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.

Enteric methane (CH4) emissions are directly related to the quantity and type of feed intake. Existing mitigation strategies, for example, the addition of legumes to grass-based diets and increased use of grains, have been thoroughly researched and applied in different production systems. In this paper, we propose a need to expand the capacity to mitigate enteric CH4 emissions in cattle under grazing conditions. The objective of this paper was to contribute to evaluate a mitigation strategy under grazing conditions of using contrasting levels of pasture quality. The study was performed with 20 heifers twice during the year: winter and spring. Each season, the study employed a crossover design with two treatments and two 5-day measurement periods. The treatments were two pastures with different nutritional values, including a pasture with a low quality (70% of neutral detergent fibre, 1% of ether extract, 8% of non-fibre carbohydrates), 9% of crude protein, 35% of dry matter digestibility and a pasture with a high quality (42% neutral detergent fibre, 1.3% ether extract, 24% non-fibre carbohydrates, 21% crude protein and 63% dry matter digestibility). Enteric CH4 emissions were measured with sulfur hexafluoride tracer technique. The dry matter intake (kg/day) was measured indirectly using titanium dioxide as an external marker. CH4 emissions from animals grazing the high-quality pasture were 14% lower expressed as % of gross energy intake, and 11% lower expressed by unit of dry matter intake (g CH4/kg). These results quantitative showed the alternative to mitigate CH4 emissions from grazing bovines exclusively through the improvement of the forage quality offered.

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.

Understanding the methane (CH₄) emissions that are produced by enteric fermentation is one of the main problems to be solved for livestock, due to their GHG effects. These emissions are affected by the quantity and quality of their diets, thus, it is key to accurately define the intake and fiber content (NDF) of these forage diets. On the other hand, different emission prediction equations have been developed; however, there are scarce and uncertain results regarding their evaluation of the emissions that have been observed in forage diets. Therefore, the objectives of this study were to evaluate the effect of the NDF content of a forage diet on CH₄ enteric emissions, and to evaluate the ability of models to predict the emissions from the animals that are consuming these forage diets. In total, thirty-six Angus steers ($\overline{x}$ = 437 kg live weight) aged 18 months, blocked by live weight and placed in three automated feeding pens, were used to measure the enteric CH₄. The animals were randomly assigned to two forage diets (n = 18), with moderate (50%, HF) NDF contents. Their dry matter intake was recorded individually, and the CH₄ emissions were measured using the SF₆ tracer gas technique. For the model evaluation, six prediction equations were compared with 29 studies (n = 97 observations), analyzing the accuracy and precision of their estimates. The emission intensities per unit of DMI, per ADG, and per gross energy intake were significantly lower (p < 0.05) in the animals consuming the MF diet than in the animals consuming the HF diet (21.7 vs. 23.7 g CH₄/kg DMI, 342 vs. 660 g CH₄/kg ADG, and 6.7% vs. 7.5%, respectively), but there were no differences in the absolute emissions (p > 0.05). The best performing model was the IPCC 2006 model (r² = 0.7, RMSE = 74.04). These results show that reducing the NDF content of a forage diet by at least 10% (52 g/kg DM) reduces the intensity of the g CH₄/kg DMI by up to 8%, and that of the g CH₄/kg ADG by almost half. The use of the IPCC 2006 model is suitable for estimating the CH₄ emissions from animals consuming forage-based diets.