Methane and carbon emissions from conventional pasture and grain-based total mixed rations for dairying

摘要

Comparisons between cows grazing pasture or fed total mixed rations (TMR) show on average less milk production from pasture but a higher methane yield per milk or milksolids production from the pasture diet. The implication that TMR are associated with relatively lower greenhouse gas (GHG) emissions than pasture required an assessment of all production costs associated with both systems because TMR requires extensive cultivation and processing of grains and silages which form the bulk of the diet.A partial life cycle analysis of GHG (excluding nitrous oxides) was carried out for separate 250 cow herds located in Waikato (pastoral grazing) and Canterbury (cropping land for the TMR). This demonstrated much higher methane and carbon dioxide emissions, (kg CO_2 equivalents/kg milk) from cows fed TMR (1.53) relative to pasture (0.84). The principal cause of the high value for TMR were carbon losses from soil cultivation which accounted for 64 percent of emissions, expressed as CO_2 equivalents compared to 24 percent from pasture. Enteric methane accounted for 28 percent of GHG from TMR and 64 percent from pastoral systems. The remaining 8 percent and 12 percent from TMR and pastoral grazing respectively, were attributed to fertilisers, cultivation and processing.