CSIRO PUBLISHING
Animal Production Science http://dx.doi.org/10.1071/AN16012
Improving forage nutritive value and botanical composition in a natural grassland using different grazing methods and herbage allowances F. Jochims A, C. H. E. C. Poli B, E. M. Soares C,D and P. C. F. Carvalho B A
Santa Catarina Research and Rural Extension Company (Epagri), Research Center for Family Farming (Epagri/Cepaf), Chapecó, State of Santa Catarina, 89801-970, Brazil. B Animal Sciences, Universidade Federal do Rio Grande do Sul, State of Rio Grande do Sul, Brazil. C Universidade Federal de Santa Maria, Santa Maria, State of Rio Grande do Sul, 97150-900, Brazil. D Corresponding author. Email:
[email protected]
Abstract. The present study aimed to define the most appropriate management for a natural grassland during winter so as to improve its botanical composition and forage nutritive value during the subsequent spring. The experiment was conducted in an 8.4-ha Pampa biome natural-grassland area and divided into 12 0.7-ha paddocks for 196 days. During the first 84 days (winter), two grazing methods (GM), namely continuous (CS) and rotational grazing (RS), and two herbage allowances (HA), namely 12 and 18 kg DM/100 kg bodyweight, were imposed on the ewes in early pregnancy, which resulted in the following four treatments: C18, C12, R18 and R12. From Day 85 onward (final third of pregnancy), all paddocks were managed with C18 so as to avoid intake restrictions. The sward was characterised by herbage mass (HM), green leaf mass (GLM), stem mass, senescent material, legume mass, forage growth (FG), canopy height, canopy density and weeds mass (other than grass and not preferred species). The qualitative characteristics were shown as neutral detergent fibre (NDF), acid detergent fibre (ADF), lignin, crude protein (CP) content and ingested crude protein. Although GLM was higher when RS was applied, HM did not differ between GM and HA when management factors were applied during winter. In spring, increased HM was observed in paddocks managed with 18% HA. Weeds mass presented GM · HA interaction, with the lowest participation in Treatment C12. The NDF and ADF levels differed between HA, in addition to being higher in 12% HA and during winter periods. In spring, the lowest NDF levels were found in paddocks under 12% HA and ADF changed only along the periods. The GM and HA applied during winter did not change the lignin content. The CP in winter differed only over the periods. Treatment C12 was applied in the winter and resulted in 8.1% more CP than did C18, R12 and R18 in the spring. There was a GM · HA · Period interaction in ingested CP, revealing that the treatments during winter influenced the quality of the ingested herbage during spring. Ewes in C12 ingested herbage with more CP than did the animals in the other treatments. Variations in grazing methods combined with HA during winter influenced the chemical and structural characteristics of the sward. High stocking rate with continuous stocking presented better chemical characteristics than did the other treatments and the use of high stocking rate in continuous grazing may have an important effect on spring pasture quality due to changes in sward structural characteristics. Additional keywords: crude protein, forage quality, neutral detergent fibre, Pampa biome, rangeland, sheep.
Received 7 January 2016, accepted 23 March 2017, published online 26 May 2017
Introduction In southern Brazil, 76% of agricultural land used for livestock production is composed of extensive production systems in which natural grasslands have been the main forage system in the past three centuries (Carvalho and Batello 2009). This region presents an important climatic variation throughout the year; frosts are common in the winter and high temperatures (above 35C) are frequent in the summer. These grasslands are formed mostly by species with C4 metabolic cycles (Bilenca and Miñarro 2004), which reduce their capacity to maintain the herbage production and nutritional quality of these species in winter (Moojen and Maraschin 2002). These natural changes, together with incorrect Journal compilation CSIRO 2017
pasture management, directly affect economic and environmental sustainability as well as animal performance in this environment (Carvalho 2013). Generally induced by low-temperature limitations or frosts, C4 metabolic-cycle plants move to a seasonal dormancy stage during winter (Sarath et al. 2014), which increases their fibre content. According to Nabinger (2002) and Heringer and Jacques (2002), when the cold season ends, the quantity of this residual herbage will determine the quantity and quality of herbage growing the following spring. Usually, low stocking rates during winter cause a preponderance of erect plants over the prostrated species during spring (Nabinger et al. 2000), which is mainly because animals www.publish.csiro.au/journals/an
