Spring defoliation of white clover seed crops. 2. Potential harvestable seed yield and seed yield components of contrasting white clover cultivars

The effects of different spring defoliation managements on potential harvestable seed yield and seed yield components of three contrasting white clover cultivars were assessed. The small-leaved cv. S184 produced more but smaller inflorescences than the large-leaved cv. Olwen and Menna, a medium-leaved cultivar. Cultivar Olwen, however, produced more ripe and brown (nearly ripe) inflorescences with more florets, seeds per floret and a higher seed yield per ten inflorescences than the other cultivars. Potential harvestable seed yield and individual seed yield components were only influenced by defoliation after bud emergence, as defoliation before bud emergence had no effect on seed yield components. Defoliation after bud emergence had a similar effect on all cultivars: the number of ripe inflorescences was unaffected by defoliation but the number of brown and therefore harvestable (ripe + brown) inflorescences was highest following defoliation three weeks after bud emergence. Florets per inflorescence, seed per floret, 1000 seed weight, seed yield per ten inflorescences and potential harvestable seed yield were not influenced by defoliation after bud emergence. Season had a significant effect on seed yield components and influenced the effect of defoliation treatments, emphasizing the importance of climate in white clover seed production. The results are discussed in relation to the spring defoliation of white clover seed crops, harvesting techniques and the provision of guidelines for optimizing seed yield.     

Effect of seed crop management on potential seed yield of contrasting white clover varieties

An experiment is described in which the effects of different spring managements on the potential seed yield and seed yield components of three white clover cultivars of contrasting leaf types were assessed. Cv. S184, the small-leaved variety, produced more but smaller inflorescences than CVS Olwen, a large-leaved variety, or Menna, a medium-leaved variety. However, cv. Olwen, produced inflorescences with more florets, seeds per inflorescence and a higher seed yield per ten inflorescences than the other cultivars. Both potential seed yield and the individual yield components were influenced by management. Cv. Olwen produced more inflorescences and a higher potential seed yield under a cutting system than under grazing systems, which reduced the number and size of the inflorescences. Cvs S184 and Menna were less influenced by management system and performed similarly under cutting and grazing. The highly significant relationship between the number of ripe inflorescences and potential seed yield showed clearly that high seed yields are only achieved if the crop is harvested when the number of ripe inflorescences is at a maximum. The relatively short duration of the period of maximum ripe inflorescences emphasized the importance of determining the correct harvesting date, although weather conditions also play an important part in deciding when to harvest. Florets per inflorescence, seed set and 1000 seed weight remained relatively constant over the harvest period, and were not influenced by harvest date. The results are discussed in relation to the management of white clover seed crops and the importance of climatic conditions during seed production.     

Spring defoliation of white clover seed crops. 1. Inflorescence production of contrasting white clover cultivars

Three white clover cultivars, S184 (small-leaved), Menna (medium-leaved) and Olwen (large-leaved), were sown at a seed rate of 3 kg ha^-1 under spring wheat cv. Tonic. In the spring of the two following years, there were three pre-bud emergence mechanical defoliation treatments on which were superimposed four post-bud emergence treatments giving a total of twelve cutting treatments. Pre-bud emergence, plots were either cut twice (at approximately two weeks before bud emergence and at bud emergence), cut once at bud emergence or not cut. To each treatment were applied four post-bud emergence treatments: plots were not cut or cut once (one, two or three weeks after bud emergence). Counts of the total number of inflorescences and of the proportions in various ripeness categories were made throughout the period of seed crop development to determine the pattern of inflorescence development and optimum harvest date. Overall, inflorescence numbers were greatest in cv. S184and least in cv. Olwen. Defoliation before bud emergence had no effect on inflorescence production; however, it was significantly influenced by defoliation after bud emergence. Although delaying the initial development of the crop, inflorescence numbers of all cultivars were highest in both years following cuts two and three weeks after bud emergence. Optimum harvest date was not affected by defoliation or cultivar, numbers of ripe inflorescences in both years reaching a peak at the end of August. The number of brown inflorescences, which may also contribute to seed yield, reached a peak in both years in late July. Seasonal differences in inflorescence production were again observed, emphasizing the difference between first and second year crops and importance of climate in while clover seed production. The implications of these differences in numbers and proportions of inflorescences in various ripeness categories under different defoliation regimes are discussed in relation to seed crop management in the UK.     

