The response of a perennial ryegrass (Lolium perenne L.) seed crop to nitrogen fertilizer application in the absence of moisture stress

Responses of perennial ryegrass (Lolium perenne L.) to nitrogen (N) fertili zer application rates and timings vary widely, because water is often limit ing. Yield response to N fertilizer application during autumn, late-winter and spring, and the associated efficiency of use of these inputs, was asses sed under conditions of non-limiting soil moisture during two, one-year lys imeter studies in Canterbury, New Zealand. There were significant (P < 0.05 ) increases in seed and herbage yields with increasing N fertilizer applica tion. Seed yields differed with year; greatest yields were 300 g m(-2) in 1 996 and 450 g m(-2) in 1997. Seed head numbers (r(2) = 0.77), seeds head(-1 ) (r(2) = 0.92) and herbage yield (r(2) = 0.92) were the major determinants of seed yield in both years. Irrigation required to maintain the soil betw een 70% and 90% of field capacity was directly related (r(2) = 0.94 and 0.9 9 in 1996 and 1997 respectively) to increases in herbage yield. Seed yield, seed quality (thousand seed weight and percentage of seed > 1.8 5 mg), efficiency of water use, efficiency of N fertilizer use and apparent N fertilizer recovery were greatest when N fertilizer was applied at a rat e of 50 kg N ha(-1), 50 or 100 kg N ha(-1) and 150 kg N ha(-1) in autumn, l ate-winter and spring respectively; further increases in spring N fertilize r stimulated vegetative growth, but not seed yield. As a management strategy, applying N fertilizer to match the N requirements of the crop during the reproductive stage of growth will result in high yi elds of high quality seed while minimizing environmental impact.