The response to nitrogen fertilizer from a cut perennial ryegrass (Lolium perenne L.) pasture in the Scottish uplands relative to efficiency of fertilizer use and provision of herbage for animals

Abstract

The dry matter (DM) responses by a perennial ryegrass pasture growing at a Scottish upland site to applications of 0–160 kg N ha⁻¹ yr⁻¹ in the sowing year and to 0–480 kg N ha⁻¹ (given in three equal dressings) in the first harvest year were measured in a small plot cut experiment. The efficiency of use of N fertilizer was expressed in three ways: as the apparent N efficiency (kg DM per kg N); as the appatent N recovery (% N recovered of that applied) and as the amount of N fertilizer required to produce pasture to meet predicted animal requirements. The latter was based on a comparison of the dietary requirements of sheep calculated from knowledge of animal nutrition and grazing efficiency, with the quantities and patterns of herbage production found in the present experiment. Apparent N efficiencies ranged between 6 and 41 kg DM per kg N, and were influenced by growth stage, season of application and level of N and P fertilization. Apparent N recoveries ranged between 55 and 63% for the first harvest year and were greatly affected by season of N application. These results are compared with those from a similar experiment which was recently carried out at 21 sites in other parts of Britain. The herbage produced from pasture without N fertilizer would have supported about 6 ewes ha⁻¹. Applications of 120 and 240 kgN ha⁻¹ yr⁻¹ would have supported 10 and 15 ewes ha⁻¹ respectively, with only a few occasions when supply did not meet demand. It was difficult to match a set stocking rate with the herbage produced from 480 kg N ha⁻¹ yr⁻¹, as depending on the animal numbers there would have been either major surpluses or deficits in herbage production at times during the grazing season. The use of N fertilizer to manipulate herbage production in upland sheep grazing systems is discussed.