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Notes: A large, and often neglected, component of grazing intake is behavioural. Grazing animals may employ two strategies to increase their intake rates during grazing: increasing bite mass and decreasing handling time. These strategies are discussed in the context of fasted and non-fasted sheep grazing white clover (Trifolium repens L. cv. Kent wild white) and perennial ryegrass (Lolium perenne L. cv. Parcour). It is concluded that within a forage species sheep have little flexibility for increasing their intake rates by altering their handling times but have some latitude for altering bite masses. Between forage species, differences in intake rates may be more strongly affected by the ‘species-specific’ requirement for mastication than by differences in bite masses per se. Comparisons are also made with grazing strategies of cattle.

Notes: A perennial ryegrass sward was managed by continuous stocking with sheep (April–September) for 4 successive years after sowing. The sward was grazed to maintain a leaf (lamina) area index (LAI) close to 1.0. Areas of the sward were released from grazing on three occasions: once during summer in the third year after sowing, and twice during spring and summer in the fourth year after sowing. There were marked changes in the structure and physiology of the continuously stocked sward following release from grazing. After several successive years of continuous stocking, the sward comprised a large population of small tillers and the small LAI resulted in consistently low rates of photosynthesis. Following release from grazing, photosynthesis increased markedly as the LAI increased but this change was associated with the loss of a large proportion of the population of tillers. There were seasonal differences in the pattern of changes in photosynthesis and tiller numbers following release from grazing which were not apparent under continuous stocking. The changes in the structure and physiology of the sward following release from grazing suggest that the net accumulation of herbage in areas of sward from which the animals are excluded, for instance using cages, may be an unreliable estimate of production under continuous stocking.

Notes: Three replicate paddocks, each of 0·235 ha, containing adjacent monocultures of perennial ryegrass or white clover [50:50 by ground area, 6 cm sward surface height (SSH) at start of experiment] were continuously stocked with three yearling and four mature non-lactating, non-pregnant Scottish halfbred ewes for 12 weeks. Herbage intake, grazing behaviour and dietary selection were measured on seven occasions. Clover SSH declined rapidly over the first 5 weeks then stabilized at 1·2–1·6 cm, whereas perennial ryegrass SSH rose slightly initially, then declined gradually. Animals initially included proportionately c. 0·6 white clover in their diet but, by the end of the experiment, this had fallen to 0·3. Total daily herbage intake declined over the 12 weeks from 1·8 kg dry matter (DM) day–1 at the start to 1·0 kg DM day–1. Total grazing time increased from 561 min day–1 to 649 min day–1 at the end of the experiment. The results suggest that, despite overall herbage depletion and a greater depletion of white clover than perennial ryegrass as a result of the initial partial preference for white clover, the animals traded-off a reduced total intake and an increased grazing time in an attempt to maintain their initial preferred dietary composition.

Notes: The spatial patterns of white clover and sward surface height (SSH) that developed In established perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pastures undercutting lent every 4 weeks to 5 cm) and gracing (continuously grazed with sheep to 5 cm) were measured. While clover cover was recorded in 1000 contiguous 5 × 5-cm quadrats down 50-m permanent transects from early spring to late autumn. Measurements of SSH were made at 10-cm intervals down the same transect. Spatial pattern was analysed using two-term local quadrat variance and patch-gap analysis. At least two scales of spatial pattern existed for white clover when defoliation treatments began. White clover was not distributed at random but found in patches (mean size = 1.1 m) where it was finely intermixed with grass. Patches, separated by gaps (regions of no clover) (mean size = 2.3 m), were in turn aggregated into ‘patches of patches’, separated by larger gaps (mean size = 4.1 m). Under grazing the pattern of patches and gaps did not alter. Under cutting, patch size increased and gap size decreased, explaining in part the greater mass and cover of white clover that arose in cut than grazed swards during the experiment. No new patches of white clover due to seedling establishment or clonal growth were observed in either cut or grazed swards. The intensity of pattern increased in both cut and grazed swards, but the increase was greater m cut swards. The initial single scale of spatial pattern of SSH of tall patches (mean size = 1.2 m) separated by short patches (mean size = 2.7 m) did not change under grazing. SSH became uniform under cutting. It is suggested that the response of plants to selective (spatially heterogeneous) grazing is a crucial factor in the development and maintenance of spatial pattern in grasslands. The importance of spatial pattern to our understanding and interpretation of plant-plant and plant-animal interactions and to the composition of temperate grasslands is considered.

