Limits to nitrogen fertilizer on grassland

The effect of nitrogen fertilizer on seasonal response of grassland, sward quality and productivity, herbage nitrate content. and soil mineral nitrogen status was studied in cutting trials lasting from one to six years. Rates of nitrogen application ranged from 0 to 160 kg N per ha per cut, totalling up to about 1000 kg N per ha per year. Plots were cut whenever a yield level of 2 to 2.5 t dry matter per ha was reached.<p/>The response to nitrogen was strongest in the first half of the season and decreased gradually thereafter. The response was influenced by residual nitrogen from previously applied nitrogen fertilizer. In the first year a positive response was obtained to higher rates of nitrogen application than in subsequent years. At rates exceeding about 500 kg N per ha per year sward quality deteriorated to such an extent that productivity decreased in the following year.<p/>As 'optimum' application a quantity of, on average, 420 kg N per ha per year was calculated. At this rate herbage nitrate content did not exceed 0.75% NO _{<font size="-1">3</font>} and accumulation of soil mineral nitrogen was minimal.