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The Rangeland Journal http://dx.doi.org/10.1071/RJ12016
Sustainable sheep management using continuous grazing and variable stocking rates in Patagonia: a case study
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Gabriel Oliva A,D, Daniela Ferrante A, Silvina Puig B and Marcos Williams C A
INTA EEA Santa Cruz, Mahatma Ghandi 1322, 9400 Río Gallegos, Santa Cruz, Argentina. Estancia Los Pozos, Casilla de Correo 74, 9400 Río Gallegos, Santa Cruz, Argentina. C Universidad Nacional de la Patagonia Austral, Libertad 304, 9400 Río Gallegos, Santa Cruz, Argentina. D Corresponding author. Email:
[email protected]
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Abstract. Adaptive rangeland management based on continuous grazing by sheep was implemented at Los Pozos station, on the Magellan tussock steppes of southern Patagonia, between 1990 and 2010. The numbers of sheep were adjusted annually following a stocking strategy tracking the herbage mass. Herbage mass was assessed by harvesting short grass species at peak herbage mass and grazing capacity was estimated based on sheep requirements. Historic production records under a traditional continuous grazing management (1930–89) were contrasted with those of the adaptive management period (1990–2010). Stocking rates of 0.59 sheep ha–1 year–1 in the period from 1930 to 1989 dropped to 0.35 sheep ha–1 year–1 in the adaptive management period. Mean herbage mass of short grasses during this latter period was 194 12 kg DM ha–1 and the stubble height of the key species, Poa spiciformis, was 24 0.7 mm. Lamb marking rates and extraction rates increased from 72 to 87%, and from 28 to 48%, respectively, under adaptive management when compared with historic management, and historic levels of sheep sales were maintained: 4405 v. 4602 animals year–1. Wool production fell from 36 102 to 26 771 kg year–1. Annual sheep production under adaptive management was less variable, as the annual coefficient of variation fell by 41% for lambing rates, by 25.8% for wool production, and by 40% for sheep extraction rates, compared with historic records, even though the inter-annual rainfall coefficient of variation increased by 25% over the same period. It is concluded that, under adaptive management, herbage production, stubble height and sheep production remained stable over 20 years. The adaptive strategy allowed a stabilisation of production and is a potentially valuable management tool in view of the expected increase in variability of rainfall predicted by global climate models for Patagonia and other rangeland areas.
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Additional keywords: domestic animal production, grazing pressure, perennial grasses, rangeland management, semiarid rangelands. Received 23 March 2012, accepted 3 July 2012, published online dd mmm yyyy
Introduction
Patagonian region, is a vast area of ~750 000 km2 between latitudes 398 and 558 in Argentina and Chile, predominantly covered by a cool semiarid steppe of grasslands or shrublands, 5 that extends into Chile around the Straits of Magellan (Paruelo et al. 2004). Sheep, mostly of Merino and Corriedale breeds, were introduced in the region in the late 1800s and gave way to the traditional stations, extensive production systems that supply wool and meat for export and domestic markets. The total 10 stock peaked at ~20 million head by 1950 in the region. There are currently ~10 000 livestock producers, most in small subsistencetype operations (INDEC 2005). Only 10% of them are mediumsize stations that run over 4000 head as our study case, but they cover a great proportion of the area, especially in the south. Most 15 of the land is grazed continuously, with little use of grazing systems, supplementation or winter feeding. Land carrying capacity was overestimated early in the production history and Journal compilation Australian Rangeland Society 2012
overgrazing at a regional scale was the rule for over half a century. In recent decades, rangeland degradation, low prices of the products and higher costs (DHV SWEDFOREST 1998) have caused stock reduction to 9.7 million head (INDEC 2005). Los Pozos was one of the first stations to introduce sheep into 5 the tussock grasslands of the Magellan Steppe in the south continental part of the region. The station has reared dual-purpose Corriedale sheep since 1880. The traditional management of this station included continuous grazing with stocking rates (SR) that were originally determined by trial and error as on other 10 Patagonian stations (Barbería 1994; Golluscio et al. 1998). Faced with lower individual sheep performance, the owners made periodic reductions in sheep numbers throughout the history of the station, a trend which was particularly severe in the 1960s. Such changes have been common in the sheep stations across 15 Patagonia (DHV-SWEDFOREST 1998; Golluscio et al. 1998; Cibils and Borrelli 2005). Most authors have interpreted this as www.publish.csiro.au/journals/trj
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the 1980s and applied widely in south Patagonia (Borrelli and Oliva 1999). The Los Pozos station applied this grazing management scheme consistently over 20 years, conducting annual assessment of herbage mass and adjusting sheep numbers. The results of the change in management could be 5 assessed in this case because the station had kept annual production records since 1930. This type of systematic information on management and production throughout a period of 80 years is exceptional in Patagonia. The objective of this paper is to demonstrate the impact of 10 20 years of adaptive management on indices of sheep production by comparing them with those of the period from 1930 to 1989 under traditional management.