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Animal Production Science
concentrate their grazing in the lower strata with a higher quality and, thus, reject the upper strata that becomes preponderant. However, the presence of photosynthetic leaves is determinant for satisfactory regrowth and defoliation intensity depends on the stocking density and the duration of grazing period, which dictates residual sward height (Gastal and Lemaire 2015). These facts emphasise the need for an adequate balance between biomass and stocking rates during winter, so as to promote better results during the following spring. Therefore, another way of managing sward structure during winter may be with different types of grazing methods (i.e. frequency control; Carvalho 2013). Roshier and Nicol (1998) claimed that overgrazing occurs on individual plants due to multiple severe defoliations and, in the same way, diet selection by grazing animals may put palatable and actively growing plants at a disadvantage (Earl and Jones 1996). For this reason, the hypothesis is that the use of rotational grazing during winter, regardless of herbage allowance, may be utilised to improve sward structure during spring. This way, rotational grazing may be adopted without meaningful effects on livestock production if the animal category used at that time has a low nutritional demand, such as in the case of ewes in early pregnancy. Thus, the main objective of the present study was to define the most appropriate management for a natural grassland during winter, so as to improve its botanical composition and forage nutritive value during the following spring when ewes have a higher feed requirement. For this purpose, different grazing methods (continuous and rotational stocking) and herbage allowances (12 and 18 kg dry matter (DM)/100 kg bodyweight) were tested. Materials and methods Experimental site and treatments The study was conducted on an 8.4-ha natural grassland area in the Experimental Agronomic Station of Universidade Federal do Rio Grande do Sul (EEA, UFRGS), Brazil. The experimental area is located in the region called Central Depression of Rio Grande do Sul, at an altitude of 46 m and at 30050 S, 51 400 W. The climate is humid subtropical (Cfa) according to the Köppen classification (Moreno 1961). The local mean precipitation is 1440 mm and the mean extreme temperatures (historical average of the past 40 years) vary between 17.5C and 30.2C (summer), 11.2C and 28.2C (autumn), 8.5C and 20.2C (winter) and 11.2C and 26.6C (spring) (Bergamaschi and Guadagnin 1990). The sward composition of the study area was measured 10 days before the experiment (June 2009) by using the Botanal method (Kohmann et al. 1985) and was composed of 32.5% Andropogon lateralis, 14.4% Paspalum notatum, 9.3% Eragrostis plana, 6% Aristida spp., 5.3% Paspalum plicatulum, 5.2% Eryngium horridum, 3.4% Coelorhachis selloana, 2.9% Paspalum dilatatum, 2.5% Piptochaestium montevidensis, 1.6% Axonopus affinis, 1.3% Sporobulus indicus, 0.8% Eragrostis eroides, 0.6% Elyonorus spp., 0.3% Stipa spp., 0.3% Desmodium incanum, 0.2% Vernonia nudiflora, 0.2% Cynodon dactylon, 1.5% Cyperaceae, 0.4% Juncaceas and 10.6% of other species. So as to standardise the plots, the experimental area was mowed 90 days before the experiment and left at rest. Afterwards, the area was divided into three experimental
F. Jochims et al.
blocks (according to the landform), with four paddocks within each block, totalising 12 experimental units (0.7-ha paddocks). The following factors were assigned: two grazing methods (GM), namely continuous (CS) and rotational (RS) stocking, and two herbage allowances (HA), namely 12 and 18 kg dry matter (DM)/ 100 kg bodyweight (BW) (12–18% HA), which resulted in the following four treatments: CS+18% HA (C18), CS+12% HA (C12), RS+18% HA (R18) and RS+12% HA (R12). The CS method consisted of keeping the ewes in paddocks without subdivisions. In the RS method, the paddocks were subdivided into six subpaddocks and the animals were kept in each subdivision for 7 days. The experiment was conducted over seven consecutive 28-day periods, which occurred from 19 June 2009 to 31 December 2009 (196 days). During the first 84 days (Periods 1–3, winter season), ewes in early pregnancy were used in the treatments previously described. The ewes lambed and lactated throughout the experiment until the end of the study. On Day 85 (final third of pregnancy, Period 4) up until the experiment ended, all the paddocks were managed under continuous grazing and 18% HA (C18). This adjustment was made to avoid possible food restrictions in the final stages of pregnancy and lactation. The same management was applied in all paddocks during the spring and the respective effects can be defined as a result of the treatments applied during the winter. Three-year-old Suffolk ewes were used in the experiment. The animals were 7–14 days pregnant and weighed 46.72 