Growth, colonization and productivity of two white clover cultivars strip-seeded into an upland Festuca-Agrostis sward

Small plots of a Festuca-Agrostis upland sward on a peaty gley podsol were strip-seeded during late June 1986 with white clover cvs Aberystwyth S184 or Menna at 4 kg ha⁻¹ and defoliated early (20 August) or late (3 September) and then frequently or infrequently (every 2 weeks or 4 weeks) until the end of September. All plots were defoliated in early November, at 3-weekly intervals during the growing season in 1987 and then grazed rotationally during 1988.
Satisfactory seedling establishment, representing 46% emergence, was achieved 5 weeks after sowing. The differential defoliation regimes had no persistent significant effects on clover development. S184 soon produced more leaves per seedling than Menna and a smaller proportion of its leaf number and weight were removed at each defoliation. Following large losses of leaves over the 1986–87 winter, SI84 had significantly more leaves per stolon than Menna; subsequently it also colonized the sward at a quicker rate. During 1987 amounts of herbage harvested (6.1 t ha⁻¹) were similar with the two clover cultivars, with S184 contributing 47% and Menna 44% of this respectively. SI84 made a larger contribution to yield during May and June but Menna was more productive during September and October. During 1988 clover populations were maintained with rotational grazing without additional fertilizer inputs.
The results show that, despite initial soil and climatic contraints, both small and medium-leaved clovers can be strip-seeded into upland swards with large subsequent benefits to yield and herbage quality. However, they also indicate the need for further experiments to determine the influence of sward morphology and defoliation regime on stolon branching rates and accumulation of growing points which, in turn, govern sward colonization.     

Slot-seeding investigations. 6. A comparison of the spread and productivity of different white clover varieties introduced into permanent pasture

Seeds of six cultivars of white clover were sown in 1983 in slots in tares of permanent pasture in soil-filled tanks in spring and in small plots in the field during midsummer. The clover cultivars investigated were the large-leaved Olwin and Milk nova, the medium-leaved Grasslands Huia and Aberystwyth S100, and the small-leaved Kent Wild White and Aberystwyth S184. Seeds from an indigenous clover population were also sown in the tanks. All herbage was defoliated at three- to six-week intervals to simulate rotational grazing and assessments were made until autumn 1984.
Leaf and stolon production and spread of stolons from the slots were more rapid in Milkanova, S184, Huia and Kent than in Olwen and S100 In the tanks, harvested yields of clover leaf and petiole were largest for Milkanova and Olwen and least for the indigenous population; both Olwen and S100 had a smaller percentage of their total weight outside the slot area than had the other varieties. In the field, Milkanova greatly out yielded all other varieties and S100 yielded the least, contributing 48 and 27%, resf actively, to total herbage yield. The yield of grass differed little between varieties in tanks but in the field it tended to be higher with the smaller than with the larger-leaved varieties. Total herbage yield was largest for Olwen and Milkanova in tanks and for Milkanova in the field. The greatest weight of stolons in tanks was produced by Olwen and in the field by Olwen and Kent, but the greatest length of stolons and number of nodes in both environments occurred in the smalt-leaved varieties, especially Kent. Least length of stolons and fewest nodes in the tanks were produced by Olwen and in the field by Milkanova.
It was concluded that all the varieties investigated could be successfully slot-seeded into permanent pastures but that the choice of variety will depend on subsequent use and management of the resulting swards.     

Out-of-season management of grass/clover swards to manipulate clover content

Two experiments are described in which the effect of grazing or defoliating mixed swards at different times over winter and spring on clover content and development was investigated. In the first experiment swards were grazed with sheep (to about 3 cm) for a short period in (a) November, (b) November, January and March, (c) March or (d) not at all, in three consecutive years. All swards were grazed intermittently during the grazing season with cattle and cut for silage once each year. Each plot received either 0 or 50 kg N ha^?1 in March. The effect of N fertilizer was to reduce clover content in each summer and clover growing point density in the third year. In two of the three years, treatments involving grazing in March had lower subsequent net annual herbage accumulation compared with the other two treatments and higher clover content in summer of the third year. Reduction in growing point density in all plots during the grazing season was associated with cattle grazing when conditions were wet, suggesting that stolon burial was implicated. Grazing with sheep in November, January and March resulted in significantly more visible (when counted in situ) clover growing points in April in year 2 and more total growing points (counted after dissection of turves) in the third year than the November grazed and ungrazed treatments which had, on occasions, higher grass tiller density. In a microplot experiment, high herbage mass standing over winter was associated with lower potential photosynthesis per unit clover lamina area and lower growing point density in March. Cutting herbage in March to 2-3 cm resulted in higher clover content and higher growing point number per unit stolon length. The latter was significantly correlated with total irradiance and red: far red at the canopy base. Potential photosynthesis of clover was not affected by cutting in March. It is concluded that growing point density can be increased by grazing or cutting during winter or spring. However, in order for these new stolons to contribute to clover yield during the summer, they have to be maintained until then by ensuring that competition from grass is minimized by keeping the sward short in winter and spring and avoiding the burial of stolons during grazing.     

