ALBERT

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: Groups of mature, non-lactating sheep and cattle grazed a Nardus stricta community during the growing season for six consecutive years from 1984 to 1989. Three unreplicated treatments were applied by continuous variable stocking to maintain between-tussock sward height at (a) 4·5 cm by cattle grazing and (b) 4·5 cm or (c) 3·5 cm by sheep grazing. Diet composition and herbage intake were measured from 1984 to 1987 on three occasions in the growing season, and live weight and stock density were recorded from 1984 to 1988. The diet of cattle usually contained more dead herbage, Nardus, sedges and rushes but less forbs and other fine-leaved grasses than the diet of sheep. Principal coordinate analysis showed different trends across years in diet composition, especially between the sheep and cattle treatments at 4·5 cm. Diet digestibility was usually higher for sheep treatments than for the cattle treatment. Diet digestibility and herbage intake increased between 1984 and 1985, and 1986; they also declined from spring to late summer. Regression of diet digestibility on independent principal components — which were derived from diet composition measurements — showed that the two most important principal components accounted for 72% of the variation in digestibility. Stock-carrying capacity (kg LW × d ha−1, calculated from live weight, grazing days and stock density) was greater on the cattle treatment than on either sheep treatment. Stock-carrying capacity also increased more in successive years on the cattle than on either sheep treatment, and it was greater on the sward maintained at 3·5 cm than at 4·5 cm by sheep. These increases in stock-carrying capacity were generally positively associated with the increase in the percentage specific frequency both of live material and of the more productive grasses in the swards. These data indicate that sheep-only stocking tends to Nardus dominance and suggest that further study using productive cattle — either alone or mixed with sheep — is needed, preferably in association with measurements of floristic change both within and between tussocks.

Notes: A factorial pot-experiment was carried out to investigate the effect of burial of stolons (stolons unburied or buried to a depth of 0·5–1 cm at day 0), with and without defoliation (plants uncut or all unfolded laminae removed at weekly intervals) on the growth of two varieties of white clover (cv. Kent or cv. Milkanova) harvested on three dates (14 d, 28 d and 42 d after burial). The soil used was a 3:2:1 mixture of clay loam:peat:sand, and there were three replicates of each variety for each treatment at each harvest date.Burial of stolons in the absence of defoliation had no effect on stolon extension, leaf appearance, or the concentration of water-soluble carbohydrate in the stolons. The number of axillary buds developing (new branches plus flowers) was increased on the new surface-growing tips of primary stolons but was reduced on branch (i.e. secondary) stolons. The proportion of branches to flowers was largely unaffected by burial.Defoliation caused substantial reductions in the concentration of water-soluble carbohydrates in stolons and stolon extension growth, a reduction in number of axillary buds developing, and in the proportion of buds which were floral, but had only a small effect in reducing leaf appearance.The combination of stolon burial and defoliation resulted in the death of secondary stolons; 42% of all secondary stolons had died by day 42, and stolon extension, leaf appearance and numbers of axillary buds developing on secondary stolons were severely reduced. The numbers of axillary buds developing on primary stolons was increased owing to more buds developing on the resurfaced stolon tips, but the increase was inadequate to compensate for the reduced growth, and death of secondary stolons.Differences between clover varieties in response to treatments were small. In general, the smaller variety, cv. Kent, was more adversely affected by burial than the larger variety, cv. Milkanova. A variety x defoliation interaction occurred for senescence of leaves; petiole senescence of cut leaves was reduced or similar to that of intact leaves for cv. Kent, but was increased for cv. Milkanova.The results are discussed in relation to treatment effects on carbon sources and sinks, and in relation to climate-soil interactions.