4.15 kg with body condition score of 2.5 0.4 (1–5 scale; Russel et al. 1969) at the beginning of the experiment. The test sheep were kept in the same paddocks for the entire experiment. Stocking rates (SR) were adjusted every 28 days by using the ‘put-and-take’ method with pregnant ewes, so as to maintain the protocol of herbage allowance during the entire experimental period. Furthermore, the SR were adjusted considering the following equation: SR = [(HM + FG · d)/d]/HA, where HM is the herbage mass (kg DM/ha), FG is the forage growth (kg DM/ha.day), d is the number of days between evaluations and HA is the herbage allowance (kg DM/100 kg BW; see description below). Pasture measurements and grazing management Herbage mass (kg DM/ha, HM) was estimated every 28 days using a double-sampling technique (Wilm et al. 1944), with eight cuts at ground level, within 50 · 50 cm quadrats and 30 visual estimations in each paddock. The cuts and visual estimates were made randomly inside the total area of each paddock regardless of the GM. Canopy height (cm) was measured using an HFRO sward stick. This evaluation was performed on the same days and places where the HM measurements were taken (five height measurements in each quadrat). The samples collected from each paddock were mixed, divided into three subsamples and weighed. One subsample was weighed and oven-dried at 55C for at least 72 h to determine DM content. The second subsample was used to determine the botanical composition and separated manually into the following components: all grass species (subdivided into green leaf lamina and stem), senescent material, legume species and weeds (species ‘not preferred’ by the ewes, which were mostly Eryngium horridum and Vernonia nudiflora).
Strategies to improve natural grassland quality
Afterwards, all components were dried and weighed separately. The weights of herbage and components of botanical composition were transformed into kg DM/ha. The third subsample was ground and used to determine the forage nutritive value. Total DM content was measured on the ground samples using an oven at 105C for, at least, 16 h, and the results analysed at 55C were corrected to total DM content (at 105C). This way, all results were explored in total DM content (105C). Mineral content was assessed through 550C incineration. Nitrogen content was determined according to AOAC procedures (AOAC 1975) and expressed as CP (N · 6.25). NDF analyses were conducted following the procedure of Robertson and Van Soest (1981). All values were corrected for ash-free content and NDF was assayed with a heat-stable amylase. ADF, excluding ash, was determined according to Goering and Van Soest (1970) and acid detergent lignin content, excluding ash, according to Van Soest and Robertson (1985). Forage growth (kg DM/ha.day) was assessed using 50 · 50 cm quadrats (Klingman et al. 1943). Two visually similar areas of sward were selected in four distinct points of each CS paddock. Herbage was cut at a ground level and removed from one quadrat for analysis, while, in the other quadrat, an exclusion cage was placed for 28 days, after which the herbage within the quadrat was also cut at a ground level and removed for analysis. The cut samples were oven-dried at 55C to determine DM content. Forage growth was the difference of the HM from the different cuts. The mean value of the four exclusion cages were transformed into kg DM/ha.day. The same methodology was used in each RS paddock, although, in each period, the cages were allocated into the last two grazed subpaddocks using two cages each. Herbage samples were collected every 28 days by using a hand-plucking technique (Euclides et al. 1992), where material similar to that which the ewes were observed to eat was collected, assuming that samples represent the ingested herbage by the ewes (three ewes per paddock). In the RS paddocks, the samples were collected between the second and fourth day of the subpaddock grazing and, in the CS paddocks, on the same days as the RS method. The samples were immediately oven-dried at 55C for 72 h and ground for analysis. Here, the samples from two paddocks were mixed so as to achieve the necessary amount for laboratory analysis, performing one replicate. Then, instead of three replications, in this analysis, only two were used (one from one paddock and the other a mixture of material from two paddocks). Experimental design and statistical analyses The experiment was performed in a randomised complete block design with repeated-measures data (seven periods) and two main factors (herbage allowances and grazing methods). This generated four treatments with three area replications each and consequently 12 paddocks that were allocated into three blocks (the hand-plucking samples are the exception since they were analysed using two replications). The block design was used due to the differences in landform and blocks had a significant effect in all variables. As a result, P-values from the blocks are not shown in the tables since the main objective of the work was not to evaluate differences caused by the different landform. Statistical