White clover contribution to continuously stocked sheep pastures in association with contrasting perennial ryegrasses

White clover content and herbage production in grass/clover swards were studied at contrasting altitudes in Wales. The lowland experiment (1) compared the performance of three clover varieties grown with four diploid perennial ryegrass ( Lolium perenne ) varieties with a range of heading dates. In the upland experiment (2) the productivity and persistence of Aberystwyth S184 (small-leaved) clover was studied in association with three perennial ryegrasses.
In experiment I. clover content and dry matter production with Aurora (very early) ryegrass was superior to that with Frances (early), Talbot (intermediate) and Melle (late). Furthermore, the performance of Kent wild white clover (small-leaved) was better than that of medium-leaved Menna and Grasslands Huia. Averaged over two years (1985 and 1986) mean clover content of Kent was 22% compared with 9% and 10% of Menna and Huia.
Performance of S184 clover during the five years (1986–90) of experiment 2 was better with Meltra (late tetraploid) than with Aberystwyth S23 (late diploid) and intermediate with Aurora. Clover stolon length and growing point number declined to a low level in 1988 but increased dramatically in 1989 and 1990 after cessation of applying fertilizer N.
Results from both sites were related to animal performance data previously published. It is concluded that small-leaved clover varieties are needed to ensure a high proportion of clover in sheep pastures and also that choice of companion ryegrass can have a significant effect on the performance of the legume component.     

Severity of damage to Trifolium repens leaves by certain invertebrate species in mixed perennial ryegrass/white clover swards: response to cultivar, cutting frequency and sward characteristics

The severity of damage by certain invertebrate species to white clover (Trifolium repens) leaves on the main stolons of plants grown in mixed perennial ryegrass/white clover swards was examined in a field experiment in the Netherlands in which two cutting frequencies (high and low) and three white clover cultivars (Retor, Alice and Gwenda) were used. The damage to the leaves was described in terms of the numbers of damaged leaves and the extent of that damage (slight <20%, moderate 20–50% and heavy >50%). The relationships between leaf damage and sward characteristics (white clover content, above-ground biomass and sward height) were evaluated throughout the growing season. Over the whole experimental period, 23·7% and 27·4% of the total number of leaves produced per stolon were damaged by slugs and weevils in the low- and high-frequency cutting treatments respectively. High-frequency cutting increased the number of leaves in the total leaf damage and moderate leaf damage categories by 21·4% and 34·8%, respectively, compared with the low-frequency cutting. The cv. Retor (medium-leaved) experienced the most severe damage by invertebrates. It had much higher leaf damage than cvs Alice (large-leaved) and Gwenda (small-leaved) at either cutting frequency, both in the total number of damaged leaves and in the different damage categories. Differences among cultivars in the number of damaged leaves and relative leaf damage occurred primarily in spring, late summer and autumn, but did not differ during the early- and mid-summer months. This study indicates that variations in leaf damage among clover cultivars were associated with differences in measured sward characteristics. Both the number of damaged leaves and the relative leaf damage were strongly negatively correlated with white clover content and biomass in spring, late summer and autumn under each cutting treatment. White clover content and biomass explained 65%, 59% and 50% of the variation in the number of damaged leaves in spring, late summer and autumn, respectively, and 58%, 57% and 45% of the variation in relative leaf damage in these three periods. Thus, sward characteristics may play a role in regulating the severity of invertebrate damage to clover leaves in addition to the primary effects of HCN.     

Effects of grazing by sheep or cattle on sward structure and subsequent performance of weaned lambs

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.     

