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Among the various sources with a potential negative impact on the environment, methane (CH 4 ) emissions from livestock origin have been highlighted as important for the agricultural sector. Research to mitigate CH 4 emissions and understand how integrated crop and livestock production systems may contribute to the reduction of greenhouse gases, is essential for the development of public policies for environmental preservation. We hypothesized that combinations of stocking methods and grazing intensities provokes differences in the quantity and quality of herbage ingested, thus altering animal production and CH 4 emissions by the grazing animal. Experiments were carried out in 2011 (Experiment 1) and 2012 (Experiment 2), when the production of pasture and CH 4 emissions from sheep were studied in a system that integrates soybean (Glycine max. (L.) Merr.) and maize (Zea mays L.) in the summer/autumn, in rotation with Italian ryegrass (Lolium multiflorum Lam.) in winter/spring. Two stocking methods (continuous or rotational) and two grazing intensities (herbage allowance: moderate and low, 2.5 and 5 times the potential daily dry matter intake, respectively) in a randomized complete block design with three replicates were studied. Lambs were used in the first experiment, while lactating ewes (all with a single lamb) were used in the second experiment. Average daily gain (ADG) of lambs was greater (P < 0.05) in continuous than in rotational stocking, regardless of grazing intensity (150 vs. 89 g day −1 and 241 vs. 209 g day −1 in Experiments 1 and 2, respectively). Ewe ADG did not differ (P > 0.05) between treatments. Live weight gain per hectare (LWGHA) showed the same response in both experiments, with greater LWGHA in moderate grazing intensity (P < 0.05). In Experiment 1, the dry matter intake (DMI) was on average 21% greater (P < 0.05) for continuous stocking than rotational stocking (1345 g day −1 vs. 1075 g day −1 , respectively), while in Experiment 2, no differences (P > 0.05) between stocking methods and grazing intensities were observed (1673 ± 83 g day −1 ). The CH 4 emissions per animal did not differ (P > 0.05) among treatments in both experiments (22.7 ± 1.0 and 39.9 ± 1.3 g day −1 , Experiments 1 and 2, respectively), but when expressed in g CH 4 kg ADG −1 emissions were on average 35 and 15% greater (Experiments 1 and 2, respectively) (P < 0.05) under rotational than continuous stocking, independent of grazing intensity (171 vs. 263 g CH 4 kg ADG −1

Simple SummaryGHGs emissions are relevant in evaluating environmental impact of farming systems. Methane (CH4) produced by enteric fermentation accounts for half of all anthropogenic emissions of GHGs in Uruguay, where ruminant production is based on year round grazing of forages. Here we compared milk production and CH4 emissions by dairy cows grazing two contrasting mixed pastures (rich in legumes or rich in grasses) using the SF6 tracer technique adapted to collect breath samples over 5-days periods. There were no differences in milk or CH4 production between the contrasting pastures, probably because of the high herbage allowance that enabled selective grazing by cows.AbstractUnderstanding the impact of changing pasture composition on reducing emissions of GHGs in dairy grazing systems is an important issue to mitigate climate change. The aim of this study was to estimate daily CH4 emissions of dairy cows grazing two mixed pastures with contrasting composition of grasses and legumes: L pasture with 60% legumes on Dry Matter (DM) basis and G pasture with 75% grasses on DM basis. Milk production and CH4 emissions were compared over two periods of two weeks during spring using eight lactating Holstein cows in a 2 × 2 Latin square design. Herbage organic matter intake (HOMI) was estimated by chromic oxide dilution and herbage organic matter digestibility (OMD) was estimated by faecal index. Methane emission was estimated by using the sulfur hexafluoride (SF6) tracer technique adapted to collect breath samples over 5-day periods. OMD (0.71) and HOMI (15.7 kg OM) were not affected by pasture composition. Milk production (20.3 kg/d), milk fat yield (742 g/d) and milk protein yield (667 g/d) were similar for both pastures. This may be explained by the high herbage allowance (30 kg DM above 5 cm/cow) which allowed the cows to graze selectively, in particular in grass sward. Similarly, methane emission expressed as absolute value (368 g/d or 516 L/d) or expressed as methane yield (6.6% of Gross Energy Intake (GEI)) was not affected by treatments. In conclusion, at high herbage allowance, the quality of the diet selected by grazing cows did not differ between pastures rich in legumes or rich in grasses, and therefore there was no effect on milk or methane production.

Methane is a major constituent of greenhouse gas (GHG) emissions from ruminants, and mitigation strategies are needed to alleviate this negative environmental impact while maintaining the environmental and other benefits of grazing systems. Forages containing plant-specialized metabolites (PSM), particularly condensed tannins, may help reduce enteric methane (CH4) emissions. However, information on in vivo CH4 emissions from cows grazing mixtures that contain bioactive herbs is scarce. Accordingly, this study compared a binary mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) against a diverse mixture of six additional species, including tannin-rich species like birdsfoot trefoil (Lotus corniculatus) and salad burnet (Sanguisorba minor), in a full-grazing dairy system. Enteric CH4 emissions were measured using the SF6 tracer technique. Cows grazing diverse mixtures increased their energy-corrected milk (ECM) yield by 4% (p < 0.001) compared with binary mixtures. However, CH4 emissions per kg ECM were also 11% greater for the diverse mixtures (p < 0.05). The very high feed quality and milk yield from both mixtures explained the low CH4 emissions recorded relative to the milk output. The addition of forbs did not provide additional benefits at these intensities, as they were maintained in low yield shares throughout.