Milk Production and Enteric Methane Emissions in Dairy Cows Grazing Annual Ryegrass Alone or Intercropped with Forage Legumes

This study evaluated the effects of reduced nitrogen fertilization and the intercropping annual ryegrass (Lolium multiflorum Lam.) with forage legumes - common vetch (Vicia sativa L.) and red clover (Trifolium pratense L.) – on milk production and enteric methane emissions in grazing dairy cows. Twelve Holstein × Jersey cows were assigned to a crossover design involving two treatments: ryegrass monoculture (RG) or ryegrass - legume mixture (RG + Leg). Methane emissions were measured using GreenFeed systems; grazing behaviour, milk yield and composition, and organic matter digestibility were also assessed. Legume inclusion contributed ~9% of the pre-grazing biomass and modestly increased crude protein content of the sward. However, cows grazing RG + Leg pastures had lower herbage mass (-214 kg DM/ha), lower herbage allowance (-6 kg DM/cow/day) and produced less milk (-2.0 kg/day; p < 0.05) and milk protein (-88 g/day; p < 0.01) than cows on monoculture ryegrass. Energy-corrected milk (ECM), methane emissions (g/day and g/kg ECM), and grazing behaviour were not significantly affected by treatment. Organic matter digestibility showed a tendency to be higher in the monoculture system (p = 0.067). These results suggest that, under subtropical grazing conditions, reducing nitrogen fertilization combined with the modest inclusion of vetch and red clover does not mitigate enteric methane emissions nor enhance animal performance. The limited legume contribution to total forage biomass may explain the absence of expected benefits. Enhanced strategies to increase legume proportion in mixed swards are needed to unlock their potential for sustainable intensification of pasture-based dairy systems.