b Forages at early reproductive phase (May) (Landau et al., 2005b).

2. Nutritive value and nutritional unbalances of Mediterranean pasture and forage crops Mediterranean grazed forages consist primarily of annual forages, which undergo abrupt changes in their nutritive value during the course of the growing cycle. Sheep usually graze from the top leafy layer downwards. Therefore, the quality of the ingested forage is higher than that on offer. As shown in Table 2, the selected components of both grass and legume forages (hand-plucked samples) have a high nutritive value during growth period. Unfortunately, they also have very high levels of crude protein (CP), which, in early stages of growth, is often characterized by a high proportion of non-protein nitrogen (NPN or fraction A) and soluble protein N (fraction B1, Licitra et al., 1996). In grasses such as Italian ryegrass these fractions occasionally make more than 40% of total selected herbage CP (Fig. 1). In this nutritional scenario, typically experienced by early to mid lactation ewes, the CP

Fig. 1. Crude protein fractions in selected plant components of four Mediterranean forages, grazed by sheep during growing phase. BM: burr medic; RY: annual ryegrass; SC: subclover; SU: sulla. Fractions are: A: NPN; B1: buffer soluble protein; B2: buffer insoluble protein – neutral detergent soluble protein; B3: neutral detergent soluble protein; C: acid detergent insoluble protein (Licitra et al., 1996).

to energy ratio is often above the required levels for milk production (approximately 110–120 g CP/Mcal NEL for milk yield ranging between 1000 and 2000 g/day). In many instances, this high ratio and the high proportion of soluble N induce very fast and excessive ammonia production in the rumen, which is converted in large amount to urea in the liver. As a result, blood and milk urea concentrations in grazing sheep are often very high during the period of active pasture growth. For example, in Sardinia the concentration of milk urea of bulk samples is often higher than 60 mg/100 ml during winter–early spring, which is a clear sign of nitrogen excess in the diet due the nutritional unbalance of pasture. The excess of CP over energy brings about: (i) increased energy requirements of the animals, due to the cost of ammonia conversion into urea (Tyrrell et al., 1970); (ii) increased N excretion, mostly as urinary urea, with environmental impact; (iii) low nitrogen utilization efficiency for milk, with dietary N being wasted as milk urea-N (Cannas et al., 1998; Landau et al., 2005a); (iv) decreased conception rates (Branca et al., 2000); and (v) possible health problems, such as laminitis, triggered by ammonia excesses and subsequent rumen alkalosis, as stated by Bertoni (1995) for dairy cattle. At large, no unique solution can be put forward to cope with this nutritional unbalance. No monoculture of any Mediterranean pasture species exists that is balanced for CP/energy for more than a short part of the grazing season. Nevertheless, the following ways can at least mitigate the excess of CP – mainly degradable protein – over energy (e.g. Peyraud and Delaby, 2006; Hoekstra et al., 2007):

1. Decreasing the herbage CP content by reducing N fertilization levels, increasing the length of grazing

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rotation, restricting access to pasture to afternoon hours, or other means; 2. Increasing the herbage energy content through a raise of non-structural carbohydrates (NSC) and in particular water soluble carbohydrates (WSC); 3. Lowering the degradability of herbage N in a way to reduce the ammonia loss at rumen level, by grazing for instance forages inclusive of condensed tannins; 4. Using supplements aimed at counterbalancing the excess of CP (e.g. high starch-low protein concentrates) or additives such as NH3 -binding components (Section 5). Points 1 and 2 are related to each other since there is a negative correlation between WSC and CP (e.g. Peyraud and Astiarraga, 1998). Points 1–4 refer to the balance between dietary nutrients (energy and protein) and the synchronization of rumen degradation of carbohydrate and N components. Management techniques used to mitigate the CP to energy ratio (point 1) will be described in Section 4. 3. Choice of Mediterranean forages for dairy sheep grazing systems Benefits could be obtained from grazing high energylow protein forages (e.g. Réarté et al., 2003), such as grass varieties rich in WSC. The WSC content of grazed plants is usually 5–20% (for grass) or 3–12% (for legumes) depending on forage species and variety, phenological stage (higher at the beginning of flowering or heading), weather pattern (higher under sunny conditions), time of day (higher in the afternoon), soil nutrition (lower with abundant N fertilization) and grazing management (higher under rotational grazing than continuous stocking; Jarrige et al., 1995). New cultivars of perennial ryegrass (Lolium perenne) have been selected recently whose WSC contents overpass by 20–40% (DM basis) that of a standard cultivar across all growing season (Lee et al., 2001). These authors found that lambs grazing a WSC-rich cultivar (cv. Ba11353) had higher average daily gain and carrying capacity, which resulted in 23% higher gain per hectare than the control. The off-take of pasture measured by the exclosure-cage method did not help to explain these results. However, in a parallel test of in vitro gas production measurement of herbage samples taken in the above study, the WSC-rich ryegrass had increased fermentation rate and glucogenic/lipogenic ratio as well as reduced ammonia concentration (Lee et al., 2003). In general, increasing the WSC in grazed herbage can be advisable for many reasons, such as the enhancement of

preference in sheep (Dove et al., 1999) and cattle (Smit et al., 2006), but the effects of WSC genetically enriched forages on intake and performance are not conclusive yet. Although the technique of spray-topping low doses of glyphosate (Siever-Kelly et al., 1999) for enhancing the WSC in annual grass-based pastures is effective, it is unattractive because it gives the perception of being hazardous for the environment. The use of forages which contain components such as tannins able to limit N degradation in the rumen (as suggested previously in point 3) is prone to success. However, one must remember that excess dietary tannins impair protein metabolism, enzyme activity, and overall sheep performance (Silanikove et al., 1994). Actually, the inclusion in sheep diet of pasture legumes containing moderate levels of condensed tannins (CT), such as the short-lived sulla (Hedysarum coronarium) and sainfoin (Onobrychis sativa) and the long-lived perennial birdsfoot trefoil (Lotus corniculatus), has recently gained consensus among scientists (see review by Mueller-Harvey, 2006). This is due to the following positive effects of such species on sheep: higher intake (in some circumstances, as reported by Molle et al., 2003), lower ammonia concentration in the rumen (e.g. Burke et al., 2002a), higher uptake of amino acids at gut level (Waghorn et al., 1987), overall lower N excretion and higher proportion of excreted N in faeces than urine (in sulla: Dentinho et al., 2006), lower emission of N and CH4 to atmosphere (Waghorn et al., 2002), improved resilience to gastro-intestinal parasites (Spiridoula and Kyriazakis, 2004), and better animal performances (review by Ramirez-Restrepo and Berry, 2005). Among these forage species, sulla, besides the moderate content of CT, displays a relatively high NSC level (Table 2), which also explains the good performance of meat sheep grazing this forage (Douglas et al., 1999). The responses of dairy sheep grazing sulla as monoculture or in association with grasses (annual ryegrass, Lolium rigidum Gaudin, or Italian ryegrass, Lolium multiflorum) are summarized in Fig. 2. These are results from studies on Sarda and Comisana ewes grazing at moderate to high stocking densities (20–80 ewes/ha) without supplementation. They highlight the potential of sulla as monoculture to improve milk performance in dairy sheep during spring (mid-late lactation phase), due to the higher intake of this forage as compared with ryegrass spp. (on average +20%). Moreover, Fig. 2 shows that sheep ingest more herbage (on average + 10%) and produce more milk when grazing sulla, associated with annual grasses either spatially, as adjacent monoculture, or temporally, as grazing sequence, than when graz-


