STUDIES IN RESPONSE TO NITROGEN

The residual effects of 59, 118 or 177 kg N/ha applied to the primary growth of a ryegrass sward were measured in two successive regrowths as yields of fresh matter (FM), DM and digestible organic matter (DOM). The first regrowths (RGl) harvested on 12 June followed primary growth (PG) harvests 8, 12, 16, 23, 29, 45 and 57 days after applying N on 16 Apr. The second regrowths (RG2) were harvested on 12 Aug., 62 days after RGl and 119 days after applying N. Significant residual responses were shown in both regrowths which increased the annual responses substantially. Responses to the first increment of N, averaged over dates of primary growth harvests, were raised from 70.3 kg FM, 9.2 kg DM, and 5.6 kg digestible OM/kg N applied, for the primary growth, to responses for primary growth+2 regrowths of 140.8 kg FM, 22.5 kg DM and 13.9 kg digestible OM/kg N applied. For the second increment of N, negligible or negative primary growth responses of 19.0 kg FM, –0.68 kg DM and –0.68 kg digestible OM/kg N applied became 179.7 kg FM, 25.6 kg DM and 16.4 kg digestible OM/kg N applied for the sum of the 3 growths. There were significant differences between the residual responses obtained for different dates of PG harvest. When PG was harvested 8 days after applying N, primary response was low or negligible; residual responses were high, and the total responses were 21.0 kg DM/kg N applied for the first N increment and 31.0 kg DM/kg N applied for the second. Similar effects were observed for FM and digestible OM. Residual responses to the second increment of N were greatest where primary growth was harvested 23 days after N application. Similar residual responses followed the primary growth harvest 29, 45 and 57 days after N application hut the total responses were reduced. It is concluded that the very high residual responses to the second increment of N can only he explained in terms of increased availability of soil N.