The frequency and severity of defoliation of individual grass tillers and clover plant units was studied in Lolium perenne-Trifolium repens swards grazed by sheep at stocking rates ranging from 25 to 55 sheep ha~' and either receiving no N fertilizer or 200 kg N ha~'. On average, sheep at the highest stocking rate defoliated individual tillers once every 4-8 d compared with once every 9-3 d at the lowest stocking rate with the reinoval of 58% and 47% of the leaf length of each tiller leaf at these stocking rates. Clover plant units were defoliated once every 4-2 d at the highest stocking rate and once every 7-7 d at the lowest stocking rate with the removal of 51% of its leaves and 12% of its stolon at the high stocking rate and 42% and 4% respectively at the low stocking rate. Differences in frequency and severity of defoliation between N fertilizer treatments were smaller than between stocking rates. Grass tillers and clover plant units were both defoliated less frequently and less severely in swards fertilized with N, though the difference in defoliation frequency between fertilizer treatments decreased as stocking rate increased. Defoliation frequency was related to the length of grass leaf per tiller or number of clover leaves per plant unit, and to the number of these tillers and the herbage on offer.

The effects of stocking rate and N fertilizer on a mixed sward of perennial ryegrass {LoHum perenne) and white clover {Trifolium repen.s) setstocked with sheep were examined. Sward produclion and composition, and sheep production were studied.Increasing the slocking rate over the range 25-55 yearling sheep ha"' reduced herbage accumulation by about 40%, whether or not N fertilizer was applied. Increasing the stocking rate increased the density of ryegrass tillers, but reduced the density of clover stolons and the clover content of the swards. Applications of N fertilizer (200 kg N ha' a"') increased herbage accumulation by about 20% but substantially reduced the clover content.Liveweight gain per animal and per unit area were greater at the lower stocking rates where the clover content and nutritive value of the diet were greatest. Wool growth per unit area was greater at the higher stocking rates. Applications of N fertilizer increased liveweight gain at stocking rates above 25 sheep ha"', but had no effect on wool produclion at any stocking rale.The results demonstrate that a stable and productive grass-clover association was maintained under conditions of scl-stocking at around 25 yearling sheep ha ' and that at this stocking rate, which appears lo be about the biological

Continuous stocking with sheep at high stocking rates may reduce the content of white clover (Trifolium repens) in mixed grass-clover swards. The present experiment was carried out to investigate Ihe effects on sward production and composition of resting a perennial ryegrass (Lolium perenne)-vjhile clover sward from grazing and taking a cut for conservationSwards were set-stocked with 25 and 45 yearling wethers ha"' either throughout a grazing season, or on swards that were rested for a 6-week period and then cut in early,, mid-or late season. In an additional treatment swards were cut only and not grazed.Net herbage accumulation was higher at the lower of the two stocking rates and was marginally increased by the inclusion ofa rest period at the high but not the low stocking rate. Clover content was higher at the lower stocking rate and was increased by the inclusion ofa rest period by 30% at 45 sheep ha"' and by 11% at 25 sheep ha"'. The effect was most marked at the end of the rest period before cutting. When rested from grazing the tiller density of ryegrass decreased although tiller length increased, and clover stolon length, petiole length and leaflet diameter increased though leaf and node number per unit length of stolon decreased; the reverse applied when the sward was returned to grazing after cutting.At the high stocking rate, rest periods in mid-season or later maintained the greatest clover content and marginally increased total net herCorrespondence: Dr R. J, Wilkins, Permanenl Grassland Division, Grassland Research Instilulc. North Wyke, Okehampton, Devon, UK, EX20 2SB bage accumulation. At the low stocking rate the timing ofthe rest period had no significant effect on total net herbage accumulation or on clover content.These results show that the combination of grazing and cutting is of benefit where the stocking rate is high enough to threaten clover survival and limit sheep performance. However, at such a stocking rate, feed reserves are at a minimum throughout the grazing season and so opportunities for resting the sward are probably low.

SUMMARYThe effects of treading and the return of excreta on a Lolium perenne-Trifolium repens sward defoliated by sheep set-stccked at high and low stocking rates were examined. Sward performance was measured inside ‘graze-through’ cages which allowed defoliation without treading and excreta return, and outside where sheep grazed either fitted with harness to prevent the return of excreta or unharnessed to allow the normal return of excreta. Live-weight gain was measured from excreta return and non-return swards. The treatments imposed had large effects on herbage growth and botanical composition.At a stocking rate of 25 yearling wethers/ha, sheep excreted about 1·1 kg N/ha/day, which increased soil N and led to an increase in herbage growth of about 26 %. The return of excreta increased ryegrass tiller density and this was partly responsible for a 26% reduction in the proportion of clover in the sward; the weight of clover was 13% loss where excreta were returned, and on this treatment stolon length at the end of the experiment was similar to that at the beginning. Doubling the stocking rate increased the N returned via excreta to about 1·3 kg N/ha/day, and this increased herbage growth by 53% but suppressed the proportion of clover by 21%, though not the weight of clover. Clover stolon length decreased during the experiment at this stocking rate, both with and without the return of excreta. Sheep live-weight change benefited from the stimulus to herbage growth where excreta were returned at the high stocking rate, but not at the low stocking rate.Treading by 25 sheep/ha increased soil compaction but had no significant (P > 0·05) overall effect on herbage growth and botanical composition. However, treading by double the number of grazing animals significantly reduced herbage growth by 10%, plant root weight by 47% and the proportion of clover in the sward by 11%.Differences in sward performance between stocking rates were due more to the difference in defoliation intensity between these stocking rates than to either treading or the return of excreta.

Four stocking rates (25-55 sheep ha~') antl two N fertilizer treatments (nil or 200 kg N ha"') were imposed on swards containing perennial ryegrass (Lolium perenne) and white clover {Trifolium repens) which were continuously stocked throughout one grazing season. These treatments gave swards that varied greatly in their content of white clover. The effects of these treatments on pasture and animal performance in the following season were examined when all plots were continuously-stocked with 25 yearling sheep ha"' and received no N fertilizer. There was slight regeneration of clover in the second year. Herbage growth ranged from 5-4 to i lOt dry matter (DM) ha"', was closely correlated with the clover content of the swards and was higher on swards that had previously received 0 rather than 200 kg Nha"'.The liveweight gain and wool production of sheep in the second grazing season closely reflected the herbage growth and clover content of the swards. As a result, animal production was greatest from swards which had previously been stocked at the lowest rate and had received no N fertilizer and ieast from swards which had been stocked at the lowest rate but had received N fertilizer.The results show that the adverse effects of over-grazing and N fertilizer application in one season may be partially rectified by lower stoc-