Notes: The relationship between net canopy photosynthesis (Pnc) measured at 400 J m−2 s-1 and leaf area index (LAI) was determined on ryegrass-dominant swards over a range of her bage masses in five grazing experiments. In three experiments the swards were continuously stocked either by sheep or by cattle to maintain a number of herbage heights and hence LAI values. In two experiments the swards were intermittently grazed with 21 -d regrowth intervals. On the continuously stocked swards measurements were made over nine periods each of 7 to 21 d in length. On the intermittently grazed swards measurements were made over six periods while the herbage was being grazed down from a high to a low LAI and over three periods during the regrowth phase.On all the swards where measurements were made while grazing was in progress, the relation of Pnc to LAI was linear. The photosynthetic rate was greater on the continuously grazed than on the intermittently grazed swards up to LAI 35 and in two experiments to LAI 5. The relationship of Pnc to LAI of swards in the regrowth phase was predominantly curvilinear and such that over much of the LAI range they had a greater photosynthetic potential than the same swards being grazed down from high to low mass. Daily rates of CO2 uptake and growth in the field were calculated for one continuously stocked experiment. Both were linearly related to LAI in the range LAI 2–4·5. The agronomic implications of the results are discussed.

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.

Notes: An experiment was carried out in which simulated swards of ryegrass (cv. S23) were grown in boxes. In the first instance the swards were cut at weekly intervals to maintain five levels of leaf area index (LAI) from LAI 1 to 4–5 in simulation of continuous grazing. Measurements were made of growth, senescence and net growth rate and of net canopy photosynthesis at constant irradiance. The results showed that the swards adapted to the defoliation regimes mainly by changes in tiller population density and pseudostem length. When the swards had equilibrated to the cutting regime growth rate increased with LAI but, since tiller density and the partitioning of growth between herbage harvested and that lost by sensecence also changed with LAI, net growth rate was constant over the LAI range 2–4·5. Maximum weight of herbage harvested was obtained between LAI 2 and 3.After 10 weeks of weekly cutting all the swards were cut back to LAI 1 and allowed to regrow. Growth rate showed almost no response to the previous culling treatments. The relationship of net canopy photosynthesis to LAI was linear for the frequently defoliated swards and curvilinear for regrowing swards. The reasons for this difference were examined.

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: 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: Measurements of rates of growth and senescence of leaf lamina per tiller and of changes in tiller population densities were made in three experiments designed to investigate the influence of sward slate on leaf turnover and net production under continuous stocking.In each experiment initially uniform swards were fenced to provide four plots on which animal numbers were adjusted twice weekly to give a series of swards maintained as nearly as possible in a steady state with respect to sward surface height (range 1.1–6.4 cm) and herbage mass (range 440–2690 kg OM ha−1). Two experiments were carried out in July–September on vegetative swards and one in May–June on a reproductive sward. Measurements were begun 3–7 weeks after treatments were started and were repeated weekly during 3–4 week measurement periods.In all three experiments the rate of lamina growth per tiller increased linearly with an increase in sward surface height and herbage mass. In the two experiments conducted in July–August this relationship was partially offset by a linear increase in the rate of senescence per tiller but net production per tiller also increased linearly in relation to sward height and mass. In the experiment conducted in May–June the rates of growth and senescence per tiller increased in parallel so that net production per tiller showed no relationship with sward condition.Tiller population densities in the July–August experiments were highest in swards maintained between 2 and 3 cm surface height and declined in swards maintained above and below this height. In the experiment in May–June tiller numbers were similar in all treatments prior to the summer solstice but diverged in a manner similar to the other experiments later in the year.The rate of lamina growth per unit area increased in a manner consistent with an asymptotic relationship and the rate of senescence increased linearly with increasing sward height and mass in all three experiments. Net production per unit area was reduced on swards below about 2.5 cm in height but was insensitive to variation in sward surface height between 2.5 and 6.0 cm (approximately 1000–2500 kg OM ha−1 herbage mass).The effectiveness of adjustments in tiller numbers and production per tiller and of changes in the balance between growth and senescence as mechanisms of sward homeostatis, together with their implications in the scope for manipulating herbage production by grazing management are discussed briefly.