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Notes: Microswards of white clover and perennial ryegrass were subjected to one of four treatments: weekly cutting to 3·5 cm, weekly cutting incorporating a period of no cutting for 6 weeks starting 27 April (early rest), weekly cutting incorporating a period of no cutting for 6 weeks starting 8 June (late rest), or cutting every 3 weeks. Two sward types were used: a mixture of white clover cv. Milkanova with perennial ryegrass cv. Melle, and white clover cv. Kent with perennial ryegrass cv. Melle. Growth measurements (leaf appearance, branching/tillering and stolon internode length) were confined to the first three treatments with records collected during contiguous 21-d measurement periods. Vertical height increments of clover and grass and red:far-red light ratios at the sward bases were also recorded at frequent intervals. At the end of the experiment population densities and unit weights were recorded for all treatments.Significant treatment effects on the rate processes were largely confined to the 21-d period immediately after weekly cutting of rested swards had resumed. On previously rested compared with weekly cut swards, clover leaf appearance rates were increased by 40% and branching rates by 164%. During the same period, grass leaf appearance rates were reduced by 50% and net tillering changed from positive to negative values. Though the rate responses were transient, effects were still apparent at harvest in September, when population density and content (proportion by population density and weight) of clover were significantly higher in the late rest treatment. The variety Kent showed a consistent, though usually nonsignificant, higher leaf appearance and branching rate compared with Milkanova, and in September was characterized by a higher population (7400 m−2 compared to 3200 m−2) of smaller units (27 compared to 46 mg/apical meristem) than Milkanova.The results are discussed in relation to defoliation effects and the role of light quantity and quality as they influence the component growth processes. Attention is drawn to the importance of canopy structure and the climatic and/or phenological differences in the relative seasonal behaviour of clover and grass, together with varietal variation within species in influencing responses to management manipulations.

Notes: The effects of two periods of rest from grazing by sheep (either April-May or May-June) on the composition of swards of both an early-flowering (Aurora) and a late-flowering (Melle) perennial ryegrass variety grown with cither a small-leaved (Kent) or a medium-leaved (Milkanova) white clover variety are described. Compared with a continuously stocked treatment, the early rest period did not affect the mass of clover in the swards (expressed as a proportion of the total) but resulted in a subsequent decrease in its population density. The late rest period doubled clover mass in the swards at the time of cutting but did not affect the subsequent population density of clover. The magnitude of the change in mass during the late rest period depended on ryegrass variety, but not on clover variety. The adverse effect of the early rest period on the subsequent population density of clover was associated with the degree of shading experienced by clover laminae at the end of the rest period. Temperature may also have had an effect here as mean temperatures were cooler during the early rest period than during the late rest period. It is concluded that the effects of the two rest periods on the clover content of the swards were primarily due to timing, and less to inherent differences between ryegrass varieties.

Notes: A series of twenty-four swards containing different proportions of white clover (0·20-0·25) and perennial ryegrass were created by using different seed mixtures, herbicide applications and previous cutting Frequencies. These swards were used to study the diet of oesophageally-fistulated wether sheep which grazed the various swards for a 30-min period after 1, 2 and 3 weeks of regrowth.The proportion of white clover in the diet was generally greater than that in the sward. Fifty-seven percent of the variation in the proportion of white clover in the diet could be attributed to the proportion of white clover in the sward. White clover and perennial ryegrass leaf and stem were grazed to the same height and the proportion of white clover in the grazed horizon of the sward explained 83% of the variation in the proportion of white clover in the diet. The proportion of white clover in the diet was greater than the proportion in the grazed horizon of the sward in week 3 of regrowth, but not in weeks 1 and 2, and greater when the proportion of white clover in the grazed horizon was lower than 0·20. Both these observations were interpreted as indicating selection for white clover by the sheep within the grazed horizon.There was a positive and linear relationship between the depth of the grazed horizon and sward height which, together with the relationship between the proportion of white clover in the grazed horizon and in the diet, would allow the prediction of the proportion of white clover of the diet from the height and the white clover content of the grazed horizon of the sward.