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.     

The effect of row spacing and cover-crop on stolon development and the seed yield components of white clover cultivars of contrasting leaf size

Three white clover cultivars, S184 (small-leaved), Menna (medium-leaved) and Olwen (large-leaved), were broadcast or sown in 15, 30 or 60 cm drills at a seed rate of 3 kg ha^-1. The three cultivars were either sown without a cover-crop or sown under a cover-crop of spring barley (cv. Crescent) or peas (cv. Countess). The effects of these methods of establishment on the stolon growth and components of seed yield were subsequently measured.
Stolon growth and development was influenced by row spacing, cultivar and season. The overall plant response at all but the widest row spacing (60 cm) was to increase stolon growth such that inflorescence production, the number and proportion of ripe inflorescences and the other seed yield components were unaffected by row spacing. Cultivars differed in their response to row spacing. Cultivar Olwen produced most inflorescences and more ripe inflorescences when broadcast, cv. S184 when sown at 60 cm row spacing and cv. Menna at 15 or 30 cm row spacing. Cultivars also differed in their response to cover-crop, with cvs Menna and S184 producing more inflorescences and more ripe inflorescences when sown under barley and peas than when pure sown. The inflorescence production of cv. Olwen was not influenced by cover-crop.
The relationship between vegetative and reproductive growth is discussed in relation to establishment, cultivar and climate and the possible implications for the establishment of white clover seed crops in the UK.     

Potential seed yield of white clover varieties

Fifteen varieties of white clover were sown in order to assess the seed yielding ability of new and potential varieties and to examine those plant factors which affect seed yield. Counts were made on number of inflorescences m^-2, proportion of ripe inflorescences and other inflorescence and seed characters. The data were used to compute potential seed yields.
Average potential seed yields were 276 and 76 kg ha^-1 in the first and second year, respectively. The decrease in the second year seed yields underlines the overriding effect of adverse weather conditions during the critical June to August period. The major effect was a reduction in the number of inflorescences produced, the number of florets per inflorescence and seeds per floret.
Some of the new listed varieties have a significantly higher seed potential than SI00, e.g. Menna (+ 38%) and Olwen (+28%), and this should facilitate the production of adequate seed supplies of British bred varieties.     

Seed production in white clover (Trifolium repens L.). II. Effect of autumn defoliation on potential seed yield and seed yield components

A field experiment was conducted to assess the effect of autumn defoliation treatments on inflorescence production, potential seed yield and yield components of white clover cv Makibashiro. Between 10 July and 10 October 1992, white clover swards were subjected to one of three treatments: monthly cutting to 3–4 cm (4DEF), cutting to 3–4 cm on 10 August and 10 October (2DEF), and no cutting (control). The total numbers of inflorescences and the proportion of inflorescences in different development categories were counted throughout the period of seed crop development to determine the pattern of inflorescence development and optimum harvest date. There were consistent significant differences in inflorescence density between treatments. Plots which received the 2DEF treatment produced significantly more inflorescences than did the no-defoliation (control) and 4DEF-treatment plots. In this particular year the optimum harvest date (date at which the proportion of ripe inflorescences and potential seed yield was highest) was 26 July, approximately 30 d after peak flowering. Defoliation treatments had no effect on optimum harvest date. However, treatments differed in potential seed yield and ripe inflorescences on this date. The 2DEF treatment gave the highest potential seed yield because there were significantly more ripe inflorescences than either the contol or 4DEF plots. Control plots produced inflorescences with more florets than the other two defoliation treatments, but the differences were not always significant. Seed number per pod was higher in inflorescences obtained from previously defoliated plots than from control plots. The 1000-seed weight was significantly lower in inflorescences developed in 4DEF plots than those developed in 2DEF and control plots. The results are discussed in relation to the management of white clover seed crops and the importance of canopy structure and light intensity for seed production.     