The effect of subsequent mangement on the success of introducing white clover to an existing sward

Two experiments, each lasting approximately 12 months, were carried out at North Wyke, Devon, in 1982-83 (A) and 1983-84 (B), to investigate various sward managements following oversowing of white clover (Trifolium repens, cv. Grasslands Huia) at 4 kg ha^-1 with a Hunter Rotary Strip-Seeder in June or July into the stubble of a permanent grass sward following conservation. Experimental managements comprised cutting, grazing with wether sheep or grass suppression by herbicide, as appropriate, in late summer/autumn (Phase I), winter (Phase II) and spring/early summer (Phase III). During Phase I, there was no differential effect on clover stolon development of lenient grazing at approximately 4-weekly intervals or topping at the same frequency to a similar height. Early in Phase II of Experiment A, grazed paddocks became so badly poached that no differences occurred between grazing either to early January or throughout the winter. Under drier conditions in Phase II of Experiment B, continuous grazing at either five (L) or ten (H) sheep ha^-1 had no immediate effect on clover stolon development, but in a silage cut in June, paddocks formerly stocked at the lower rate yielded 40% more DM than those at the higher rate. Experiment A compared the use of a grass-suppressing herbicide, propyzamide, applied at 0.6 kg a.i. ha^-1 in either October or February; in Experiment B it was applied in October. Prophyzamide applied at either time in Experiment A increased the clover content of herbage regrowing after the end of the experimental period from 16% to 36% (s.e.d. ± 3.9). In Experiment B, October application raised the clover contents of herbage cut in June 1984 from 10% (H) and 17% (L) to 32% (s.e.d.±5.9), and stolon lengths per m² at the end of the summer period from 33 (H) and 56 (L) to 86m (s.e.d. ± 11.7). However, the effect of spraying propyzamide on subsequent herbage yields was erratic, and appeared to depend on the incidence of frost after application. In Phase III of Experiment A, continuous grazing was compared with a silage cut in June. At the end of the experiment there were 31 m m^-2 of clover stolon in silaged areas compared with only 2.5 m m^-2 following grazing (s.e.d.±6.6). Clover content and herbage yields were also significantly higher following conservation. In Experiment B in the same period, rotational grazing with a 14- or 35-day recovery interval was compared with a silage cut in June, with or without 100 kg N ha^-1 applied in March. Application of N to the conservation treatment reduced clover stolon length per unit area, and in the regrowth in the post-experimental period the conservation treatment without N had the largest clover content (31% compared with 16-23% for other treatments, s.e.d. ± 3.6)     

Effects of an autumn defoliation on overwintering, spring growth and yield of a white clover/grass sward

An established sward of binary mixtures of meadow fescue (Festuca pratensis) and white clover (Trifolium repens) (either AberHerald, Grasslands Huia or Sandra) was subjected to (A) no further defoliation, (B) a defoliation in late September or (C) a defoliation in late October after four harvests had been taken during the grazing season. About a tonne of dry matter (DM) was removed by the autumn defoliations. There were two levels of nitrogen application in spring, either 0 or 90 kg ha^?1. The development of grass and clover morphology and population sizes from early autumn until the first harvest the following year was followed by regular sampling of the above-ground material. Stolons were analysed for total non-structural carbohydrates (TNCs), and the temperature at stolon level was continuously recorded. There were no interactions between autumn defoliation, clover cultivar or nitrogen treatments on any of the parameters studied. White clover growing-point numbers and stolon morphological characteristics were reduced in size during the winter and did not recover during the spring. A defoliation in late September resulted in the greatest reduction, whereas there were no differences between the other two treatments. The grass tiller population increased from early autumn until the last sampling occasion in May, but both autumn defoliations resulted in a smaller increase. Defoliation in late September had the greatest impact. The TNC content of white clover stolons fell from about 350 g kg^?1 to 150 g kg^?1 DM from late autumn until late April. There were small differences between the treatments, but a defoliation in late September resulted in a significantly lower level in late autumn. The temperature amplitude at stolon level was consistently greater in plots defoliated in late September. Total DM harvested in spring was 4367, 2564 and 3536 kg ha^?1, of which 388, 352 and 460 kg ha^?1 was white clover, from treatments A, B and C respectively. It is concluded that an autumn defoliation may affect the overwintering of white clover negatively, but that the effect on the grass may be even more detrimental.     