Fig. 2. Index of intake (bars) and milk responses (dots) of ewes grazing either Italian ryegrass pastures only (baseline, index = 100), or i) sulla pastures in conterminal association with Italian ryegrass (50:50 by area) grazed in succession (‘rationed complementary grazing’, SURG) with 3 or 6 h access on sulla in the morning and the rest on ryegrass paddocks, ii) sulla pastures in conterminal association with Italian ryegrass (50:50 by area) with free access during the daytime (COMO), and iii) sulla monoculture pastures (SUMO). Error bars indicate S.E.M. (Sources: Molle et al., 1998, 2000, 2003; Di Miceli et al., 2005; Bonanno et al., 2007). Means and S.E.M.

ing grass monoculture. Burke et al. (2002b) found that lambs fed fresh sulla or its mixture (50:50 on DM basis) with either white clover or lucerne had higher daily gain and feeding efficiency, measured as intake to liveweight gain ratio, than those fed grass-based pastures. They also found that the adjacent monoculture (50:50) of sulla with grass pasture gave intermediate results in comparison with the corresponding monoculture-based diets (Burke et al., 2002a). Rutter et al. (2005) recently assessed the preference for sulla by using adjacent monocultures of annual ryegrass and sulla grazed by Sarda lactating sheep with access to the pasture for 22 h daily (except for the milkings at 8:00 and 16:00). These authors showed that preference for sulla was almost complete in the first two grazing hours in the morning, but decreased rapidly as time spent on the pasture progressed. On average, the preference for sulla, expressed as percentage of total grazing time, was 74%, which is close to the value of 70% usually found in sheep grazing white clover-perennial ryegrass as adjacent monocultures (Rutter, 2006). This behaviour has been attributed to the accumulation of CT in sheep rumen during the grazing process. For example, in lambs submitted to preference trials indoors, Villalba and Provenza (2002) found that, after the ingestion of tannins, the animals reacted to post-ingestive malaise by exploiting alternative feeding stations where tannin-rich feeds were less frequent. In a more recent study, Giovanetti et al. (2006) focused on lactating sheep grazing a sulla monoculture at flowering – when CT concentration peaks. The sheep were either drenched with water or with 100 g/day

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of polyethylene glycol (MW 4000), which specifically binds to tannins. Interestingly, the latter group exhibited longer grazing time (P < 0.07) than that exposed to CT effect. Although sulla is an outstanding legume forage to be incorporated in a grass-based grazing system, it does not adapt well to acidic and sandy soils. Thus, alternative forage legumes can be envisaged for inclusion in pastures for dairy sheep in association with grasses such as the ryegrass spp. However, species such as the berseem clover (Trifolium alexandrinum), Persian clover (Trifolium resupinatum) or the self-regenerating burr medic (Medicago polymorpha) are less suitable than sulla or sainfoin to counter the N excess in lactating sheep diet, while providing good herbage production as monocultures (see review by Rochon et al., 2004). The very high CP to energy ratio of burr medic (Table 2) can bring about large wastes of N when this species is grazed either as monoculture (Molle et al., 2002) or grass–legume binary mixture (Molle et al., 2007), as revealed by high milk urea concentration (often >60 mg/100 ml). Alternative forage species belonging to the daisy (Asteraceae) family can be usefully included in pastures for dairy sheep. Chicory (Cichorium intybus) is an interesting short-lived perennial forage with a tap rooting system, which allows this species to extend its growth cycle to the end of spring, even without irrigation. Nutritionally speaking, this forage is a good source of NSC, such as inulin; it has a relatively low fiber content and lower CP content than legumes in late-spring (Table 2), when it could be the only green forage available under rainfed conditions. Moreover, chicory contains plant secondary metabolites (PSM), such as phenolic compounds and sesquiterpene lactones, which are thought to elicit a positive effect against gastro-intestinal parasite infestation (e.g. Athanasiadou et al., 2006). Sarda dairy sheep rotationally grazing chicory monocultures yielded as much milk as those grazing sulla monocultures for three grazing seasons (Sitzia et al., 2006). In another study, late-lactation sheep grazed three pasture types based on chicory, the annual safflower (Carthamus tinctorius—a daisy plant), and burr medic in May–June (Landau et al., 2005b). Chicory-grazing sheep produced more 6.5% fat-corrected milk (P < 0.10) and had higher body weight gain than the other groups. They also had significantly lower milk urea concentration (MUC), probably due to the lower CP content (Table 2). Safflower was associated with a lower milk protein content. Safflower, as well as another daisy plant, the garland (Chrysanthemum coronarium), contains terpenes, which can partially impair rumen function. Terpenes were evoked to explain the very limited intake and poor performance of sheep fed

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fresh garland at flowering stage as sole feed (Addis et al., 2005). In contrast, when this forage species was grazed by late-lactation sheep as a mixture with annual ryegrass and burr medic, garland represented around 30% (DM basis) of the diet and the resulting milk performance was as good as that obtained with the binary mixture without garland (Cabiddu et al., 2006a). Results of recent preference trials have clarified that acclimatized sheep can counteract the effect of a toxic metabolite with that of another toxin ingested, provided the detoxifying mechanism is not the same for both toxins (Iason and Villaba, 2006). Examples of complementary toxic PSM are tannins and saponins, which explain the bloat curbing effect of tannin-containing legumes. Other classes of complementary toxins are nitrates and oxalates from one hand, tannins, terpenes and oxalates to the other, as reviewed by Provenza et al. (2007). 4. Grazing management of Mediterranean pastures The spatial distribution of different forages in a cultivated pasture (e.g. a grass (G) and a legume (L) in Fig. 3) can range from intimate mixture (case a) to monocultures in different paddocks (case d). An intermediate case is a patchy distribution of the two species within the same paddock, such as adjacent monocultures to which animals have free access (case b). Another possibility is to have temporary fencing (e.g. electric movable fences) between adjacent monocultures, with grazers having access to them in succession (case c). Time on pasture can be then split in two or more meal ‘blocks’. This is the simplest possible grazing circuit (Dumont

et al., 2001). Each option has its own costs and benefits. Scientific evidence has been recently accumulated suggesting that, probably due to higher cost of selection, intimate mixtures (case a) tend to reduce intake and performance of sheep compared with a patchy distribution of the different forages, as found in meat sheep grazing white clover and perennial ryegrass or subclover and perennial ryegrass conterminal monocultures (see Champion et al., 2004 and review by Chapman et al., 2007). Intimate mixtures have complex dynamics that are often conducive to dominance of one species over the others and often to weed spreading. Adequate fertilization, weed control and alternative uses, such as hay production at a specific phenological phase, are all more difficult to perform under these conditions. Under free choice of adjacent strips of different species (case b) sheep express their preference until the availability of the preferred forage goes down to a ‘switch point’, below which the previously less preferred forage begins to contribute more to sheep diet, often at the same proportion of the preferred forage (Harvey et al., 2000; Rook et al., 2002; Prache and Damasceno, 2006; Prache et al., 2006). This preferential pasture depletion, although mitigated along the defoliation process, can be conducive to under-grazing of the less preferred forage, as found by Molle et al. (2000) with sulla-annual ryegrass and by Prache and Damasceno (2006) and Prache et al. (2006) with perennial ryegrass-fescue as paired monocultures. In contrast, in the grazing circuit (which is the basis of traditional shepherding), sheep express their preference only up to an extent, which does not result in marked under-grazing of the less preferred forage if the time of access to each species is adequately tuned. In this case,

