Use of Nitrogen Fertilisers on Grassland for Milk Production

The response of grass to fertiliser is variable in terms of the yield attained for a given rate of N or the yield increment for each unit of N applied. Variability arises from a range of factors and interactions between these amongst which environmental factors, climate and soil, are of primary importance. ADAS, GRI and other research and educational establishments have been conducting jointly a series of experiments on representative lowland sites throughout the country to assess the variation in the response of S23 perennial ryegrass and the extent to which climate and soil influence yield and response to N. This paper is a preliminary report of one of these experiments illustrating the range of variation in annual yield and response to N.

Marked differences in yield and response have been obtained between sites and between years on individual sites. Yields without fertiliser N ranged from less than 0*5 t/ha to 6 t/ha : at 450 kg N/ha yields ranged from 7 t/ha to over 16 t/ha : and the yield increment obtained from 450 kg N/ha ranged from 7 t/ha to 13 t/ha.

The response to N is directly related to the potential yield at a site and the amount of N available from the soil. In the absence of fertiliser N yield is dependent on the supply of soil N and climatic variation has a minimal effect on yield. The influence of climate on yield, and on response to fertiliser N increases with increasing application of N. The yield at 600 kg N/ha is indicative of the potential yield of a site and shows a general relationship with moisture status of a site. The variability is complex and subtle and ultimately elucidation of the influence of environmental variables will be dependent on a detailed examination of moisture and temperature changes within individual cuts.

From a practical standpoint satisfactory and almost linear responses have been obtained up to 300 kg N/ha. Yield and response are lower in the drier eastern areas than in the moister western areas. An understanding of the influence of environmental factors on the response of grass fertiliser N will provide the basis for the effective use of fertiliser N on grassland.