A life cycle assessment of high sugar grasses on pasture-based dairy farms

Pasture-based dairy systems are associated with polluting emissions, such as methane (CH4), ammonia (NH3) and nitrous oxide (N2O), while the relatively low use of concentrate feeds constrains scope for dietary intervention. In this context, high sugar grasses (HSG) have been proposed to enhance nitrogen (N) utilization efficiency in the rumen and consequently reduce N excretion. The use of HSG could thus reduce the carbon and N footprints of milk production. We use life cycle assessment (LCA) integrated with a detailed dietary model to investigate the potential for using HSG to reduce polluting emissions and improve the environmental footprint of milk. A previously-constructed LCA model describing dairy farm production on pasture was employed. Two feeding scenarios were considered, using either conventional perennial ryegrass or HSG. A meta-analysis provided data to formulate a diet with a daily dry matter intake of 2.6 kg of concentrate feed/cow, and provided N excretion estimates. The Cornell Net Carbohydrate and Protein System was then used to simulate the feeding scenarios and to calculate data on CH4 emissions for lactating cows fed on pasture during the grazing season (6 months per year). The HSG scenario resulted in a 19% reduction in N excretion (g/kg of milk), leading to similar reductions in eutrophication and acidification burdens. Carbon dioxide equivalent greenhouse gas emissions from the HSG scenario were 3.1% higher per hectare owing to a 1.5 kg/cow higher milk yield (not statistically significant), but emissions intensities (g CH4/kg milk) were 3.4% lower. In conclusion, our results suggest that the use of HSG may reduce nitrogen excretion of cows, together with associated eutrophication and acidification footprints of milk production, with more minor effects on the carbon footprint. Acknowledgements: Support was provided through the Sêr Cymru NRN-LCEE project Cleaner Cows