Two 1·0 ha plots of a late‐heading diploid perennial ryegrass (var. Contender) and a late‐heading tetraploid ryegrass (var. Condesa), and two 1·4 ha plots of the tetraploid with Aberystwyth S184 small‐leaved white clover, were direct sown in May 1987. Over the three years 1988–90 they were continuously stocked by Mule ewes with Suffolk‐cross twin lambs, from early April to the end of August, at a target sward surface height (SSH) of 4–6 cm on one set of plots (constant swards) and, on the other set, al 4–6 cm rising after June to a target 6–8 cm (rising swards). The heights were achieved by variable stocking. Fertilizer N was applied only to the grass plots at the rate of 150‐ 180kgN ha‐1 annually. SSH was mainly within the target 4–6 cm, after higher initial heights at turnout in 1988and 1990. Mean heights of the constant swards (April‐ August) averaged 5·53, 4·43 and 5·04cm in the three years. The rising swards (July‐August) increased in height over the constant swards by an average of 0·88, 0·48 and 0·55 cm, in successive years. Clover content of the herbage mass dry matter in the grass/clover swards increased over each grazing season to average 13·0, 26·5 and 21·2% in the three years, with a high mean stolon density of 130 in m‐2 in August 1990. Ryegrass tiller densities in year 3 were 23% higher in the diploid than in the tetraploid swards, which had 43% more than the 10000 tillers m‐2 of the tetraploid ryegrass/clover swards. It is concluded that the combination of a densely stoloniferous small‐leaved clover with the open growth habit of a tetraploid ryegrass can achieve swards of high clover content under continuous sheep stocking.

A comparison of diploid and tetraploid perennial ryegrass and tetraploid ryegrass/white clover swards under continuous sheep stocking at controlled sward heights. 2. AbstractPerformance of continuously stocked Mule ewes nursing Suffolk-cross twin lambs over three grazing seasons,

Seven varieties of white clover (Trifolium repens L.), of varying leaf size from large to small, were sown with perennial ryegrass (Lolium perenne L.) in 1985, at an upland site, and compared over three harvest years under a cutting management (5-6 cuts), and under continuous grazing by sheep. Fertilizer N input totalled 80 kg ha" ' in both the cutting and the grazing trial. The same varieties were included in two official National List (NL) trials sown the same year at a nearby site; these comprised a yield trial with 6-7 cuts and a persistency trial mown very frequently (17 cuts) to simulate intensive grazing.At the upland site the large-leaved varieties, Milkanova and Blanca, were the highest yielding under cutting (mean 3-3 t DM ha"'), and the small-leaved SI84 and Kent the lowest yielding (mean 2-3 t DM ha~'). The rankings were reversed under grazing (corresponding yields 1-1 and 2-1 t DM ha"'). The medium-leaved varieties Donna, Menna and Grasslands Huia behaved similarly to the large-leaved varieties. Ranking order in the cutting trial was similar to that in the NL trial.Ground covers after three harvest years Correspondence: Mr G. Swift. Scottish Agricultural College, Crop Systems Department, Bush Estate, Penicuik, Midlothian EH26 OPH. UK.differed significantly only under grazing, when the small-leaved varieties had a 2-3 times greater cover than the larger-leaved varieties. A poor separation of the varieties in the NL persistency trial suggests that defoliation was not sufficiently severe to simulate intensive grazing.These comparisons indicate that the performance of clover varieties under the cutting regimes used should not be extrapolated to continuous sheep grazing.