Fig. 3. Examples of spatial distribution of two complementary forage species, e.g. a grass (G) and a legume (L).


the best option is to offer the more preferred forage (e.g. the legume) in the morning and the less preferred forage (e.g. the grass) in the afternoon (Rutter, 2006). It is likely that the intake of grass is boosted in the main evening meal by the post-ingestive effect of legumes (via probable accretion of volatile fatty acids in rumen pool) as well as by the need to increase the intake of dietary fiber in order to ruminate it at night, thus postponing the uptake of nutrients when grazing is impaired by darkness and predator hazard. Interestingly, likewise legumes in simple grass–legume grazing circuits, the intake of PSM-enriched foods and total diet are usually higher when PSM are offered in the morning than when they are offered in the afternoon. This has been shown in a recent study on sheep where the intake of PSM-enriched foods (inclusive of tannins, terpenes or oxalates) was 20% higher in the morning than in the afternoon meal (Papachristou et al., 2007). Although it is possible to alternate monocultures established in permanent paddocks from day-to-day or period-to-period (case d), this practice is less preferable than the others, due to nutritional reasons. Indeed, the advantage of diverse diets in comparison with monospecific diets has been recently found by Champion et al. (2004) in sheep grazing grass and legumes as intimate mixtures, conterminal or separate monocultures. This was confirmed by Cortes et al. (2006) who compared different spatial combinations (cases a–c in Fig. 3) of two grasses (L. perenne and Festuca arundinacea) with their monocultures (case d, in Fig. 3). The authors found longer grazing time and higher intake in the sheep offered bispecific diets compared with those offered their preferred monoculture (L. perenne). From an environmental point of view, the proposed spatial distribution of forages is in general sustainable regarding the N leaching hazard. By rotating the monocultures (cases b–d), N accumulated underneath legume swards can be efficiently up-taken by the subsequent grass crop. In the case of intimate mixture (case a), however, a more immediate and – possibly – efficient transfer of N from the legume to the grass component is expected. Whatever the spatial distribution of forages, the grazing method can range from a continuous to an intermittent approach. Utmost limits are “one day grazing per season”, under extreme strip-grazing management, and “all pasture all season” grazing (continuous set or variable stocking). Studies conducted on sheep under adequate stocking rates have shown minor improvement in animal performance (e.g. Marley et al., 2007), if any, due to rotational grazing in comparison with continuous stocking (see review by Molle et al., 2004). This is in line with results from grazing cattle, in which the milk

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performance per cow and per unit land were compared (Peyraud and Delaby, 2006). Rationed grazing (limiting the time for access to the pasture) can also be taken into consideration, to provide an adequate feeding transition (e.g. from stall-feeding to all-day grazing regimens) and to mitigate the trampling effect on vegetation, particularly during wet climate conditions. Trampling and mudding the pasture biomass bring about soil compaction, impaired water utilization and herbage growth, and enhancement of water and soil nutrients’ runoff. However, it must also be recognized that rationed grazing may limit the herbage intake by sheep, particularly if herbage sward height or herbage allowance, or both, are low. Iason et al. (1999) showed that when herbage sward height was kept as low as 3 cm, a restricted time for access to pasture of 9 h/day had a marked negative effect on herbage intake, notwithstanding the increase in intake rate in the sheep submitted to the restricted access regimen. On the other hand, this decrease in intake did not occur when sward height was kept at 6 cm. Bonanno et al. (2007) fed unsupplemented Comisana dairy sheep on Italian ryegrass, sulla, or their 50:50 (by area) conterminal monocultures, with daily access of either 8 or 24 h/day. They found that herbage DMI and milk yield were both higher in the groups with unconstrained daily access to the pasture, irrespectively of pasture type. Also Latxa milked ewes set stocked on a natural pasture for 7 h/day showed significantly, although limitedly, higher milk performance in three out of four years than counterparts allowed to access the pasture for 4 h/day. Since the intake rate in sheep given shorter access time to pasture was higher than in those with longer access time, the herbage intake was similar between the two groups (Perojo et al., 2003; Garcia-Rodriguez and Oregui, 2003; Garcia-Rodriguez et al., 2005). The effect of restricted access time to the pasture is modulated not only by herbage accessibility but also by the horizontal and vertical distance walked daily by the flock between sheep house and pasture paddocks. In milked sheep, Ligios et al. (1997) found similar milk performance of Sarda ewes grazing Italian ryegrass with high herbage mass on offer during lactation and submitted to 0, 4.5 or 9 km/day of horizontal walking. However, in this experiment access time was not constrained. Also in sheep stall-fed ad libitum and submitted for the last 40 days of pregnancy to 0, 650 or 1300 m/day of horizontal walking, Decandia et al. (1996) were unable to detect any negative carryover effect of exercise on lamb birth weight, lamb daily gain and milk yield during the suckling period. In contrast, milk yield tended to be higher in the exercized dams.

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Stocking rate and density usually have an overwhelming influence on pasture and animal responses as well on the environmental impact of the whole grazing system. For example, short- to medium-term grazing intensity is crucial for modulating herbage allowance, sheep intake and, consequently, milk performance. The basic criteria for a sensible choice of grazing methods and stocking rates for different types of Mediterranean pastures, particularly forage crops, have been discussed by Molle et al. (2004). Less is known about the optimal grazing management of Mediterranean natural pastures, woodland or brushland. This would imply the implementation of grazing methods and pressures, which warrant adequate nutrition, conservation of pasture resource from year-toyear, and maintenance of pasture biodiversity. Aiming to reach these goals, a method has been recently proposed under conditions of heterogeneous pasture with woody vegetation (Agreil et al., 2004), based on the finding that sheep grazing shrubby rangeland are able to compensate for decreased herbaceous biomass by browsing more (Agreil et al., 2005). Providing sheep access to ‘functionally heterogeneous’ plants (inclusive of woody species) by adequate sheperding allowed them to reach intake levels much higher than those predicted for such kind of low-digestibility dietary components. Overall, this study suggested that the implementation of an intermittent or rotational grazing of heterogeneous pastures can be successful in terms of both sheep nutrition and landscape preservation. This is in contrast with the widely accepted statement that low-production grazing lands (rough grazing) are unsuitable for intensive grazing methods. The classical approach (lenient grazing under continuous stocking), however, tends to promote higher frequency of less preferred, so-called ‘unpalatable’ species – eat the best and leave the rest –, which requires the use of expensive and/or unsustainable weeding techniques (e.g. herbicide spreading, mechanical control). Weed species featured by ‘low palatability’ often contain PSM that have some anti-nutritional effects. Although information about this subject is scarce, results of fundamental research and some grazing experiments (Provenza et al., 2007) indicate that in order to curb this process the instant stocking density should be increased, so that sheep are forced to familiarize with the less palatable forages and ‘train’ their detoxification systems. The earlier the exposure in sheep life-time is, the longer the benefits last during their productive career. Such approach can be suitable for adapting replacement lambs – maybe with some adult sheep acting as ‘guide’ – to biodiverse pastures containing potentially noxious plants or plant parts. This intensive (short duration) rotational management (Provenza et al., 2006) could be realized through the

