Afrikaans: Stikstofbemesting vir Intensiewe Weidingproduksie Nitrogen (N) is the nutrient taken up in largest quantities by pasture plants from the soil. Its availability, together with temperature and moisture supply, are usually the major factors determining the productivity of pastures. Responses of grasses to applied N The responses of pastures to fertilizer N have been studied in scores of research trials both locally and overseas. In South African research, the major focus has been on the N requirements of ryegrasses, kikuyu and Eragrostis curvula (weeping lovegrass), with limited work being carried out on other species such as cocksfoot, fescue and Digitaria eriantha (Smuts fingergrass). For the relation between grass DM (dry matter) yield and fertilizer N applied, a characteristic response curve is obtained, an example of which is presented in Figure 1. When N is applied there is usually an initial near-linear response (A in Fig. 1), a phase of sharply diminishing response (B) and a point (C) beyond which N has little or no effect on yield. The amount of DM produced for each kilogram of N applied within zone A depends largely on the species under consideration, the frequency of defoliation and growth conditions. Tropical grasses generally produce more DM per unit of N than do temperate grasses. In field trials, Eragrostis curvula, for example, has produced up to 60 kg DM per kg N applied, but irrigated Italian ryegrass only between 25 and 34 kg DM per kg N applied. In the United Kingdom, perennial ryegrass produced an average of 23 kg DM/kg N over an N application range of 0 – 300 kg N/ha. It must be emphasised that data such as these are averages over the season and conceal wide variations in response efficiency within the season. For example, in perennial ryegrass the spring response is two to three times greater than at other times of the year. Milk production response On intensive dairy pastures, the additional feed produced in response to N fertilization is ideally converted into milk production. A typical conversion ratio is about 15 kg pasture dry matter per kg milk-solids, or roughly one kg pasture dry matter per liter of milk. In South Africa currently, the value of pasture dry matter in dairy farm operations is estimated to be approximately R2000/ton. In overseas studies, it has been estimated that the response in terms of milk production ranges from 9 to about 16 kg milk per kg fertilizer N applied. This arises not because of any significant increase in yield per cow, but from an increase in stocking rate, i.e. cows per hectare. Type of fertilizer Urea and LAN (limestone ammonium nitrate) are the two most important forms of fertilizer N used on pastures, with other products such as ammonium sulphate being used in lesser amounts. Grasses take up N in both the ammonium and nitrate forms; however, since ammonium (including the N in urea) is converted to nitrate within a few weeks in well-aerated non-acidic soils at temperatures above about 5˚C, most of…
