Factors affecting the response of cut grass to the nitrogen content of dairy cow slurry

Applications of dairy cow slurry and mineral fertiliser were made at a range of nitrogen (N) rates, in either spring or summer, to grass plots cut for silage. A total of twenty-seven sites on medium textured soils were used over 3 years covering a range of climatic conditions in England and Wales. The efficiency of utilisation of the slurry N was calculated by comparing herbage DM yields from slurry applications with those from fitted response curves to ammonium nitrate N. Although location, rainfall, slurry DM and soluble N content appeared to influence the efficiency of slurry N on occasions, rate and time of application were the most important at each site. The efficiency of slurry N averaged over all experiments ranged from 38% for a low rate of application in spring down to 17% for a high rate in summer, the combined effects of slurry N and fertiliser N being additive. The apparent recovery of slurry N by the crop ranged from 28% to 9%. The residual effects of slurry N on herbage yields later in the same season were consistently low.