Notes: This study used both experimental evidence and a mathematical model to address some differences in interpretation in the literature on the relationship between sward height and the bite dimensions (bite depth, bite area and bite mass) of sheep grazing contrasting vegetation types. Individual non-fasted sheep were released onto small areas (10 × 10m) of white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) and bite dimensions were measured as they grazed across patches (0·7 × 0·7 m) of predetermined sward surface height (SSH). Sward heights were 4, 7, 10, 13 and 16cm for white clover patches and 4, 6, 5, 9, 11·5 and 14cm for ryegrass patches. Four sheep were assigned to each plant species and each sheep grazed one patch of each height (five patches/sheep). Bite depth, bite area and bite mass increased linearly with SSH in both white clover and ryegrass. At a given SSH, bite depth was similar in white clover and ryegrass, but bite area and bite mass were greater in white clover than in ryegrass. The linear relationships observed between bite mass and SSH contrasted with the asymptotic relationships observed in some other studies, but it is suggested that different relationships may arise because of methodological differences between studies. Furthermore, when linear relationships for bite mass were compared with asymptotic relationships for bite mass in a mechanistic model of animals grazing from ryegrass-white clover pastures it was demonstrated that the nature of the relationship had relatively little effect on the relationship between intake rate and SSH. This was because intake rate depended on the fundamental mechanistic relationship between bite mass and prehension bite rate. This relationship meant that the greater bite masses found when linear relationship were assumed were associated with reduced prehension bite rates and thus the effect on intake rate was relatively small. In addition, the predictions of the model regarding the bases of diet selection by animals grazing ryegrass-white clover mixtures were simplified, and stabilized, when linear relationships were assumed.

Notes: A study was made to characterize the effect of the duration of regrowth on the pattern of changes in the major physiological processes involved in the net accumulation of herbage, and so to provide a rational basis for optimizing production under rotational grazing.During regrowth following a severe defoliation, rates of canopy photosynthesis, and so the rate of production of new leaves, increased rapidly but there was a delay before there was a corresponding increase in the rate of leaf death. Although the amount available for harvest (crop live dry weight, W) continued to increase as the duration of regrowth was extended from ‘short’ (12–13 days), through ‘medium’ (19–23 days) to ‘long’ (30-34 days), there was not a continued increase in the average growth rate – the increase in the weight of the crop, (W – Wo), divided by the duration of regrowth, t, In this study, the average growth rate (based on changes in the weight of lamina alone) increased as the duration of regrowth was extended from 12-13 to 19-23 days but changed little as the duration of regrowth was extended from 19-23 days to 30-34 days. In spring and summer, elongating stems increased the average growth rate (of lamina plus stem) up to 30-34 days but the accumulated stem material could not reliably be harvested by sheep.A mathematical model was used to explain why the average growth rate is characteristically insensitive to the duration of regrowth beyond a given minimum duration. For practical purposes, we suggest from the results of this study that regrowths of at least 14 days but less than 28 days will be effective in achieving not only close to the maximum average growth rate of highly digestible material, but also in sustaining a densely tillered, leafy sward which regrows rapidly from severe defoliation and is more reliably harvested (utilized) by sheep.

Notes: A 10-week grazing experiment was conducted on a perennial ryegrass sward with lactating ewes and their twin lambs. Three paddocks were rotationally grazed with rest periods of from 4 to 5 weeks. Sward surface heights at the start of each grazing were 145, 259 and 250mni for treatments RG1, RG2 and RG3. A further four paddocks were maintained by continuous variable stocking (CS) at sward surface heights (SSHs) of about 30, 60, 90 and 120mm. Sward and animal measurements were made on the two different grazing managements as the RG swards were grazed down, giving measurements at similar sward heights for treatments RG and CS.There was less green leaf and the total herbage mass present under RG was less than on CS swards at the same sward heights, demonstrating the differences in structure between rotationally and continuously grazed swards.Regression analysis of animal factors on sward factors showed that grazing behaviour was more highly correlated with green leaf mass than SSH or any of the other sward measurements. On the RG swards, maximum intake per animal was reached at about 1500 kg green leaf mass ha−1. A SSH of 60mm allowed the CS ewes to achieve the highest intake rate, but at this height the ewes on treatments RG2 and RG3 were restricted to approximately half this rate. The results suggest that green leaf mass or leaf area index, rather than sward surface height, could be used as a rational basis to relate intake of herbage to sward state for swards changing rapidly in leaf to stem ratio.