use of electric fences or close shepherding control. It is important to highlight that specific studies on milked ewes are required before suggesting the field application of this technique. Another approach for grazing heterogeneous pastures aiming at maintaining or improving species diversity, while obtaining economically viable animal performance, is the use of more than one herbivore species (mixed grazing or co-grazing). A successful example of simultaneous and sequential co-grazing of sheep and cattle has been recently given by Fraser et al. (2007) working on temperate herbaceous swards. Differently, on heterogeneous pastures including heather brush, in NW Spain, Celaya et al. (2007) found that, in spite of a certain dietary overlap, sheep showed a better coupling with goats than with cattle, in terms of animal performance. The discrepancy between these studies can be probably explained by differences in both vegetation types and grazing species or breeds. Defining an adequate stocking rate for each co-grazing species is rather difficult and should take into consideration both dietary overlap and pasture composition, as suggested by Animut and Goetsch (2008, this issue). Whatever the pasture type (forage crops or natural pasture) is, grasses usually are the primary component of dairy sheep diet under most of grazing conditions. This is particularly the case during the early phase of pasture growth (winter), when grasses overcome by far the growth rate of legumes and forbs, usually more sensitive to low temperature constraints. Under these conditions, applying proper management techniques to grass-based pastures can mitigate the CP to energy ratio unbalance. Limiting the level of N fertilization and increasing the frequency of N applications can influence positively dairy sheep nutrition and N farm-gate balance. In cattle-based grazing systems, a sensible reduction of fertilization rate on a perennial-ryegrass based pasture sharply reduces herbage CP and mildly reduces herbage CP and DM digestibilities as reviewed by Peyraud and Delaby (2006). Milk performance is little affected if the CP of the herbage on offer is above or equal to 150 g/kg DM (Peyraud and Delaby, 2006). The reduction of N fertilization is probably the most powerful tool to curb N emissions in cattle systems based on temperate permanent pastures. Indeed, the negative effect of a reduction of N fertilization level on milk production per unit area is more than counterbalanced by its positive effects on the N balance per unit of area. For example, reducing N input from 300 to 100 kg/ha lowered the milk yield per hectare by 20%, while the N surplus per hectare was reduced by 66% (Peyraud and Delaby, 2006). Lengthening the recovery period in a rotationally grazed pasture,


particularly if based on grasses, can cause a mild but significant reduction of herbage CP content. Unfortunately, data on the implementation of these techniques on Mediterranean pastures grazed by dairy sheep are lacking. However, it can be argued that there is probably limited scope for reducing N fertilization input to non-irrigated forage crops or pastures under Mediterranean climate. In fact, N fertilization levels usually range between 50 and 100 kg N/ha, because water is the main limiting factor for pasture production. Hence, in sheep farming systems, the product of stocking rate by the length of grazing season, which has a high impact on N surplus per land unit (Peyraud and Delaby, 2006), is expected to be more associated with cropping intensity, proportion of irrigated land and supplementation level than to N supply as fertilizer. 5. Supplementation of grazing sheep Concentrates often represent a high proportion of energy intake in grazing dairy ewes, particularly during lactation, but the main question is: how important is concentrate supplementation for grazing sheep nutrition and performance? By pooling the results of different experiments on dairy sheep, the milk response to concentrate supplementation was evident (Fig. 4) but relatively modest in comparison with estimates based on literature focused on dairy cattle. For example, the milk responses of dairy cattle and dairy sheep, in terms of milk net energy (Mcal) output per kg of concentrate, were 0.45 (Delaby et al., 2003) and 0.30 Mcal of milk per kg concentrate (this review), respectively, being the milk response of cattle 20% higher than that of sheep. Among the many reasons that could explain this difference, there are differences in size, morphology, physiology and genetic merit between these species, differences in supplementation level and quality of concentrates, dif-

Fig. 4. Milk response (g milk/kg concentrate) of dairy ewes grazing or stall-fed fresh forages, supplemented with fibrous (NDF 28–52%, NSC 24–48%) or starchy (NDF 21–28%, NSC 48–60%) concentrates at different levels (From Addis et al., 2005; Avondo et al., 1995; Cabiddu et al., 2006b; Decandia et al., 2007a; D’Urso et al., 1993; Marques and Belo, 2001).

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ferences in herbage mass and allowance as well as the differences in the quality of the herbage actually consumed by the two species in the reviewed studies. 5.1. Effects of pasture condition on substitution rates and animal performance In cattle grazing high quality pastures, the supply of concentrates often results in high substitution rates, especially when sward height is high (Bargo et al., 2003; Delaby et al., 2003). As a consequence, the total increase in DMI and energy intake, and the milk response can be small. Similarly, in sheep the substitution rate between high quality pastures and concentrate is often high (Bocquier et al., 1988). For example, in a study with Sarda ewes in early and mid lactation, continuously stocked on Italian ryegrass paddocks maintained at different sward heights (30, 60 and 90 mm compressed sward height) and supplemented with 500 g/day of whole corn per ewe, the substitution rate increased as sward height increased, under some circumstances exceeding 100% (Fig. 5, Molle et al., 1997). This effect was associated to a decrease of the milk response as the sward height increased and lactation progressed, although the latter effect was confounded with pasture maturity. Therefore, for ewes grazing good-quality pastures, the main role of concentrate supplementation should be, besides that of increasing milk yield and sometimes milk protein, to improve the balance among nutrients in the diet and to synchronize energy and N supply for optimal microbial growth. This would in turn reduce N wastage at animal scale and improve or maintain the health status of the ewe. Substitution of herbage with

Fig. 5. Substitution rate of grazed herbage by corn grain (%) in dairy ewes continuously stocked on Italian ryegrass pastures at different sward heights. The sward height was measured by a weighed square grass meter (Molle et al., 1997).

