Environmental impact of grazed grass/clover patures

Summary

Grazed clover/grass pastures are important for animal production systems and the clover component is critical for its contribution to nitrogen (N) inputs via biological fixation of atmospheric N2. The resource efficiency and environmental emissions for clover/grass pastures can differ from that of N-fertilised grass-only pastures. Clover N2 fixation uses photosynthetically-fixed carbon, whereas N fertiliser production consumes fossil fuels and has net greenhouse gas (GHG) emissions. Clover has a higher phosphorus (P) requirement than grass and where extra P fertiliser is used for clover/grass pastures the risk of P loss to waterways is greater than for grass-only pastures. Nitrogen leaching from grazed pasture increases exponentially with increased N inputs and urine-N contributes 70-90% of total N leaching. However, the few studies comparing clover/grass and N-fertilised grass-only pastures at similar total N inputs indicated similar N leaching losses. Nitrous oxide (N2O) emissions from grazed pastures due to N-cycling of excreta are similar for clover/grass and N-fertilised grass-only pastures at similar total N inputs. However, the grass-N systems have additional specific losses from the fertiliser-N application. Thus, total N2O emissions are generally higher for N-fertilised grass pastures than for clover/grass pastures. A summary of various whole-system and Life Cycle Assessment analyses for dairy farms from various countries indicated that at similar total N inputs, clover/grass pasture systems can be more efficient than N-fertilised grass systems per kg of milk produced from an energy use and GHG perspective whereas results for nutrient losses to waterways were mixed and appear to be similar for both systems. In practice, other management practices on farm, such as crop integration and winter management options, can have a larger overall effect on environmental emissions than whether the N input is derived from N fertiliser or from N2 fixation.