EFFECT OF REGROWTH INTERVAL ON THE PRODUCTIVITY OF SWARDS DEFOLIATED BY CUTTING AND GRAZING

Over three grazing seasons (1984-1986) a sward of perennial ryegrass, cv. Talbot, which received a total of 336 kg N ha-1 each season, was cut or grazed with ewes at 3- or 4-week intervals on a rotational basis. Sward productivity was higher under cutting than under grazing irrespective of the interval between defoliations. Under cutting, mean herbage organic matter (OM) yields over both intervals were 8.66, 9.62 and 8.17 t ha-1 in 1984, 1985 and 1986 respectively while under grazing the corresponding yields were 7.65, 8.63 and 7.50 t ha-1. The mean annual yield of herbage defoliated at 3-week intervals was 7.50, 8.64 and 7.20 t OM ha-1 compared with 8.80, 9.60 and 8.46 t OM ha-1 for swards defoliated at 4-week intervals in the three years respectively. The nitrogen (N) content of both the available and the residual herbage was consistently higher under grazing than under cutting. Available herbage contained 31.3 and 27.7 g N kg OM-1 and residual herbage 26.1 and 22.7 g N kg OM-1 under grazing and cutting respectively. The mean yield of N under cutting was 284 kg ha-1 compared with 304 kg ha-1 under grazing. Defoliation interval had no effect on N yield, the overall mean yield being 294 kg ha-1 under both 3- and 4-week defoliation intervals. The effect of the treatments on tiller population was slight and inconclusive. The process of grazing reduced yield probably as a result of damage to the sward through trampling; the positive effect of excretal N on yield was minimal on account of the short grazing periods.