Results for years 4–8 of a long‐term grazing experiment on swards of a diploid perennial ryegrass (Lolium perenne), var. Contender (D swards), a tetraploid ryegrass, var. Condesa (T swards) and Condesa with S184 white clover (Trifolium repens) (TC swards), direct sown in May 1987, are presented. The swards were continuously stocked with sheep from 1988 to 1990, as previously reported, and for a further 5 years, 1991–95, at a target sward surface height (SSH) of 4–6 cm. Control of sward height was successfully achieved by variable stocking, except in 1993 when paddocks were set stocked and the resulting mean SSH was 9·3 cm. Grass swards received on average 160 kg N ha−1 year−1; grass/clover swards were mainly not fertilized with N with the exception that they were given 30 kg N ha−1 as a remedial mid‐summer application during a period of low herbage mass on offer in 1994 and 1995. Mean white clover content of the swards fell from 18·2% of herbage dry‐matter (DM) in 1992 to 8·5% in 1993, whereas stolon lengths fell from 120 to 58 m m−2. A return to lower sward heights in 1994–95 resulted in an increase in white clover content to 12·8% by the final sampling in August 1995. Perennial ryegrass content of the grass swards remained high throughout (mean 96·7% in 1995). Perennial ryegrass tiller densities recorded in August 1991, 1993 and 1994 showed consistently significant (P < 0·001) sward differences (3‐year mean 16 600, 13 700 and 10 100 perennial ryegrass tillers m−2 for the D, T and TC swards). In 1994, the year after lax grazing, a low perennial ryegrass tiller density (9100 m−2) and low white clover content (mean 4·3%) in the TC swards resulted in a much lower herbage bulk density than in the grass swards (April–July means 72, 94 and 44 kg OM ha−1 cm−1 for the D, T and TC swards). There was a consistent 40 g d−1 increase in lamb liveweight gain on the TC swards over the T swards, except in 1994. In that year there was a reduction in lamb liveweight gain of 33 g d−1 on the TC swards and a significant increase in ewe liveweight loss (117 g d−1) associated with low herbage bulk density despite optimal sward height. Lamb output (kg liveweight ha−1) on TC swards reflected white clover content, falling from a similar output to that produced from grass given 160 kg N ha−1, at 18% white clover DM content, down to 60% of grass + N swards with around 5% clover. A 6% greater output from the T than the D swards was achieved mainly through higher stocking rate. The experiment demonstrated a rapid, loss in white clover under lax grazing, and showed that the relationship between performance and sward height is also dependent on herbage density. High lamb output from a grass/clover sward was only achieved when the clover content was maintained at 15–20% of the herbage DM.

The effect of grazing clover on ewes and lambs was studied using nitrogen-fertilized grass (G) and grass/white clover pastures (GC) containing proportionately 0-18 clover. Carcasses of lambs grazed on GC were significantly heavier than carcasses of lambs grazed on G pastures. An interaction with weaning occurred. Carcasses from lambs grazed on GC for 53 days post weaning were 2-3 kg heavier (F < 0-01) but lambs slaughtered at weaning had similar carcass weights. Diet had no effect on relative joint components of carcasses. The effect of clover in the diet on carcass weight was attributed to higher levels of protein retention. Analysis of the fatty acid composition of intramuscular and subcutaneous and perirenal lipids indicated small but significant effects of diet. Lean tissue lipids of lambs grazing clover showed significant increases in C18:2 and reduced C20 : 5 fatty acids. In tissue fats there were small increases in C14 : 0, C16 : 0 and C18 : 2 with reduced C18:1 content. Twin suckled lambs grazing GC for 94 days from turn-out in early April showed increased live-weight gain over lambs on G of 336 v. 287 glday (s.e.d. 84; P < 0-001). Post weaning live-weight gain of lambs was 173 and 221 g/day on G and GC respectively. Sward height was maintained at 5-18 and 5-24 (s.e.d. 0-075) cm on G and GC paddocks by adjusting ewe numbers. GC pastures carried proportionately 0-82 of the stock on G pastures but output of lamb was similar at 1289 and 1247 kg/ha for G and GC respectively. Results shoioed that the production penalty of lower stocking rate associated with grass/clover v. grass fertilized with 190 kg nitrogen per ha was ameliorated by higher lamb live-weight gain and carcass weight without change in joint composition or nutritionally significant change in fatty acid composition of carcass tissues.