The impact of increasing the length of the cattle grazing season on emissions of ammonia and nitrous oxide and on nitrate leaching in England and Wales

Ammonia (NH₃) emissions from cattle are much less when they are grazing than when they are housed. The urine excreted during grazing may rapidly infiltrate soil whereas it remains on the surface of impermeable floors and yards. If the average grazing season for the UK herd could be extended from 6 to 8 months, NH₃ emissions from cattle could potentially be reduced by ca. 15% (of the total for all livestock) if the cattle spend all of the extra grazing days outdoors. The main objective of this desk study was to assess the potential of extended season grazing to reduce NH₃ emissions from UK cattle farming. The impacts on nitrate (NO₃^?) leaching and nitrous oxide (N₂O) emissions were also estimated. A simple process-based model was developed to quantify the potential for extending the grazing season. A farm-scale model of NH₃ emissions at the farm-scale, based on published emission factors for UK agriculture, was used to estimate NH₃ emissions. Losses of NO₃^? following slurry spreading were estimated using the MANNER model, while NO₃^? leaching and denitrification losses during grazing were taken from output by the NGAUGE model. We conclude that one month’s extra grazing (based on the animals being outside for all of that month, day and night,) may reduce NH₃ emissions from slurry-based systems by ca. 9% and for FYM-based systems by ca. 7% compared with losses from the current ca. 180-day winter housing period. However, in practice cattle are not outdoors all day during the extended grazing period. If it is assumed that cattle graze for an average of 4 h per day over the extended period, then the monthly reduction in NH₃ emissions may be only ca. 1–2%. At all sites most of this conserved N was predicted to be lost as NO₃^?. For slurry-based systems this could be at least 80%. For FYM-based systems, for which there was less potential to conserve NH₃, the increase in NO₃^? leaching was always greater than the NH₃ conserved. The effects on direct emissions of N₂O were estimated be negligible, if grazing began earlier in spring or perhaps some reduction when grazing continues for longer in autumn. We conclude that extending the grazing season will increase NO₃^? leaching and that further studies are needed to fully evaluate the potential for reducing emissions of NH₃.