The effect of cutting for conservation on a grazed perennial ryegrass-white clover pasture

Continuous stocking with sheep at high stocking rates may reduce the content of white clover (Trifolium repens) in mixed grass-clover swards. The present experiment was carried out to investigate the effects on sward production and composition of resting a perennial ryegrass (Lolium perenne)- white clover sward from grazing and taking a cut for conservation. Swards were set-stocked with 25 and 45 yearling wethers ha^?1 either throughout a grazing season, or on swards that were rested for a 6-week period and then cut in early, mid- or late season. In an additional treatment swards were cut only and not grazed. Net herbage accumulation was higher at the lower of the two stocking rates and was marginally increased by the inclusion of a rest period at the high but not the low stocking rate. Clover content was higher at the lower stocking rate and was increased by the inclusion of a rest period by 30% at 45 sheep ha^?1and by 11% at 25 sheep ha^?1 The effect was most marked at the end of the rest period before cutting. When rested from grazing the tiller density of ryegrass decreased although tiller length increased, and clover stolon length, petiole length and leaflet diameter increased though leaf and node number per unit length of stolon decreased; the reverse applied when the sward was returned to grazing after cutting. At the high stocking rate, rest periods in mid-season or later maintained the greatest clover content and marginally increased total net herbage accumulation. At the low stocking rate the timing of the rest period had no significant effect on total net herbage accumulation or on clover content. These results show that the combination of grazing and cutting is of benefit where the stocking rate is high enough to threaten clover survival and limit sheep performance. However, at such a stocking rate, feed reserves are at a minimum throughout the grazing season and so opportunities for resting the sward are probably low.     

Effect of defoliation on flower initiation in white clover in summer

Flowering behaviour of plants of six cultivars of Trifolium repens (Kent Wild White, Grasslands Huia, S100, Blanca, Olwen and Milkanova) growing in field plots at the Grassland Research Institute, Hurley, was recorded in mid-July. Half of each plot had been defoliated in early May, the other half in mid-June. In all cultivars defoliated in May, inflorescence initiation had stopped even though the natural photoperiod at that time was well above the critical daylength for initiation Defoliation in mid-June had led to a renewal of inflorescence initiation in all cultivars.
The results confirm that the stimulatory effect of defoliation on inflorescence initiation in plants that have stopped initiation in long days, which has previously been observed in Grasslands Huia in controlled environments, also occurs in the field and in other cultivars.     

White clover seed production from mixed swards: effect of sheep grazing on stolon density and on seed yield components of two contrasting white clover varieties

A trial was carried out over two harvest years to assess the effect of sheep grazing and closing date on stolon density and seed yield components of two contrasting white clover varieties and to determine the potential for producing clover seed from mixed swards in an integrated livestock/seed production system.
Overall, the small-leaved cv. S184 had a higher stolon density at closing and harvest than the large-leaved cv. Olwen, but only significantly so in 1987. Stolon density at closing was generally increased by delaying closing. Varieties also differed in their response to closing date. Stolon density of cv. S184 in both harvest years, generally increased with later closing whilst cv. Olwen was less influenced by closing date and at all dates in 1987, and all but one date in 1986 was not significantly different from the ungrazed treatment.
Delay of closing significantly increased stolon density at harvest on all closing dates in 1987 but had no effect in 1986. Varieties responded similarly to a delay in closing but the magnitude of increase of cv. Olwen was less than that of cv. S184.
The seed yield components of both cv. S184 and cv. Olwen were influenced by closing date, but the effect differed between years. Although there was some initial increase in inflorescence production, delaying closing after inflorescence buds appeared on the stolon reduced inflorescence number of cv. Olwen in both years but reduced inflorescence number in cv. S184 in 1986 only. However, inflorescence size, seed yield per inflorescence and the proportion of ripe inflorescences at harvest was reduced in both years. Cultivar S184 produced more inflorescences than cv. Olwen in both years and in both years tolerated later closing than cv. Olwen.
The effect of sheep grazing and closing date is discussed in relation to stolon removal and the suitability of particular leaf types for this system of seed production.     