the result of high SR leading to a reduction in herbage mass and productivity (Soriano 1956) but some have emphasised that the main problem was inadequate distribution of sheep in heterogeneous paddocks and continuous, instead of rotational, grazing systems that led to range degradation (Paruelo et al. 1992; Golluscio et al. 1998). Some producers, on the other hand, have interpreted these trends as a consequence of climate change with reduced rainfall (Andrade 1998; Caceres et al. 2006). In the late 1980s, faced with reduced individual sheep performance, the Los Pozos owners decided to implement a different management scheme based on quantitative estimates of herbage mass. The initial forage evaluation and grazing plan was developed in 1989–90. Based on the premise that the most deleterious effect on rangeland is a too-high SR in relation to the herbage mass (Heady and Child 1994), the grazing plans developed for Los Pozos by P. Borrelli (pers. comm.) consisted of an adaptive scheme based on year-round continuous grazing with variable (tracking) SR that were adjusted yearly on the basis of assessments of herbage mass. This approach was developed in
Los Pozos is located in the Dry Magellan Steppe (Fig. 1), 30 km north of Río Gallegos, Santa Cruz, Argentina (698190 W, 518290 S). The climate is maritime, with an annual rainfall of
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Fig. 1. (a) Location of Los Pozos station in south Patagonia; (b) map of the cadastral division, including the city of Río Gallegos (circle) – the shaded area indicates the Dry Magellan Steppe; and (c) Landsat TM image of the station including the paddocks.
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Grazing capacity, defined as ‘the number of animals that produce the greatest return without damage to the physical resources and in concert with other values received from the land’ (Heady 1975), is a concept that is considered to be equal to carrying capacity (Scarnecchia 1990) and was estimated annually. The total herbage mass of short grasses, graminoids and forbs was used in these estimations without taking into account losses due to trampling or other factors. As short grasses, graminoids and forbs represent ~29% of the aboveground net primary production of the Magellan Steppe (Cibils et al. 2005; Ferrante 2011), these assessments are roughly comparable to those obtained with well known methodologies, such as double sampling (Tothill et al. 1978), pastoral value (Daget and Poissonet 1971) or range condition matrix (Dyksterhuis 1958), that estimate total herbage availability but include a use factor of ~30%, as is common in many semiarid regions (Holechek 1988). The sheep units used were ewe equivalents (EEq), that consider the energy requirements of a 49-kg ewe shorn in spring (September) that produces a 20-kg lamb after 100 days of lactation, which is estimated at 2.79 MCal day–1. Assuming a mean digestibility of the herbage of 0.55%, these requirements were estimated to be 513 kg DM of herbage EEq year–1 (Borrelli 2001). The methodology used to assess herbage implies that each sheep has an extra allowance of tussocks and short shrub forage that is not quantified. One EEq equals 1.01 ewe equivalents used in New Zealand (Cornforth and Sinclair 1984), 1.54 dry sheep units used in Australia (Committee on Agriculture and Resource Management 1990; McLaren 1997), and 0.12 animal units used in the United States (SRM 1989). The number of grazing sheep was adjusted every year based on the estimated grazing capacity of each paddock. Total numbers of sheep on the station varied accordingly as the sum of the capacity of each paddock. The station owners chose to maintain SR slightly below estimated grazing capacity. The impact of grazing was monitored using a key species residue technique. A key species is defined as ‘a forage species whose use serves as an indicator to the degree of use of associated species, and because of its importance, must be considered in any management program’ (SRM 1989). In this case stubble height measurements of P. spiciformis (formerly Poa dusenii) (Giussani 1993) were used. The species is a dominant, preferred short grass with a total cover of ~10% in the
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The Santa Cruz range assessment method (Borrelli and Oliva 1999; Borrelli 2001) developed by the Instituto Nacional de Tecnología Agropecuaria was used to estimate herbage mass. This methodology is a harvest technique (Sala and Austin 2000), based on the estimation of herbage on offer by clipping 0.2-m2 samples at peak herbage mass (January) at predetermined sites distributed across a paddock. Eight samples were taken in uniform paddocks, and this number increased to 12 when less uniform paddocks were evaluated. Only short grasses, graminoids and forbs were harvested. The residual stubble height of P. spiciformis, was recorded at each site, and usually 10 pasture height measurements were obtained per herbage sample. Clipped herbage was dried and weighed, and this value was considered the herbage mass. Stubble height was used as an indicator of grazing intensity, and a value of at least 20 mm was set as a target to regulate grazing pressure. The method (Borrelli and Oliva 2001b) has been successfully applied in south Patagonia but is not widely used outside this region. It does not take into account the forage of lower nutritive value produced by tussocks or woody plants. Short grasses and herbs have the
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highest nutritive value (Lara and Cruz 1987), and constitute 73% of the diet of sheep in the summer (Posse et al. 1996; Pelliza et al. 1997). Sampling of herbage was performed annually in 10 paddocks of the station, that ranged between 1200 and 4700 ha, 5 with a mean of 2700 (standard deviation of mean 1579) ha. These management units were relatively uniform so that the sampling sites were distributed systematically, following existing tracks. In each paddock, three permanent sampling sites were established, and three quadrats harvested from each site. The 10 samples were oven-dried for 48 h at 408C and weighed. Total herbage mass of each paddock was estimated as the mean herbage mass (kg DM ha–1) of the sites weighted by its area.
