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Notes: Cattle and sheep can create and maintain a mixture of relatively tall and short patches in grass swards through selective grazing. In swards that are grazed by cattle this heterogeneous structure can result in the frequencies of height measurements having a skewed distribution that has variously been better described by the double-normal distribution the gamma distribution and the Weibull distribution than by the more common normal distribution. The fit of these statistical distributions, and the adequacy of the potentially useful log-normal distribution, to sward height frequencies were tested in sown temperate swards grazed by sheep and compared within a single sward. It was concluded that the single-normal and Weibull distributions were inadequate and that overall the log-normal and gamma distributions had the best fit to the measurements.

Notes: Perennial ryegrass/white clover pastures grazed by sheep and receiving either no fertilizer N (No) or 120 kg N ha−1 year−1 (N0) were maintained with surface heights of 2·5, 3·5 and 5·0 cm for over four years. The treatments were replicated.The white clover (WC) population was greatest in the N0treatment, and declined during the study. Between-year variation in WC was negatively related to rainfall and positively related to temperature, WC as a proportion of the total plant population decreased during the summer in the Nl treatment. The perennial ryegrass (PRG) population was greater in the Nl treatment, declined during the study and both within and between years was positively related to temperature. The population density of the unsown grasses was highest in the N-fertilized treatment and in the swards maintained at the lowest heights (these treatments also had the highest stocking rate); it increased during the study, within-years being positively related to temperature and between-years being positively related to rainfall.The WC stolon extension rate was largely unaffected by N fertilizer application and was greatest in the taller swards. Leaf appearance rate was unaffected by N fertilizer application and sward height; it was positively related to temperature and negatively related to rainfall.Branching rate was greater in the N0 treatment with significant sward height effects confined to a negative relationship with local sward height within treatment plots on one occasion; it was negatively related to rainfall. The ground level red:far red light ratio was negatively related to local sward height. The total live weight of sheep carried in the No treatments was 0·7 of that in the N1 treatments.Expected photomorphogenic responses by we were confined to stolon extension. It was concluded that on the poorly drained clay-loam soil used in this study the effects of sheep, in interaction with climatic factors, had an overriding effect on clover branching rate and the ultimate species composition.

Notes: In a range of perennial ryegrass/white clover swards, variation in the surface heights of the grass and clover components, the rates of increase of these surface heights and the specific leaf areas (SLAs) of ryegrass and white clover were described for 1 year. The swards were of an early- or late-flowering (Aurora or Melle respectively) perennial ryegrass variety growing with either a small- or a medium-leaved (Kent or Milkanova) white clover and were either continuously stocked by sheep or continuously stocked apart from a rest period in April-May (Aurora) or May-June (Melle).The surface heights of grass and clover were not affected by the variety of their companion species, and the surface heights of the two clover varieties were similar. The grass was always taller than the clover, although the magnitude of the difference between the species varied with time of year and the timing of the rest period.Before the summer solstice the rate of increase in height of grass was greater than that of clover except at cool temperatures (5°C) and warm temperatures (16°C), and in the unrested Melle sward. After the solstice the rates of increase in height, particularly of clover, were lower than the rates seen at similar temperatures before the solstice.Overall, the SLAs of both clover varieties were greater than those of ryegrass when grown with Aurora but not when grown with Melle, and the SLAs of both species increased during the year. By October the SLAs of both grass varieties were less than those of their companion clovers.The results are discussed in relation to their implications for the species composition of the swards.

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 effects of sheep grazing, with or without a 7-week rest period before ear emergence, on species balance in four perennial ryegrass/white clover swards were assessed.Swards of an early (Aurora) or a late (Melle) flowering ryegrass were grown with either a small-leaved (Kent) or a medium-leaved (Milkanova) clover.The clover lamina, expressed as a proportion of the total lamina mass, was greater with Kent than Milkanova. This proportion was not changed by resting the Aurora (earlier rest) swards, but was increased by resting the Melle (later rest) swards. A greater proportion of the clover lamina mass was near the top of the canopy in the Milkanova than in the Kent swards, and during the summer than in spring or autumn. The densities of clover growing points were greater in the Kent than the Milkanova swards and were increased by the late rest period. This effect persisted over the winter.Stocking density was higher on the Kent than the Milkanova swards. The effects of the two rest periods, and the variety combinations, on pasture composition are interpreted as being principally a result of the differences in temperature between the earlier and later rest periods and of the different relationships between temperature and the growth of ryegrass and clover. Varietal characteristics, such as the vertical distribution of laminae, were also shown to be important.

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.