Effects of white clover cultivar and companion grass on winter survival of seedlings in autumn-sown swards

The aim was to study the effects of white clover cultivar and combinations with perennial ryegrass cultivars on seedling establishment in autumn‐sown swards and on winter survival of seedlings. Large‐leaved white clover cv. Alice and small‐leaved white clover cv. Gwenda, and an erect and a prostrate perennial ryegrass cultivar were sown in autumn in pure stands and as four binary grass‐clover mixtures. Mixtures of white clover cv. Huia and Aberherald with perennial ryegrass were also sown. Companion grasses had no significant impact on the establishment of white clover. The number of seedlings of white clover cv. Alice in mixtures (335 m^?2) was higher than cv. Gwenda (183 m^?2) and pure swards had similar white clover population densities as mixed swards. White clover cv. Huia tended to have more seedlings than Aberherald (355 and 205 m^?2 respectively). No stolons were produced prior to a severe winter, because of the late sowing date. Winter survival of clover seedlings was 0·56 in mixtures and 0·69 in pure stands, irrespective of white clover or companion grass cultivar. Stolon development of white clover in autumn is often considered essential for overwintering survival and spring growth. In this study, there was considerable survival of the non‐stoloniferous tap‐rooted seedlings of all four clover cultivars despite a severe winter.     

Frequency of stocking rate adjustments required on contrasting upland perennial ryegrass pastures continuously grazed to a sward height criterion from May to July

The use of sward height as a criterion for determining the time and extent of stocking-rate changes on continuously grazed swards was investigated over a 2-year period (1985–86) in a sheep production experiment. Swards of three contrasting perennial ryegrass ( Lolium perenne L.) varieties were established with and without Aberystwyth S184 small-leaved white clover ( Trifolium repens L.) at an upland site (310–363 m) in mid-Wales. From spring (late April) until weaning (mid-July) the pastures were continuously stocked with Beulah Speckled Face ewes and Suffolk cross lambs. During this period sward heights of 4 ± 0.5 cm were obtained and maintained by regular adjustment of animal number on individual paddocks. Grass-only swards received 160 and 200 kg N ha⁻¹ and the grass clover swards were given 80 and 75 kg N ha⁻¹ in 1985 and 1986 respectively.
Differences were observed between the treatments in sward height profiles over the season necessitating contrasting adjustments to stocking rates. Mean stocking rate necessary on early flowering Aurora (22 6 ewes ha⁻¹) was respectively 27% and 17% higher than on late-flowering Aberystwyth S23 and Meltra (tetraploid) ryegrasses; mean stocking rate on grass-only swards was 19% higher than on the grass-clover pastures.
It is concluded that sward height is a useful criterion on which to make adjustments to stocking rates to compare the potential performance of contrasting swards, under continuous grazing. The infrequent adjustments required to maintain a constant sward height, especially on the late flowering diploid perennial ryegrass variety on which many upland pastures are based, suggest that the criterion of sward height could be successfully employed on farms as an aid to efficient grazing management.     

The influence of autumn management and companion grass on the development of white clover over winter in mixed swards

Three experiments designed to investigate different facets of autumn management on white clover stolon development are described. The effects of defoliation interval (2, 4, 6 and 8 weeks during 16 weeks from 27 July) were investigated. The shortest interval resulted in the shortest length of stolon material per unit area but cutting interval had no effect on growing point density nor on hardiness of stolon tips evaluated in October, December and January.
Chemical grass suppressants were employed to reduce grass biomass during winter in two experiments to evaluate the influence of grass on white clover development. One experiment involved varying grass tiller density by spraying a perennial ryegrass/white clover sward in October with three rates of three chemical suppressants (Clout, Kerb and Checkmate). Although tiller and clover growing point density were inversely related in January, the overall relationship was not strong.
Clout at l·5kg a.i. ha⁻¹ was sprayed in October on one of two subplots in each of twelve grazed grass/white clover plots that had been maintained at 7 or 9 cm from July to October then grazed to 3–4 cm with sheep. Sward height had no effect on clover population density but the shorter sward had a greater mean node number per secondary stolon branch. By March, suppressing grass resulted in more than double the stolon population density, a higher proportion of plants with tertiary and quaternary branches, and on marked stolons, five times more branches and 60% higher dry matter (DM) produced during winter but with shorter petioles compared with clover in untreated plots.
It is concluded that white clover has the capacity to branch during a mild winter and as stolon branch numbers can suffer a net loss as a result of the presence of the grass canopy, management that controls grass growth during winter should aid over-wintering and improve persistence of white clover.     

Sward composition, animal performance and the potential production of grass/white clover swards continuously stocked with sheep

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.     