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concentrate can also be used to increase stocking rates or lengthen the grazing season where pasture availability is limited. These goals can be achieved by defining the appropriate dose, quality and distribution techniques of concentrates. 5.2. Interaction between animal characteristics and concentrate supplementation The absolute response to supplementation with concentrates can vary depending, among others, on the genetic merit and lactation stage of the sheep. There is a general shortage of data on these relationships in grazing sheep. In dairy cattle, the milk response to concentrate supplementation seems to be affected negatively by the stage of lactation and positively by the genetic merit of the animals, as reviewed by Bargo et al. (2003) and Delaby et al. (2003). The lower milk response in latelactation than in early-lactation is probably because as lactation progresses there is a homeorhetic change in the hormonal status of the animals, which favors the partitioning of dietary energy, especially that from starchy concentrates, towards body reserves deposition rather than milk production (Peel and Bauman, 1987). This mechanism seems to be more evident in low genetic merit cows (Peel and Bauman, 1987; Bargo et al., 2003). Similarly, Kennedy et al. (2003) observed lower substitution rates and higher milk responses in high than in low genetic merit cows. Since dairy sheep have not been subjected to the same intense genetic selection applied to dairy cows, it is likely that the 20% lower milk response of sheep compared to cattle previously reported is mainly due to differences in genetic merit. 5.3. Effects of quantity and quality of concentrate supplementation on sheep performance The definition of the optimal dose of concentrates can be complicated by the possible interaction between concentrate dose and concentrate quality. According to the dataset summarized in Fig. 4, as the supply of starchy concentrates increased from less than 300 g/day to above 400 g/day, the milk response of ewes actually decreased, while as the supply of fibrous concentrates increased, the milk response increased. In dairy cows, when starchy concentrates were compared with fibrous concentrates, Bargo et al. (2003) reported inconsistent effects on milk yield. Similarly, when comparing barley, corn, beet pulp and soybean hulls-based concentrates fed as supplements (600 g/day) to lactating sheep grazing Italian ryegrass, significant differences among treatments were hardly found for herbage intake and milk production,

although milk yield tended to be higher in the fibrous concentrates than in the starchy ones (Decandia et al., 2007a). A negative role of starchy concentrates in diets offered ad libitum to stall-fed mid- and late-lactation dairy sheep (Cannas et al., 2002, 2003) was explained by the fact that sheep are proner than dairy cows to divert energy to body fat deposition when concentrates that stimulate gluconeogenesis and insulin response (e.g. those rich in starch) are used. Concentrates rich in digestible fiber, in contrast, do not stimulate gluconeogenesis and insulin action, because acetate is their main fermentation product. In fact, in many experiments such concentrates stimulated milk yield more than starchy concentrates in ewes in mid- or late-lactation (Cannas et al., 2002, 2003; Bovera et al., 2004; Zenou and Miron, 2005). Differently, in early-lactation ewes, the positive effect of fibrous concentrates is absent or less evident (Cannas et al., 2002, 2003; Bovera et al., 2004; Zenou and Miron, 2005), probably because in periods of negative energy balance the high levels of growth hormone reduce the responsiveness of peripheral tissues to insulin (Peel and Bauman, 1987). The use of high levels of starch supplement can be risky in grazing sheep in late-winter or spring (midlactation), since grass NDF can still be low, while WSC can be as high as 20% DM. The provision of a small amount of hay (ca. 300 g DM per ewe), even when pasture is available, is considered a way to let animal meet their fiber requirements and to prevent sub-acidosis in sheep supplemented with starchy concentrates. As mentioned before, high-quality pastures generally provide N intake in excess to ruminal requirements. In this case, the utilization of concentrate rich in protein can further increase this unbalance. Excessive N intake may also be associated with inefficient microbial synthesis and, sometimes, with insufficient intestinal supply of microbial protein. Avondo et al. (2002) reported that herbage intake in ewes grazing pastures rich in CP was negatively correlated with the concentration of CP in the supplements, suggesting that ewes are able to sense their own nitrogen status and self-regulate, when allowed, their nitrogen intake. Unfortunately, the effect of CP supplementation on sheep grazing good-quality pasture is not easy to predict. Indeed, in lactating ewes grazing pastures fairly rich in CP (up to 19% CP), milk yield was significantly higher in ewes supplemented with concentrates rich in rumen degradable protein than in those supplemented with concentrates having lower degradable protein content, while there were no effects of the undegradable protein supply (Wilkinson et al., 2000). This was explained by the fact that the ratio between


rumen degradable protein and fermentable energy of the pasture was too low to maximize microbial activity. However, even the estimation of this ratio might not always be sufficient to explain the complex interaction between rumen N and energy supply and milk production. For example, in Assaf ewes milk protein concentration, but not milk or casein yield showed a quadratic response to ratios of rumen degradable organic matter (an estimate of rumen fermentable energy) to degradable protein ranging from 4.5 to 5.3 (Landau et al., 2005a). In order to mitigate the unbalance between energy and CP in sheep grazing lush pasture, one could think about supplementing them with small amounts of concentrates containing tannins (e.g. chestnut tannins) or other rumen NH3 -release modulating agents (e.g. terpenes, essential oils). However, information about this subject is still scanty for grazing sheep. For example, when 90 g/kg DM of quebracho soluble tannins were added to a supplementary concentrate fed at a level of 450 g DM/day, milk yield did not differ between tannin-fed and control dairy ewes, while milk fat content was lower in tannin-fed ewes (Garcia-Rodriguez and Oregui, 2003). Similarly, when Merino lambs fed pelleted diets were supplemented with 20 g/kg DM of chestnut tannins, no effects on average daily gain and carcass characteristics were observed (Frutos et al., 2004). In contrast, when there is a clear rumen N shortage in the rumen, the supplementation of the diet with proteinrich concentrates generally induces positive effects. For example, when Sarda ewes fed iso-energetic supplements of soybean meal or corn grain grazed mature pastures of low N content, pasture intake was higher with the soybean meal supplement than with the corn grain supplement (Molle et al., 1995). 5.4. Group feeding and strategies to account for animal variability Medium-large flocks of dairy sheep are characterized by high variability in milk yield and a certain variability in lactation stage. Thus nutrient requirements can vary markedly within the flock. However, in pasturebased systems usually all the animals of the same flock receive the same amounts of concentrate at milkings. The most frequent result of this feeding technique is that the most productive animals are under fed, while the least productive ones are over fed. Since most of the milk is produced by the ewes which have production levels higher than the mean production of the flock (Bocquier et al., 1995), underfeeding these animals reduces their milk yield, with serious economic