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analysis was conducted separately for winter (first 84 days, Periods 1–3) and spring (Days 85–196, Periods 4–7). A variance analysis was performed, including the effects of the GM, HA and Period and the interactions among them. When significant differences were detected among the means, they were compared using the Tukey’s test at 5% significance level (P 0.05 was considered significant, while 0.05 < P 0.10 was considered a trend). Tests were performed using MIXED procedure of SAS 9.2 software (SAS 2001). Results Botanical composition and stocking rate There were no treatment · period interactions (P > 0.10; winter and spring) for herbage mass (HM), green leaf mass (GLM), stems mass and senescent material (Table 1). During winter, no significant (P > 0.10) difference in HM among GM and HA was observed, although HM decreased 43% from the first to the third period (P 0.05; Table 1). In the spring, there were differences between HA and GM for the HM. Paddocks managed with RS in winter presented 10% more HM in spring than did paddocks under CS during winter. Paddocks managed with 18% HA during winter presented more HM during spring than did paddocks managed with 12% HA during winter. In spring, HM increased over the periods in which there was greater HM during the last two periods (P 0.05). During winter, GLM tended to be greater in 18% HA than in 12% HA (P = 0.074), whereas, during spring, GLM tended to be greater in paddocks that were managed with RS in winter (P = 0.059; Table 1). Stem mass, GLM, and senescent material decreased during winter, while only GLM and stem mass increased during spring (P 0.05); senescent material were similar over spring (P > 0.10; Table 1). During spring, differences in stem mass for HA and GM (P 0.05) were observed; stem mass was 18% greater in paddocks managed under RS than under CS during winter, and 17% greater in paddocks managed under 18% HA than under 12% HA during winter. During winter, there were no significant (P > 0.10) effects on weeds mass. However, during spring, there was GM · HA interaction for weeds mass and differences were observed among periods (P 0.05). This interaction showed that paddocks managed under Treatment C12 during winter had the lowest weeds contribution to HM during spring (48.1 kg DM/ha). Meanwhile, weeds contribution to HM was similar among the other treatments (84.4 kg DM/ha in C18, 79.2 kg DM/ha in R12 and 90.8 kg DM/ha in R18). During spring, weeds contribution to HM increased (P < 0.05). Weeds mass presented an average of 67 kg DM/ha at the end of winter (Period 3) and an average of 94.9 kg DM/ha at the end of the trial period. There was no treatment · period interaction for legume mass (P > 0.10; Table 2). A trend (P = 0.075) for changing the contribution of legume mass to HM was observed during spring; however, no treatment effects in legume mass contribution were observed. During winter, no difference in forage growth (FG) was detected among treatments (P > 0.10; Table 2). There was also no FG during winter, regardless of the treatment, in addition to an increase in forage losses in the periods (see negative FG, Table 2; P 0.05). However, during spring, FG
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Animal Production Science
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Table 1. Herbage mass, green-leaf mass, stem mass and quantity of senescent material in natural grassland managed during winter with different grazing methods and herbage allowances with pregnant ewes Herbage mass, green-leaf mass, stem mass and senescent material are expressed as kg DM/ha. Differences between factors in the column are indicated by different lowercase letters: a–c indicating a significant difference (P 0.05), and x–z indicating a trend (0.05 < P 0.10); significant differences among periods in the column are indicated by different uppercase letters (Tukey’s test) Parameter
Herbage mass Winter Spring
Green leaf mass Winter Spring
Stem mass Winter Spring
Senescent material Winter Spring
Continuous stocking Rotational stocking
1905 1975
1169b 1305a
Grazing method 772.7 437.8y 796.2 466.2x
337.0 338.7
192.0b 234.5a
706.6 748.1
458.0b 498.0a
12 18 s.e.
1880 2000 83.8
1198b 1286a 30.3
Herbage allowance (%) 755.4y 443.1 813.4x 460.9 21.9 10.2
331.8 343.9 14.3
193.3b 233.3a 6.6
704.3 750.4 28.1
477.6 479.6 13.1
1 2 3 4 5 6 7 s.e.
2483A 1938B 1399C – – – – 66
– – – 1170B 1152B 1304AB 1342A 43
– – – 394.9B 425.1B 503.0A 485.2A 14.4
419.5A 330.6B 263.5C – – – – 17.5
– – – 181.2C 173.2C 217.6B 281.2A 9.4
842.1A 754.5A 585.4B – – – – 34.4
– – – 505.6 461.1 484.1 463.6 18.5
Grazing method (GM) Herbage allowance (HA) Period (P) GM · HA GM · HA · P
0.373 0.130