Management options for increasing white clover contents of swards, without resowing

Two field experiments were carried out at North Wyke, Devon in 1985 (Experiment A) and 1986 (Experiment B) to investigate the effectiveness of either cutting or rotational sheep-grazing managements for raising the clover content of clover-depleted swards. Subplots were pretreated in March with (a) propyzamide at 0·4kg a.i. ha^-1, (b) chlorpyrifos at 0·72 kg a.i. ha^-1 and methiocarb at 0·22 kg a.i. ha^-1, (c) carbofuran at 1·3 kg a.i. ha^-1, or (d) not so treated, in order to reduce grass tiller density, control insect and mollusk pests, or control all invertebrate pests respectively (a-c), Carbofuran was not applied to swards that were to be grazed subsequently. The propyzamide pretreatment (a) significantly reduced the quantity of herbage dry matter (DM) grazed and the silage DM yields in both years, but raised the numbers of active clover buds, and clover stolon density and its weight in 1986, though not in 1985, The pesticide package (b) raised the quantity of herbage DM grazed in both years, and the silage DM yield in 1986, Carbofuran (c) raised silage yields in 1985. Neither pretreatment (b) nor (c) significantly affected clover performance. In comparison with sheep grazing, cutting showed a trend to higher DM yields, and significantly raised clover stolon density and weight in both years, and active bud numbers in 1986. The periodic sheep grazing management included recovery intervals of 14 d and 35 d. In 1986 (but not 1985) the longer recovery interval raised herbage DM consumption, but had no effect on clover development. The cutting management included nitrogen inputs of either 100 kg ha^-1 in March, or none. N input raised annual DM yields in 1986 (but not 1985) but did not affect clover DM yields or performance in either year. The experiments at North Wyke were supported by on-farm experiments using exclosure cages at ten sites in 1985 and nine in 1986, in Yorkshire, Wales, the Midlands and Devon. In both years, application of propyzamide as in treatment (a) reduced DM yields (P<0.001) and raised the proportion of clover (P<0.001) in May harvests. The density of active clover buds (P<0.05), stolon density (P<0.001) and stolon weight (P<0.001) were increased by October. A combined carbofuran and methiocarb treatment significantly (P<0.001) increased herbage yields, but did not affect measures of clover performance. Unlike the main experiments, a comparison of grazing (outside the cages) and cutting management (within the cages) showed no effect on clover development. It was concluded that cutting, or rotational sheep grazing with a long recovery interval, would promote clover development in the clover depleted sward. Though successful in the overall assessment, application of propyzamide gave highly variable results on different sites and was not sufficiently reliable.     

Effects of continuous sheep stocking and strategic rest periods on the sward characteristics of binary perennial grass/white clover associations

Five binary perennial grass/white clover (Trifolium repens, cv. Menna) mixtures were evaluated over a 3-year period under continuous sheep stocking together with the imposition of a rest period for either an early or a late conservation cut; the experiment with plot sizes of 0·16 ha was replicated three times. The grass species and cultivars used were Merlinda tetraploid and Magella diploid perennial ryegrass (Lolium perenne), Prairial cocksfoot (Dactylis glomerata), Rossa meadow fescue (Festuca pratensis) and Goliath timothy (Phleum pratense). The greatest total lengths of white clover stolon developed in the meadow fescue (171·6 m m^?2) and timothy (151·9 m m^?2) associations compared with those in tetraploid perennial ryegrass (98·6 m m^?2), diploid perennial ryegrass (91·9 m m^?2) and cocksfoot (74·6 m m^?2) (s.e.d. 16·4, P < 0·001). On average, the proportion of white clover stolon that was buried was between 0·86 and 0·89 and this was more abundant in late than early season. Whereas timothy persisted, the persistence of meadow fescue was low under any of the managements tested and this was markedly reduced by the third grazing season. In the diploid perennial ryegrass sward, a late June to early August rest period for conservation enhanced white clover stolon length. An early April to late May rest period greatly reduced total white clover stolon length in both diploid perennial ryegrass and tetraploid perennial ryegrass associations (diploid perennial ryegrass-unrested 89 m m^?2, early rest 56·1 m m^?2, late rest 130·7 m m^?2; tetraploid perennial ryegrass - unrested 125·1 m m^?2, early rest 71 m m^?2, late rest 99·7 m m^?2; s.e.d. 19·19, P < 0·001). The numbers of white clover stolon growing points per unit stolon length were greatest when the sward was rested during late June to early August ?55·9 m^?1 stolon length compared with 45·7 m^?1 for an April to late May rest and 46 m^?1 in the absence of a rest (s.e.d. 2·59, P < 0·001). Likewise, the percentage of stolon above ground was greatest with the late June to early August rest ?15·78% compared with 10·61% for the April to late May rest and 7·69% for no rest (s.e.d. 1·569, P < 0·001). The complementary percentages of buried stolon indicate the important role this fraction has and the need to study stolon behaviour in grazing studies generally. It is concluded that, in relation to perennial ryegrass as a companion grass, meadow fescue and timothy allow better white clover development and cocksfoot less. However, other attributes have to be considered, for example the poor persistence of meadow fescue and the slower regrowth of timothy, both of which allow the invasion of weed grasses, or the lower acceptability of cocksfoot to livestock. The timing of the rest period before the conservation cut can influence white clover development considerably, but the effects differed with different companion grasses.     