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consequences for the farm. In addition, animals which are too fat or too thin at mating display a lower fertility and prolificacy than those with adequate body condition (Rassu et al., 2004). For these reasons, a subdivision of the flock in two or more feeding groups should be taken into consideration. However, very little research has been carried out on the strategies to account for animal variability in grazing sheep. Even if several studies have been carried out on dairy cattle fed total mixed rations (e.g. Sniffen et al., 1993), this research is not much relevant for grazing sheep, for which the forage to concentrate ratio varies from one animal to another within the same flock. This is because variations in the amount of concentrate supplied can be, to a certain extent, compensated by variations in pasture intake in sheep, while in dairy cows fed total mixed rations the forage to concentrate ratio is fixed within each feeding group. To our knowledge, supplementation strategies for grazing ewes which account for animal variability and criteria to be used for the division of the flock into two or more groups have been addressed only by Bocquier et al. (1995). They reported that the groups should be set up preferably during late pregnancy, by separating the ewes with single birth from those with twin and triplets using ecographic diagnosis. They also suggested that during lactation the groups should be periodically checked and reorganized considering the milk produced or the body condition score of the animals (Bocquier et al., 1995). However, the frequent reorganization of the groups might pose problems of competition and adaptation among sheep, with possible losses of milk. Milk production is probably the easiest criteria to use to create subgroups in flocks with animals in the same stage of lactation. It also represents the main economic objective of the farm. Body condition score could be a more appropriate criteria in flocks characterized by large variation in the stage of lactation. Anyhow, the two criteria are correlated, especially after the first months of lactation. In practical terms, there are no particular problems involved in dividing the flock in two or more groups, when the farm is well organized and the pastures are subdivided in paddocks. Different groups can be kept in different paddocks, with the most productive ewes grazing on the best pastures, for the whole period for which different feeding regimes are deemed necessary. Sometimes the high yielding group grazes the paddocks ahead, being followed by the less productive or dry group (leaders-followers grazing technique). The groups should be milked separately and different doses or types of supplements should be supplied to each group.

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This is not the case if sheep graze open lands and pasture partitioning is not feasible. In this situation, one possible technique is to graze sheep as a single flock and to subdivide it in two or more productive groups, identified by marks of different colours on the fleece, before each milking. This requires the use of a corral with exits leading the ewes into different sheepfolds depending on the colours marked on their fleece. The different groups should be milked separately and fed the appropriate dietary supplements. This technique simplifies grazing management but requires additional labour before each milking. As electronic identification of sheep is increasing, automatic barrier management systems will probably appear in the next years. A third option, valid for all types of farms, is to use milking parlours equipped with electronic feeders that can supply individual doses of concentrate, according to the milk production of each ewe. Bocquier et al. (1995) reported that the utilization of this type of equipment did not affect milk yield, compared to ewes kept in a single group, but allowed to save 50 kg of concentrate per ewe per lactation. 5.5. Choice of the concentrate feeding time for optimal nutrient synchronization While the literature is rich in experiments testing the effects of different doses and, to a lesser extent, types of concentrates on the performance of grazing animals, much less research has been carried out on the effects of feeding time and nutrient synchronization on such responses. In sheep, Henning et al. (1993) showed that microbial yield and efficiency were not affected by the degree of energy and N synchronization when pulse doses of nutrients were supplied, while microbial activity was markedly improved when there was a steady supply of energy and N in the rumen. Witt et al. (2000) observed that the synchronization of dietary energy and N supply of stall-fed ewes did not improve milk yield but significantly reduced plasma urea concentration throughout the day. A marked improvement in microbial efficiency and reduction in rumen ammonia was observed in pasturefed male sheep when barley grains were fed just before or 2 h before grazing compared with barley supplementation 4 or 6 h before grazing (Trevaskis et al., 2001). In conclusion, it seems that improving the synchronization of energy and N availability is not expected to strongly influence milk yield. Nevertheless, adequate concentrate feeding schedules are beneficial because they can reduce the N excesses, and thus rumen ammonia, and blood and milk urea concentrations.

Even though in literature milk yield and the health status of the animals are not often affected by N excesses, it should be noticed that results of short-term experiments might not fully represent what happens in sheep subjected to N overload for months, as it often occurs in many farms. In such conditions, marked improvements in the nutritional status of dairy sheep (with lower incidence of diarrhea induced by N-excess and, in some cases, with higher milk yield) were observed when concentrates were distributed three times instead of twice a day, with one distribution of concentrate being given just before grazing on lush pastures in addition to the two normally given at milking time (Cannas, 2004). At field level, it is hard to achieve the appropriate feeding time and level of concentrate supplementation in grazing ruminants on an empirical basis. Most feeding systems and diet-balancing systems for ruminants are based on static models and thus are not able to make such predictions. Recently, Imamidoost and Cant (2005) published a dynamic model to estimate feeding time and level of concentrate supplementation in high-producing grazing ewes. Despite the low accuracy of some of its predictions, this model was able to predict with good accuracy the variation of the pasture to concentrate substitution rate, as the supply of concentrate increased, and the optimal number and time of supply of concentrate meals. 5.6. Effect of concentrate supplementation on N pollution at farm scale As previously mentioned, increasing the proportion of concentrates in sheep diets, particularly in those based on cereals, is an effective way to balance energy and N allowances and thus curb the release of N in the excreta by sheep. Giovanetti et al. (2007a) found that lactating sheep fed pelleted diets ad libitum had lower milk urea concentration and N excretion and higher N utilization efficiency (i.e. N in milk/N intake) with starch-based diets than with digestible fiber-based diets. However, Decandia et al. (2007a) were unable to confirm these results in grazing sheep offered a moderate supplementation level (600 g/day) of the same pellets. One of the possible reasons for the apparent contradiction between these studies was that although the level of supplementation in the grazing study was relevant, it was not sufficient to elicit the effects of dietary carbohydrate source and level observed in the previous study. Although at animal scale the use of concentrates rich of NSC generally limits N release, at paddock and farm level this effect can be smoothed or, at high supplementation levels, sometimes counterbalanced by the probable


increase of the stocking rate allowed by the herbagesaving effect of the supplement. This could result in higher N surplus per land unit (Peyraud and Delaby, 2006). Concentrates are usually extra-farm inputs, but even when produced on the farm the environmental advantage of their use is questionable, particularly when based on irrigated highly fertilized maize. On the contrary, the concentrate supply techniques that improve the synchronization of energy and protein availability might reduce N waste at both animal and system level. As mentioned above, small amounts of tannins via supplement could be utilized to decrease the N wastage at animal but also system scale. Dosing lambs with chestnut tannin, Sliwinski et al. (2002) confirmed the possible benefit of this approach to reduce rumen NH3 concentration, even though this reduction was associated with only a slight reduction of urine N losses (as % of N intake) and a limited increase of N utilization efficiency. This unclear effect might have been due to the very low tannin dietary concentrations used in the experiment (1–2 g/kg DM). The antimicrobial activity of essential oils against several pathogenic bacteria has been extensively demonstrated (Wallace, 2004). For this reason, in the last years much research has been carried out in vitro and in vivo to test the ability of essential oils to reduce rumen ammonia production in protein-rich diets. In general, it seems that while in short-term studies many essential oils were able to reduce ammonia production in vitro, by inhibiting the deamination of amino acids, the same effects were not observed in in vitro long-term studies or in vivo, probably as a result of shifts of microbial populations or of the adaptation of microbes to essential oils (Benchaar et al., 2007). The action of saponins seems to be more effective, compared to that of essential oils, mainly for their antiprotozoal activity and for their selective action against some bacterial species (Wallace, 2004). Indeed, the addition of Yucca schidigera, a plant species rich in saponins, to the diet of wethers significantly reduced rumen ammonia concentration and urinary N excretion, while it increased microbial N supply and efficiency (Santoso et al., 2006). Similarly, in wethers the addition of extracts of Y. schidigera or Q. saponaria, both rich in saponins, to diets based on ryegrass hay reduced rumen ammonia and volatile fatty acids concentrations without affecting fiber digestibility (Pen et al., 2007). In goats, the addition of saponins from Biophytum petersianum reduced rumen ammonia concentration and urinary N excretion (Santoso et al., 2007). However, while in most experiments the addition of saponins reduced rumen ammonia concentration and urinary N excretion and improved

