Quantity and Timing of Fertiliser N for Grass and Grass/Clover Swards

Although the use of fertiliser nitrogen on grassland has been recognised for a considerable time as an effective means of increasing the output of milk per unit area, relatively low levels are still used on a national basis. This may be due to the lack of experimental work quantifying the response in milk output over a wide range of nitrogen levels. The data available has been reviewed and shown to be in line with the responses reported in grass dry matter from cutting experiments. These latter responses have been used to calculate optimum levels of nitrogen input for varying ratios of the value of milk to the cost of nitrogen fertiliser. These indicated an optimum nitrogen level of 540 kg/ha at 1973 prices which declined to 475 kg/ha if the cost of fertiliser was increased by 40 per cent. If fertiliser costs increased by any more than 100 per cent above 1973 prices, relative to the value of milk, the optimum nitrogen levels decline dramatically.

As season effects the response to nitrogen it is important to distribute the nitrogen within the season in a pattern which takes this into account. The initial indications are that nitrogen levels should be reduced considerably during the early spring and autumn periods.

The effects of high nitrogen levels on animal health are examined and it is concluded that there is no evidence to indicate any ill effects on health or fertility which cannot be overcome by good management.