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239 mm distributed evenly throughout the year (Burgos 1985). Temperatures are low, with a mean of 12.78C in summer and 1.48C in winter (De Fina et al. 1968). Winds, mainly from the west, are constant, with a yearly mean of 27 km h–1 and frequent gales in spring and summer. Rainfall records were obtained from the Río Gallegos airport (Servicio Meteorológico Nacional, Buenos Aires, Argentina), which is located 30 km from Los Pozos. The station occupies 27 200 ha in a landscape that includes terraces of the Gallegos river in the south and a series of sedimentary plateaus of Tertiary origin reaching 150 m above sea level (Anchorena 1985). Soils are borolic haplargids (Salazar Lea Plaza and Godagnone 1990), with a 10-cm depositional layer of sand that is rich in organic matter in the surface, and abundant pebbles cemented with clay in the deeper horizons. The plateaus are flat and impeded drainage generates a series of lagoons (‘Pozos’) that give the station its name. These closed basins bear water bodies of different sizes but only a few of them are permanent. In dry periods the amount of water declines or disappears, and wind erosion generates extensive plumes of airborne sediments that are often salty. The vegetation is dominated by tussocks of Festuca gracillima. The spaces between the mounds of these tall caespitose grasses are covered with a diverse array of short grasses dominated by Poa spiciformis, graminoids (Carex spp.), forbs and short, prostrate shrubs, mostly Nardophyllum bryoides (Faggi 1985). About 70% of the plant cover consists of perennial species, annuals are few and have a low nutritive value for sheep. Annual aboveground herbage mass is ~980 kg DM ha–1 with ~286 kg DM ha–1 of short grasses and forbs (Cibils et al. 2005; Ferrante 2011). The station has 10 large management units and several smaller paddocks. Good quality drinking water is provided by bore holes and springs in the lower parts of the lagoon basins. Los Pozos produces lambs and wool, and 83% of the flock consists of Corriedale ewes.
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Magellan tussock grasslands (Faggi 1985; Humano et al. 2005). The height of 20 individuals per sampling site, totalling 60 per paddock, was measured annually from the crown of the plant to the modal height of the leaves using a ruler. Previous studies indicate that this species accumulates ~50% of the aerial biomass at a height of ~20 mm. If the measured stubble height is kept at 20 cm, the sheep use only 50% or less of the harvested herbage, and smaller residues are considered to indicate overgrazing (Cibils et al. 2001). Sheep production data were obtained from the historic records of the station. Flock numbers included all categories of sheep remaining on the station after lambing and selling. Lambing rate (%) was estimated from lambs marked (January) and ewes mated (May of the previous year). Extraction rate (%) was estimated as the ratio of total numbers of sheep sold (including rams, wethers, lambs and ewes) to flock numbers, expressed as a percentage. Total clean wool weight sold was estimated from greasy wool weight multiplied by the percentage of clean wool. This percentage was obtained from analysis of the wool by the Consejo Agrario Laboratory from 1972 to 2010. Prior to 1972, no data on wool analyses were available so the percentage of clean wool during that period was estimated at 50%. This was considered typical of wools received in the Consejo Agrario Laboratory before the 1970s. The relationships between SR, lamb marking rate (LM) and clean wool production (WP) were explored using the SAS/Stat version 9.0 statistical package.
mean annual rainfall was observed throughout the 80-year record, although a dry period was evident during the 1960s, when the mean annual rainfall was only 160 mm. Four dry years (rainfall