Notes: The effects of sheep grazing on the development and production of heather swards were studied over a period of 3 years in an experiment in which sheep numbers were adjusted to remove 0, 40% or 80% by weight of the current season's shoots in summer and in autumn.The number of shoots grazed increased with the level of grazing but not in direct proportion. The length of shoot grazed was greater at the 80% level of grazing. Heavy autumn grazing was associated with frequent uprooting and breaking off of shoots and with a high incidence of shoots grazed down to the previous season's wood. The siting of new season's shoots was influenced by the amount of shoot length grazed in the previous season. Grazing behaviour of sheep within a plot was affected by variation in heather cover with the sheep showing a tendency to graze near bare areas.After 3 years of grazing, standing crop was reduced by both levels of grazing whether in summer or autumn. New shoot production was unaffected on plots which had had the 40% level of shoot removal but was reduced by half on plots which had had the 80% level of shoot removal whether in summer or autumn.A clipping experiment was also carried out where 0, 40% or 80% of the length of current shoots was removed in May, early July and Septeinber. Standing crop was unaffected by May clipping but was reduced on plants with a history of 80% shoot removal in July or September. New shoot production was only reduced on clipped plants which had been subjected to 80% removal in September.The role of overwintering green shoots and carbohydrate reserves in the support of new shoot growth was considered.The implications of the results in relation to heather management are discussed.

Notes: The effects of sheep grazing on the development of and production from heather swards and on intake and diet selection by grazing Scottish Blackface wether sheep were studied in an experiment in which sheep numbers were adjusted to remove 0, 40% or 80% by weight of current season's shoots in summer and in autumn for a period of 5 years. All combinations of season and level of grazing were provided. The plots were divided after 2 years and grazing was continued for a further 3 years on one subplot but discontinued on the other.Sward productivity during the 5 years of grazing was unaffected at the 40% level of shoot removal but at the 80% level was reduced by up to 66%.After a season's uninterrupted growth at the end of the experiment heather cover and herbage mass were lower and current season's shoots as a proportion of total mass were higher on those treatments which received the greatest severity of grazing. Weight of current season's shoots was unaffected on treatments which had received the 40% level of shoot removal but was reduced by 40% and 50% on treatments which had received the 80% level in summer and autumn respectively. On the rested subplots sward recovery was such that no treatment effects remained after 3 years except with respect to 80% shoot removal in autumn where herbage mass was reduced compared with other treatments.Intakes of digestible organic matter per sheep were higher at the 80% than the 40% level of grazing in the fifth year of the experiment. This was ascribed to the ingestion of new shoot growth from the twig bases on the 80% level of removal treatments. Intake and digestibility were higher in the summer on those treatments which had received the 80% level of removal in previous autumns. Over the 5 years of the experiment there was a small decline in intake and digestibility values, with the decline being greatest in the summer and at low levels of grazing.Floristic changes differed on the rested and grazed areas. On the rested subplots in the fourth year after grazing stopped, heather height was lower and cover by grasses, sedges and herbs was greater on treatments which previously had received the highest levels of utilization. On bare areas the rate of recovery of heather was rapid where shoots as well as seedlings contributed to recovery growth. On the grazed subplots, with the exception of the low-growing ruderal Rumex acetosella, cover due to grasses, sedges and herbs remained low as species other than heather were selectively grazed.Relationships among stocking rates, grazing behaviour and heather utilization are discussed and guidelines for the recognition of overgrazing are outlined.

Notes: Extensification (a reduction in fertilizer inputs and stocking rate of grassland) is seen as one way of increasing the conservation value and of reducing the environmental impact of upland sheep production in the UK, but little is known about the consequences of such a change. This study determines the changes in animal production over ten years following the introduction of four extensive grazing management strategies to perennial ryegrass/white clover pastures at two upland sites. Fertilizer-free treatments were maintained with sward heights of: 4 cm (treatment 4/4U) or 8 cm (8/8U) during the whole of the grazing year, 4 cm during summer and 8 cm during autumn (4/8U) and 8 cm during summer and 4 cm during autumn (8/4U). A control treatment that received 140 kg N ha−1 year−1 was also maintained with a sward surface height of 4 cm (4/4F). Scottish Blackface sheep grazed all treatments.The 4/4F treatment carried the greatest number of animals (3746 grazing days ha−1 year−1); the 4/4U and the 8/8U treatments carried 0·73 and 0·43 of this number respectively. The number on the 4/8U treatment was similar to that on the 4/4U while the 8/4U treatment carried 1·41 of that on the 8/8U treatment (0·61 of 4/4F). Mean individual animal performance was greatest on the 8 cm swards and tended to be lowest on the 4/4F treatment. However, the 4/4F treatment produced the greatest live weight of lamb (623 kg ha−1 year−1) with the 4/4U producing 0·77, and the 8/8U producing 0·55, of this amount. Although there was year-to-year variation in agricultural output, it was concluded that the lower levels of sheep production that result from a change to extensive systems of grazing management can be maintained for at least 10 years.