Notes: The study was designed to test the hypothesis that grazing management in early season could alter sward structure to facilitate greater animal performance during critical periods. The effects of grazing a mixed perennial ryegrass/white clover sward at different sward surface heights, by cattle or sheep, in early season on sward composition and structure, and on the performance of weaned lambs when they subsequently grazed these swards in late season were determined. In two consecutive years, from mid-May until mid-July, replicate plots (three plots per treatment) were grazed by either suckler cows and calves or ewes and lambs at 4 or 8 cm sward surface heights (Phase 1). From mid-August (Year 1) or early August (Year 2), weaned lambs continuously grazed, for a period of 36 d (Year 1) or 43 d (Year 2) (Phase 2), the same swards maintained at 4 cm (treatment 4–4), 8 cm (treatment 8–8) or swards which had been allowed to increase from 4 to 8 cm (treatment 4–8). Grazing by both cattle and sheep at a sward surface height of 4 cm compared with 8 cm in Phase 1 resulted in a higher (P 〈 0·001) number of vegetative grass tillers per m2 in Phase 2, although the effect was more pronounced after grazing by sheep. Sheep grazing at 8 cm in Phase 1 produced a higher number of reproductive tillers per m2 and a greater mass of reproductive stem (P 〈 0·001) than the other treatment combinations. The mass of white clover lamina was higher under cattle grazing (P 〈 0·05), especially on the 8-cm treatment, and white clover accounted for a greater proportion of the herbage mass. These effects had mainly disappeared by the end of Phase 2. On the 4–4 and 8–8 sward height treatments the liveweight gain of the weaned lambs was higher (P 〈 0·05) on the swards previously grazed by cattle than those grazed by sheep. The proportion of white clover in the diet and the herbage intake also tended to be higher when the weaned lambs followed cattle. However, there was no difference in liveweight gain, proportion of white clover in the diet or herbage intake between swards previously grazed by cattle or sheep on the 4–8 sward height treatment. It is concluded that grazing grass/white clover swards by cattle compared with sheep for the first half of the grazing season resulted in less reproductive grass stem and a slightly higher white clover content in the sward, but these effects are transient and disappear from the sward by the end of the grazing season. They can also be eliminated by a short period of rest from grazing in mid-season. Nevertheless these changes in sward structure can increase the performance of weaned lambs when they graze these swards in late season.

Notes: In two experiments with ryegrass/white clover mixtures, the proportion of clover was measured before and after cuts which removed 8-75% of above-ground biomass. Cutting was found to reduce the proportion of clover leaf area in the crop in both experiments, sometimes by as much as two-thirds, and the proportion of clover dry weight, by up to half, in one of them. That is, the harvested material contained a greater proportion of clover than did the sward before the cut. This disproportionate removal of clover was due to clover having a greater proportion of its leaf near the top of the canopy than grass. It showed that preferential removal of clover occurs as a result of the purely passive selection by a mower, not only as a result of grazing by animals which may be capable of active as well as passive selection.Despite the disadvantage to clover of losing more of its leaf area than grass, and in some cases more of its dry weight also, when the mixture was cut, the clover content of the sward did not decrease during the growing season as a whole. This was because, where no nitrogen fertilizer was applied, clover had a greater relative growth rate (RGR) than its companion grass during the growth periods between cuts and this increased its percentage of the mixture. Even where nitrogen was applied, clover equalled the RGR of grass and maintained its proportion of the crop, except in one instance.

Notes: The potential 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–1 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–1 (plus twin lambs); lamb growth rates were 223, 268, 295 and 260 g d–1and so total lamb live weight gain was 1054, 920, 630 and 1148 kg h a–1. 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 presence of a small and declining clover content, the output of the mixed grass/clover sward managed in this way was 80%, 80% and 82% of that of a grass sward supplied with 420 kg N ha–1 in 1985, 1986, and 1987 respectively and, similarly, 83% of the output in 1987 of a grass sward receiving 210 kg N ha–1.