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microbial protein synthesis and efficiency and sheep performance, in some cases it reduced microbial protein synthesis and efficiency (Wallace, 2004). As stated by Wallace (2004), the effects of saponins on ruminants are complex and depend on the diet and the saponins used. For these reasons, more research is needed to clarify when their use in the field is advantageous. 6. Monitoring nutritional unbalance in grazing sheep To effectively manage the complexity of the dairy sheep grazing systems in the light of new scientific achievements, decision support tools are required. Collecting a sample which is representative of the diet ingested by sheep is a challenge because of their selective behaviour. Measurements or subjective estimates of pasture variables, such as the sward height, herbage mass or allowance, and pasture botanical composition, are time consuming and do not provide an accurate picture of expected grazing system response unless the pasture consists of homogeneous, actively growing monocultures or simple mixtures. Residual sward height is often used as global indicator of pasture availability (e.g. NRCS, 1997), but its accuracy for a specific pasture should be adjusted to the animal genotype, as herbivore species, and breeds with different sizes often have different sward height optimal profile (Osoro et al., 2002). Under heterogeneous pasture conditions inclusive of woody vegetation subjective visual assessment of the availability of different putative feeding sources (particularly the parts of plants which make the most of intake during paddock depletion) is pivotal to the grazing tactic (Agreil et al., 2004). Evaluation of the biomass available in paddocks and its quality could be also based in the future on on-farm or remote-imaging technologies (e.g. Shut et al., 2005), as suggested also by preliminary results on Mediterranean pastures (Fava et al., 2007). Indeed, although a well-based mechanistic feeding system focused on dairy sheep has been recently released (Cannas et al., 2004), its ability to predict intake of grazing sheep has to be improved. Novel empirical prediction systems of small ruminant intakes based on regression analysis, using pasture and animal production characteristics, such as milk yield and composition, body weight and body condition score, as independent variables represent an important step forward in this direction (Avondo et al., 2002, for sheep; Decandia et al., 2005, for goats). Mechanistic prediction systems of grazing sheep intake such as that by Baumont et al. (2004) are promising for a more significant and long-lasting

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scientific advancement but, in order to become practical for dairy sheep in Mediterranean environment, they need to integrate appropriate ‘key’ relationships, which are currently lacking. Milk urea concentration (MUC) is an effective gauge of protein nutrition in grazing sheep, as previously found by Cannas et al. (1998) under confinement conditions. In a 3-year study, lactating sheep grazed three binary mixtures consisting of annual ryegrass, and either burr medic, subclover (Trifolium subterraneum) or sulla. Pooling data (N = 72) of the average group dietary CP percentage and plotting them against the average MUC, a linear relationship which explained 0.55 of total variation was found (Molle et al., 2007). According to this equation, for CP dietary levels ranging between 15 and 20% DM, MUC varies from 32 to 43 mg/100 ml. In the same study a moderately strict linear relationship between CP/NEL and milk urea was found: CP (g/Mcal NEL )=1.62 MUC(mg/100 ml) + 68.44; NEL R2 = 0.55. A more fundamental research has confirmed that MUC is the best single predictor of the ratio between CP and NEL , since the relationship between MUC and CP concentration is modulated by the dietary energy level (Giovanetti et al., 2007b, Fig. 6). Interestingly, there is a good agreement between the slope of the regression above reported and that of Fig. 6, which suggests that relationships across different feeding regimens have similar trends. A side-achievement of the study by Giovanetti et al. (2007a) has been the finding that MUC is well related with N excretion as urine and N utilization efficiency. Validating this relationship in grazing sheep will make MUC an outstanding variable for monitoring, besides nutrition, the environmental impact of dairy sheep industry. However, its practical application could be risky outside the specific genetic (animal and for-

Fig. 6. Relationship between CP/NEL and milk urea in stall fed dairy sheep (Giovanetti et al., 2007b).

age) and environmental conditions wherein they were built. While the above results are promising, monitoring pasture intake and quality by analyzing chemical attributes of faeces holds potential because faecal samples are easily obtained, and they are always representative of the animal and the diet selected on pasture. The organic matter digestibility of a tropical pasture has been shown to be correlated (R2 = 0.74) with faecal nitrogen concentration (Boval et al., 2003). The information encompassed in faecal samples has been further exploited when faecal Near Infrared Reflectance Spectrometry (FNIRS) pioneered at Texas A&M by Lyons and Stuth (1992) for cattle was applied to sheep. In the FNIRS methodology, chemometric analysis enables to predict a wide array of dietary attributes, relying on the characteristics of faecal spectra (see review by Landau et al., 2006). The technology has allowed to predict in confined sheep: dietary CP content with high precision (R2 > 0.95) and accuracy in a range between 0.24% (Decandia et al., 2007b) and 1.1% (Li et al., 2007); OMD with moderate precision (R2 in a range between 0.80 and 0.90) and accuracy between 1.5% and 3.4%; fiber fractions, in particular lignin with good precision (R2 = 0.96) and accuracy (0.27%). In addition, ADF and NDF insoluble CP, i.e. dietary CP of low availability, could also be predicted (Decandia et al., 2007b). The first attempt to determine botanical composition by FNIRS (Walker et al., 1998) showed potential to estimate the dietary percentage of Euphorbia esula L. in sheep fed diets with four different forage ingredients, featuring an accuracy of 5% and a R2 value of 0.96. However, NIRS equations in general, and FNIRS equations in particular, cannot be extrapolated beyond the conditions represented in calibration samples (Coleman et al., 1995). FNIRS equations established for goats fed three species of browse and concentrate (Landau et al., 2004) were not robust to conditions in which free-ranging goats ingested diets with more than ten ingredients, including the three browse species used for calibrations. However, in a further study on goats Landau et al. (2005b) found that external validation of FNIRS equations of dietary CP and in vitro digestibility was successful if the structure of predicted population was similar to that of calibrating population. In other words, FNIRS can be successfully used for grazing under farm conditions only if a wide dataset encompassing a wide array of botanical and chemical diet composition is available, from which calibrations are calculated ad hoc to assess dietary attributes (see review by Landau et al., 2008). The complexity of establishing FNIRS equations under grazing situations has been overcome in grazing goats by using feeding observational


data (e.g. qualitative and quantitative bite counting) as reference values to calibrate equations (Glasser et al., in press) and similar work needs to be done for dairy sheep. To summarize, the FNIRS method is non-invasive and environmental friendly, and provides quickly accurate information as long as calibrations are judiciously used. 7. Practical application and future research needs A general strategy for feeding dairy sheep on pasture entails an adequate choice of forages as well as effective grazing and supplementation management. A detailed description of year-long feeding models under different scenarios is beyond the scope of this review, since aspects such as the effect of nutrition on reproductive performance were deliberately overlooked here and are discussed elsewhere (e.g. review by Rassu et al., 2004). However, for practical purposes, an outline of a general feeding strategy to be implemented on field is sketched in Fig. 7. It depicts the management of autumn-lambing dairy sheep in a semi-intensive grazing system based on non-irrigated Mediterranean forages. For simplification

