Abstract
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.Understanding 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.
Highlights
The growing global concerns on climate change among other environmental issues have moved researchers and farmers to include environmental impacts together with productivity when evaluating and optimizing farming systems, as reported by Johnson et al [1], Pinares-Patiño et al [2], Johnson et al [3]and Vlaming et al [4]
Animals fed with legumes had a lower proportion of energy lost to methane than those fed with grasses according to Waghorn et al [6] because legumes promoted higher intakes and production of the animals
Herbage mass above 5 cm (2,165 kg Dry Matter (DM)/ha on average) and sward height (29.5 cm on average) were similar between pastures (Table 1). Botanical composition of both swards was significantly different as intended (P < 0.0001): Legume sward presented a higher proportion of legumes (60% of herbage mass above 5 cm as Medicago sativa and Trifoliun repens) while Grass sward presented a higher proportion of grasses (76% of herbage mass above 5 cm as Lolium multiflorum)
Summary
The growing global concerns on climate change among other environmental issues have moved researchers and farmers to include environmental impacts together with productivity when evaluating and optimizing farming systems, as reported by Johnson et al [1], Pinares-Patiño et al [2], Johnson et al [3]and Vlaming et al [4]. The growing global concerns on climate change among other environmental issues have moved researchers and farmers to include environmental impacts together with productivity when evaluating and optimizing farming systems, as reported by Johnson et al [1], Pinares-Patiño et al [2], Johnson et al [3]. Reducing emissions of GHGs in livestock production is a global priority to mitigate climate change, and a top priority concern for countries with grazing systems. Uruguayan ruminant production systems are predominantly pasture-based with approximately 75% of agricultural land within Uruguay dedicated to pasture. To combine sustainable dairy production and environmental conservation, the quantity and quality of forage given to dairy cattle should help mitigate methane emission. Animals fed with legumes had a lower proportion of energy lost to methane than those fed with grasses according to Waghorn et al [6] because legumes promoted higher intakes and production of the animals
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