Notes: The morphological characteristics of Trifolium repens, grown in association with either Lolium perenne or Holcus lanatus and grazed by sheep to maintain sward surface heights of 3 cm, 5 cm and 10 cm, were measured. The lamina area, petiole length, internode length and internode weight of T. repens were significantly greater when H. lanatus was the companion grass than when L. perenne was the companion grass. All of these characteristics, together with the number of laminae per stolon, were also greater in the taller swards than in the shorter swards. The number of branches on each shoot did not vary significantly with either companion species or sward height treatment. Except for the effects of the treatments on the number of branches, the results are generally consistent with the known effects of variation in foliar shading on the morphology of T. repens.

Notes: Rates of leaf extension and senescence were measured over three-day intervals on neighbouring sets of caged and uncaged tillers on several occasions during summer 1983, both when grazing animals were present and also when they were absent. The weather was dry and drought conditions developed as the season advanced.Comparison of data from caged and uncaged tillers indicated that some consistent differences occurred. Loss of information through grazing (affecting 30% of uncaged, larger than average, tillers), contributed to the significantly reduced estimates of leaf extension of ungrazed, uncaged tillers compared with caged tillers. Senescence rates were higher in the open plot than under the cages; old leaves were grazed on only 8% of uncaged tillers so that data loss was less important as a cause of bias. In the drought conditions prevailing, it was thought that caging in the presence of grazing animals resulted in reduced moisture stress and contributed to the above results.Comparisons of caged and uncaged tillers when grazing animals were absent were made to examine the environmental effects of caging and the slight difference in tiller marking procedure. No differences were found in leaf extension rates, though a trend towards higher senescence rates for the uncaged populations was evident. It was concluded that the criteria limiting the use of cages (a period of protection less than the average interval between both leaf appearance and repeated defoliation) should be extended to include a time limit set by the need to avoid bias associated with the environmental effects of caging and cessation of grazing. To define this time limit better in particular circumstances, the extent and direction of bias in the estimation of leaf extension and senescence rates as affected by period of caging, tiller handling procedures, sward and climatic conditions requires further investigation.In the case of vegetative tillers, estimates of leaf extension rates can be made considerably more precise by using tiller size variables (pseudostem length, total green leaf length, leaf width) as covariates. A suggestion for the number of tillers to be measured per transect is given. However, no such increase in precision can be made for estimates of senescence rates.

Notes: Two contrasting decision rules for adjusting stock density to control the sward height of continuously grazed grass swards were tested and developed. One rule calculated the adjustment as a percentage of the existing stock density (Percentage Rule); the other related the adjustment to estimates of the bulk density of the grazed horizon and individual animal intake (Absolute Rule). Both decision rules related the adjustments to the change in sward height over the previous week and to the deviation from the target value. The decision rules were tested by continuously grazing lactating ewes and then dry ewes after weaning with a target sward height of 4 cm on 0·33-ha plots with stock density adjusted by varying animal numbers. The Percentage Rule failed to increase the stock density sufficiently rapidly when grass growth was accelerating, leading to increasing deviation in sward height from a 4 cm target value. The Absolute Rule succeeded in controlling sward height once the bulk density term had been correctly adjusted. Pre-emptive adjustment of stock density in anticipation of a change in grass growth rate marginally improved the control that was achieved by the Absolute Rule.

Notes: The relationship between the rate of canopy gross photosynthesis and LAI was investigated for ryegrass swards released from grazing after a period of continuous stocking to maintain the pasture at LAI 2–3. The regrowing swards were allowed to increase to about LAI 5 over a period of about 2 weeks while the rate of photosynthesis was measured and compared with that for continuously stocked swards maintained at LAI values between 2 and 6. Gross photosynthesis was found to increase more rapidly than LAI on the regrowing swards than on the steady-state control pastures. However, this response appeared to be influenced by seasonal factors. The potential increase in net COi uptake was estimated to range from 0 to 22%. The response was attributed to an increasing proportion of young photosynthetically-efficient leaves near the top of the canopy, modified by seasonal changes in the rate of tillering. The agronomic implications of the results are iscussed.