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reasons, the pasture growth curve shown in the body of Fig. 7 encompasses both natural pasture and forage crops, although the specific growth profile of these pastures does not usually overlap (e.g. Guerin and Gautier, 2004). Pasture growth refers to two main classes, i.e. grass and other species (legumes and forbs such as Asteraceae), highlighting the need for a diverse diet throughout the production cycle. These classes represent forage species, which have complementary nutritional values and contain moderate levels of a variety of secondary compounds, referred to as antinutritional until recently. In practice, a high-quality grass such as Italian ryegrass could be complemented by legumes like sulla or sainfoin and Asteraceae like chicory. Establishing these forage crops can be of interest for non-irrigated farms aimed at medium to high production per unit of land (about 750–1500 kg/ha of milk). A prerequisite for choosing forage species is that under local conditions they show adequate agronomic performance, namely biomass production and its distribution within and across grazing seasons (persistency). Attention should also be directed to the economic viability of forages at farm scale and their effect on food quality, inclusive of ‘nutracines’

Fig. 7. Outline of a general strategy for the feeding of autumn-lambing dairy sheep grazing semi-intensive, non-irrigated Mediterranean forages. Data and curves are only indicative. Fibrous concentrates refer to concentrates based on digestible fiber sources such as soy hulls and sugar beet pulps.

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(i.e. food components having putative beneficial effects on consumers’ health, such as CLA and polyunsaturated fatty acids) and sensory properties. The use of forages with double purpose such as those adequate for both grazing and hay or silage making and, possibly, winter cereals should be kept in mind in order to decrease the import of extra-farm feed resources. The offer of the above complementary forages, as adjacent monocultures or grazing circuits, is more beneficial for the nutrition of lactating sheep than that intimate mixtures of the same species. In any case, some degree of freedom should be given to sheep in order to let them adjust their diets. The grazing methods described in Fig. 7, which are for guidance only, basically refer to systems where the presence of forage crops is relevant (at least 30% of the farm area). Even when supplementation of grazing sheep with hay and concentrates based on cereals increases milk yield only slightly, it is often helpful for keeping the stocking rate above the carrying capacity allowed by pasture during slow growing periods (autumn–winter, Fig. 7). Digestible fiber-based concentrates provide better milk responses, particularly in mid-late lactation sheep, while high to medium protein-rich concentrates based on legumes such as soybean or peas or dehydrated lucerne can compensate for the CP shortage in standing-hay and stubbles during summer (Fig. 7). Tactics aimed at adapting the above general strategy to the ever-changing climate and pasture conditions should be based on a sensible use of the monitoring tools described in Section 6, such as sward height, body condition score and milk urea. Although progress has been made in the feeding and grazing management of dairy sheep, relevant research areas are still undermined. For example, the relationship between sward height and intake or performance of dairy sheep has been explored only for a few forages and under a narrow set of grazing conditions (see review by Molle et al., 2004). There is also urgent need of research on grazing circuits for dairy sheep and automated monitoring and shepherding of flocks. ‘Remote shepherding’ has been suggested by the use of remote-controlled gates or virtual fencing (Anderson, 2007). An intriguing but also distant scientific target is the decoding of sheep vocalization in order to monitor and possibly control flock management. For example, in the future we might be able to lead flocks from milk-shed and back along the grazing circuit by the re-play of ‘key-bees’. Advancements in the acoustic monitoring of feeding behaviour (e.g. Ungar and Rutter, 2006) could be regarded as a first step in that direction. Studies on the synchronization of energy and N availability in the rumen by tuning the administration time of the concentrate relative to grazing time are

also warranted, since data on sheep are scanty. The same applies to the use of supplementary tannins, essential oils or other NH3 -binding agents. All these areas urge new research efforts if the release of N to the environment has to be effectively curbed. Besides research, technology transfer to farm scale is also required for enhancing grazing management feasibility. For example, widening the FNIRS spectra database and standardizing NIRS equipment at regional and broader scale is a sensible target, if grazing livestock management has to make a quicker move from ‘art’ to ‘technology’. Acknowledgment The authors gratefully acknowledge the skilful assistance of Dr. Ana Helena Dias Francesconi for editing the manuscript. References Addis, M., Cabiddu, A., Pinna, G., Decandia, M., Piredda, G., Pirisi, A., Molle, G., 2005. Milk and cheese fatty acid composition of sheep fed different Mediterranean forages with particular reference to CLA cis9–trans11. J. Dairy Sci. 88, 3443–3454. Agreil, C., Meuret, M., Vincent, M., 2004. Grenouille: une méthode pour gérer les ressources alimentaires pour des ovins sur milieux embroussaillés. Fourrages 180, 467–481. Agreil, C., Fritz, H., Meuret, M., 2005. Maintenance of daily intake through bite mass diversity adjustment in sheep grazing on heterogeneous and variable vegetation. Appl. Anim. Behav. Sci. 91, 35–56. Anderson, D.N., 2007. Virtual fencing – past, present and future. The Rangeland Journal 29, 65–78. Animut, G., Goetsch, A.L., 2008. Co-grazing of sheep and goats: benefits and constraints. Small Rumin. Res. 77, 127–145. Athanasiadou, S., Gray, D., Younie, D., Tzamoloukas, O., Jackson, F., Kyriazakis, I., 2006. The use of chicory for parasite control in organic ewes and their lambs. Parasitology 134, 299–307. Avondo, M., Licitra, G., Bognanno, M., Keshtkaran, A.N., Marletta, D., D’Urso, G., 1995. Effects of the type and level of supplementation on grazing behaviour of lactating ewes in a Mediterranean natural pasture. Livest. Prod. Sci. 44, 237–244. Avondo, M., Bordonaro, S., Marletta, D., Guastalla, A.M., D’Urso, G., 2002. A simple model to predict the herbage intake of grazing ewes in semi-extensive Mediterranean systems. Livest. Prod. Sci. 73, 275–283. Bargo, F., Muller, L.D., Kolver, E.S., Delahoy, J.E., 2003. Invited review: production and digestion of supplemented dairy cows on pasture. J. Dairy Sci. 86, 1–42. Baumont, R., Cohen-Salmon, D., Prache, S., Sauvant, D., 2004. A mechanistic model of intake and grazing behaviour in sheep integrating sward architecture and animal decisions. Anim. Feed Sci. Technol. 112, 5–28. Benchaar, C., Calsamiglia, S., Chaves, A.V., Fraser, G.R., Colombatto, D., McAllister, T.A., Beauchemin, K.A., 2007. A review of plantderived essential oils in ruminant nutrition and production. Anim. Feed Sci. Technol., doi:10.1016/j.anifeedsci.2007.04.014.

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