The implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK. 4. The effect of seasonal pattern of nitrogen fertilizer application and annual stocking rate

Greyface (Border Leicester x Scottish Blackface) ewes, mated in October, we re housed in winter and turned out after lambing in late March to one of fo ur treatments replicated three times on perennial ryegrass (Lolium perenne) -dominated pastures. On three of the treatments the stocking rate was 12.5 ewes plus lambs ha(-1) annum(-1) (SR12.5), whereas on the fourth it was 15. 0 ewes plus lambs ha(-1) annum(-1) (SR15.0) There were two levels of applie d fertilizer nitrogen (N), 152 kg N ha(-1) annum(-1) (N-152) and 205 kg N h a(-1) annum(-1) (N-205). Three patterns of nitrogen application were used: predominantly in spring (E), predominantly in autumn (L), and mainly in spr ing and autumn (E + L). Grazed sward surface height was controlled within t he range 3.5-5.5 an during spring and summer, and supplementary feed was of fered when sward height was below 3.5 cm. The effects on animal performance, yield of silage and requirement for supp lementary feed were measured over 3 years (1986-88). Management of the swar d height within the specified limits resulted in similar levers of individu al animal performance for all treatments, but treatment SR15.0N205E + L pro duced a significantly (P < 0.05) greater output of lamb per hectare. There was no evidence to suggest that, by increasing the amount of N fertilizer a pplied in the autumn, ewe reproductive performance was increased. Neither t he treatment with the lower level of N applied in the spring (SR12.5N152L) nor the high stocking rate treatment (SR15.0N205E + L) was self-sufficient in winter feed (silage) production. SR15.0N205E + L also required more supp lementary feed in both the lactation (38.1 kg ewe(-1)) and the mating (9.1 kg ewe(-1)) periods. SR12.5N152L required the second greatest amount of sup plementary feed during lactation (36.1 kg ewe(-1)), whereas SR12.5N205E + L required the least supplementary feed over both periods (27.8 kg and 4.8 k g ewe(-1)). Taking all treatments together there were significant (P < 0.05 ) differences between years in lamb weaning weight (kg), lamb output (kg ha (-1)), yield of silage (kg ewe(-1)) and supplementary feed required (kg ewe (-1)), arising mainly from a lower level of herbage production in one year. Treatment SR12.5N152E generated the highest gross margin per hectare.