Perennial ryegrass pastures were maintained at sward surface heights (SSH) of 30, 60, 90 and 120 mm by continuous variable stocking using lactating ewes and their twin lambs in spring and non-lactating (dry) ewes in autumn.The effects of SSH on ingestive behaviour, herbage intake, animal performance and sward structure and production were measured in spring, and again in autumn, when an additional SSH of 20 mm was also established.

The poteniia] productivity of perennial ryegrass/ white clover swards (GC) under continuous stocking management was assessed by comparing their performance, when grazed by sheep at sward surface heights of 3, 6 and 9 cm, with that of an all-grass sward (G) maintained at 6 cm and fertilized with 420 kg N ha " '. The grass/ clover swards received no nitrogen fertilizer. The different grazing treatments had a marked effect on animal performance. In the first year for example, for treatments GC3, GC6, GC9 and G6-420 respectively, mean stocking rates to weaning were 19-7, 14-3, 8-9 and 18-4 ewes ha " ' {plus twin lambs); lamb growth rates were 223, 268, 295 and 260 g d "' and so total lamb liveweight gain was 1054, 920, 630 and 1148 kg ha"'. The relative performance of the treatments was similar in all three years. All three grazing treatments had a similar effect on the composition of the grass/clover swards. Clover content increased in 1985, and was sustained in 1986 and 1987 during the main grazing season, although a marked decline in clover content during the winter led to a progressive long-term decline in both the proportion and the amount of clover.It is suggested that a management based on maintaining a sward surface height close to 6 cm {as in all-grass swards) leads to optimum performance in grass/white clover swards grazed using continuous stocking with sheep. Despite

The effect of daily herbage allowance on herbage intakes and growth rates of lambs grazing perennial ryegrass and red clover was investigated in two experiments. Herbage allowances defined as g herbage D.M./kg live weight (LW)/day were controlled by varying the areas of plots grazed for 2 days by groups of six lambs.In the first experiment five herbage allowances in the range 20-120 g D.M./kg LW/day were offered on two areas of a perennial ryegrass (cv. S. 23) sward that received nitrogen fertilizer applications of 39 or 78 kg N/ha/28 days. In the second experiment five herbage allowances in the range 30-160 g D.M./kg LW/day were offered on perennial ryegrass (cv. S. 23) and red clover (cv. Hungaropoly) swards.Asymptotic curves were fitted to describe the relationship between herbage allowance and daily intake of herbage. In Expt 1 nitrogen fertilizer rates did not affect the yield of herbage or animal performance. In Expt 2 intakes were higher on the clover sward than on the ryegrass sward at the higher herbage allowances.The asymptotic curves to describe the relationship between herbage allowance and growth rate of lambs differed widely between periods. Growth rate of the lambs increased linearly with increase in digestible organic matter intake. Live-weight gain per unit of digestible organic matter intake was higher on the red clover than on the ryegrass.The conclusion is drawn that if the herbage present to ground level is not more than three times the daily intake of the animals, intake of herbage of the animals may bo restricted.

1. Scottish Half-bred ewes carrying twin foetuses were fed individually to make live-weight gains in the last six weeks of pregnancy of (1) 20%, (2) 10% and (3) 0% of their live weight in week 14 of pregnancy. In lactation the ewes were fed ad libitum. The lambs were removed 12 to 16 hr after parturition and the ewes were machine-milked twice daily for the first six weeks of lactation. 2. Total birth weights per ewe of twin lambs from the treatments were (1) 10-10 kg, (2) 9-44 kg and (3) 8-18 kg and differed significantly. 3. The level and pattern of voluntary intake in lactation did not differ significantly between the treatments. Total dry-matter intakes in the six weeks of lactation were (1) 121-9 kg (2) 105-9 kg and (3) 109-5 kg. 4. The pregnancy treatments affected the level of milk production and the shape of lactation curves. The total yields in the first six weeks of lactation were (1) 58-8 kg, (2) 43-5 kg and (3) 26-9 kg. Higher contents of fat and protein and the lower content of lactose in the milk from treatment-3 ewes on days 1 and 3 of lactation indicated a slower onset of lactation in these ewes. Between days 7 and 35 of lactation the contents of fat and SNF were lowest on treatment 3 but the differences were not significant. 5. The live-weight changes in lactation, which were in inverse order to the gains in late pregnancy, were (1) 3-4 kg, (2) 5-5 kg and (3) 9-5 kg.

A comparison was made between the seasonal pattern of net herbage accumulation under cutting and the pattern of intake under continuous variable stocking on a Lolium perenne sward, which was well supplied with nitrogen fertilizer and irrigated to minimize moisture stress.The seasonal pattern of production under monthly cutting showed characteristic asymmetry with a marked peak in production in late May/early June, a rapid decline in June followed by an increase in rate in August and the total amount harvested over the season was 9-28 t organic matter ha"'.By contrast, the seasonal patterns of production on swards grazed by ewes to maintain sward surface heights of 30, 50 and 70 mm all differed fundamentally from that observed under cutting. Production was more evenly distributed throughout the season under these grazing treatments which, it has been proposed, should optimize performance. Total amounts harvested by grazing were 10-84, 1012 and 8-51 t OM ha"', respectively. The different patterns of production under cutting and grazing were considered to be associated with differences in the expression of reproductive development in the grass crop and the maintenance of a smaller leaf area per unit ground area under grazing.The results of the study suggest thai it is not appropriate to regard the seasonal pattern of net Correspondence: R. J. Orr, AFRC Instiiuie for Grassland and Animal Production, Hurley, Maidenhead, Berks SL6 5LR. UK.accumulation under a standard cutting regime as representing the potential production of grass under continuous stocking. Moreover, in practice, the more uniform seasonal pattern of production seen in swards maintained on the basis of a controlled sward surface height makes it easier to match the supply of herbage to the appetite of grazing animals as the season progresses.