Animal production evaluation of herbage varieties.

Perennial ryegrass/white clover (Lolium perennel Trifolium repens) pastures of three white clover varieties were compared at UK lowland (Plas Gogerddan, PG) and upland (Bronydd Mawr, BM) sites over three harvest years (1989–91) under continuous variable stocking (ewes and lambs until weaning and lambs thereafter). Mean annual lamb output from small-leaved S 184 (1179 kg ha^?1) was 29% greater than that from prostrate small-leaved AberEndura at PG with medium-leaved Huia also giving 19% more output than AberEndura. At BM, output from S 184 (863 kg ha^?1) was 19 and 14% greater than that from AberEndura and Huia respectively. The differences in lamb output between the small-leaved varieties owed to a combined effect of higher individual lamb growth rates and greater stock-carrying capacity, both of which were more pronounced in the post-weaning period. The higher output from S 184 relative to Huia at BM was attributed to higher stocking rate, particularly after weaning. Clover productivity and persistence were also studied under three cutting-only managements at PG. Performance under a treatment cut at 2–3 cm every 10 d (T1), which is synonymous with assessment of persistence in UK National List testing, was poor with clover growing point number of only 498 m^?2 compared with 4906 m^?2 on the grazed sward in autumn 1991. Although productivity and clover content under T3 (cut at 3–4 cm every 42 d — similar to National List yield regime) gave the same varietal ranking as lamb production, there was an under-estimation of the small-leaved varieties, particularly AberEndura, relative to Huia. An intermediate treatment (T2), cut at 3–4 cm every 21 d, also over-estimated the performance of the medium-leaved variety. Herbage data from ground level sampling every 21 d using exclosure cages on the grazed swards were also poorly related to lamb performance. These results highlight the effect of clover variety on lamb production, which appeared to be independent of leaf size, and also confirm the existing problems associated with assessment of white clover varieties using a cutting regime.     

Effects of cultivar and cutting frequency on dynamics of stolon growth and leaf appearance in white clover grown in mixed swards

The hypothesis that dynamics of growth, branching of stolons and appearance of leaves are important for the persistence of white clover ( Trifolium repens ) in mixed swards was tested. The effect of cutting frequency and white clover cultivar on stolon and leaf dynamics was studied throughout the growing season in a field experiment. Mixtures of perennial ryegrass ( Lolium perenne ) and white clover cultivars with different leaf sizes, cvs. Alice, Gwenda and Retor, were evaluated for white clover persistence at two cutting frequencies. Stolon dynamics, stolon survival, leaf and node appearance rate, branching, flowering and stolon elongation rate were analysed and related to white clover content and yield.
There were clear seasonal fluctuations in stolon and leaf characteristics. Stolon elongation rate and appearance rates of nodes, leaves and branches declined in autumn. Temperature and irradiation explained a major part of the variation of stolon elongation rate and leaf appearance rate and 25% of the variation in stolon branching rate.
Significant and consistent differences in yield and botanical composition were found between mixtures. Mixtures with cv. Alice had the highest total and white clover yield and the highest white clover content, whereas mixtures with cv. Retor had the lowest yield and the lowest white clover content. Stolon elongation rate and stolon internode length were significantly higher in cv. Alice than in the other white clover cultivars, whereas cv. Retor had a significantly higher percentage of leaves with damage caused by insects and slugs than the other cultivars. Little support was found for the hypothesis that branching characteristics are a key component of competitive success in mixed pasture, as most measured branching characteristics were not associated with differences in white clover yield between cultivars and cutting regimes.     

The effect of stolon burial and defoliation early in the growing season on white clover performance

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.