THE EFFECT OF NITROGEN-FERTILIZATION UPON THE HERBAGE PRODUCTION OF TALL FESCUE SWARDS CONTINUOUSLY GRAZED WITH SHEEP .1. HERBAGE GROWTH DYNAMICS

The effect of level of nitrogen application upon the dynamics of herba ge growth in a continuously grazed sward of tal fescue was investigate d during two successive years. In order to obtain a large range of swa rd structural conditions, the experiments were carried out with two co ntrasting cultivars: cv. Clarine and cv. Barcel, and, in Year 2, with two different sward heights or leaf area indices (LAIs). During each o f five experimental periods (2-3 weeks), swards received either optimu m (N2) or deficient (NI) N applications, were maintained at their targ et LAI, and leaf growth was measured on labelled tillers. With continu ously defoliated tillers, N shortage had only a small effect on the le af elongation rate compared with tillers protected by cages. The leaf production per tiller was only slightly reduced by N shortage, and it was mainly by the means of a reduction in tiller density that the N de ficiency resulted in reduced herbage growth per hectare. These results indicate that, in continuously grazed swards, in contrast with result s previously found in intermittently defoliated swards, leaf elongatio n is not the only important component of difference in herbage growth and that the promotion of tillering rate is an additional pathway for N response in such management regimes.