Effect of plant density on stolon growth and development of contrasting white clover (Trifolium repens) varieties and its influence on the components of seed yield

Plants of two contrasting white clover varieties (cv. Aberystwyth S184 and Olwen) were planted in the field in spring in each of 2 years at four densities (9, 25, 49 and 100 plants m⁻²) in 1-m² plots. The effect of plant density on stolon growth and development and the components of seed yield was subsequently measured.
Stolon growth and development was influenced by plant density, variety and year. At low plant densities both white clover varieties produced longer primary stolons than at higher densities. Plant density, however, had no significant effect on the number of inflorescences at harvest. At the high stolon densities there were significantly fewer reproductive nodes per primary stolon than at the lower plant densities. Plant density did not significantly affect any other seed yield components, but the number of inflorescences at harvest, florets per inflorescence, seed set per floret and 1000-seed weight were all significantly influenced by both variety and year.
The relationship between the vegetative and reproductive growth of white clover is discussed in relation to plant density, variety and climate and the possible role of defoliation managements on inflorescence development.     

The effect of blending white clover varieties and their contribution to a mixed grass/clover sward under continuous sheep stocking

The effect of blending small- and medium-leaved white clovers together in a mixture of varieties was examined under continuous sheep stocking over a period of three years. Four varieties were used, S184 and Gwenda, small-leaved varieties suitable for intensive sheep grazing, together with medium leaved varieties Menna and Donna, which are mainly used in general purpose seed mixtures for medium term leys. The small-leaved varieties were blended with Menna or Donna, sown with a commercial grass mixture and managed as near as possible to farm practice. Although the leaf size of Gwenda is only slightly greater than that of S184, blends based on these two small-leaved varieties behaved differently. In spring of the first harvest year the yield of Gwenda and of the clover in mixtures containing Gwenda was 27% greater than the yield of the same mixtures which contained S184. As the season progressed this difference decreased. In terms of total annual yields and saving in fertilizer N, the benefits of including white clover in a seed mixture were more pronounced when Menna was mixed with S184 and Gwenda, rather than when Donna was used, although both varieties are in the medium-leaf category. It was concluded that the slower establishment and the lower clover yield of S184, when compared with larger leaved varieties, can be overcome by blending with a variety that is slightly larger in leaf size, such as Menna, although the choice of variety may depend on sward management and its persistency under grazing.     

Forage yield and persistency of Trifolium repens × Trifolium nigrescens hybrids under rotational sheep grazing

Introgression of reproductive traits from the annual, profuse flowering, ball clover (Trifolium nigrescens Viv.) into white clover (Trifolium repens L.) is one breeding strategy to improve seed yields of T. repens that must be achieved without sacrificing agronomic performance and persistency under grazing. The yield and persistency of hybrids between white clover and T. nigrescens were compared under rotational sheep grazing over three harvest years. The hybrids included the backcross (BC) 2 and 3 generations produced using white clover as the recurrent parent. The large‐leaved T. repens variety Olwen, medium‐leaved varieties AberDai and Menna and the small‐leaved variety S184 were sown as controls. Hybrids and control varieties were sown with a perennial ryegrass companion; between April and the end of October in each harvest year the plots were rotationally grazed with sheep with clover and perennial ryegrass (DM) yield and the proportion of clover present measured over the growing season. The clover and total DM yields of the BC2 and BC3 were generally comparable with the small‐ and medium‐leaved varieties within the experiment and significantly greater than the yields of the large‐leaved variety Olwen. Throughout the 3 years of the experiment the BC2 maintained a clover content above 0·30 and comparable with the small‐leaved varieties, while the clover content of the BC3 was comparable with the small‐ and medium‐leaved varieties in the first and third harvest years. No significant difference in perennial ryegrass production was observed when grown with the backcrosses or the control varieties. Differences in stolon and growing‐point density were observed at the end of the experiment with the density of the BC2 and BC3 less than the small‐leaved variety S184 but, in common with the medium‐leaved varieties, greater than the large‐leaved variety Olwen. The implication of these results for the use of this material in future experiments and in the white clover breeding programme is discussed.     

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.     

The effect of time of harvest on seed production of three red clover cultivars

Small plots of red clover cv. Sabtoron, S123 (diploids) and Hungaropoly (tetraploid) were harvested for seed production at two-week intervals from 19 August to 17 October inclusive in 1981, inflorescence appearance rate, bee density and components of yield having been monitored throughout the summer. Inflorescence appearance rate reached a maximum at the end of July/early August for Hungaropoly and Sabtoron and during mid-August for S123. Bee density followed a similar pattern. Florets and seeds per inflorescence and 1000-seed weight decreased as flowering progressed except during the first three weeks in July. Potential seed yield was calculated from the components of yield for the harvests taken on 3 and 18 September. Losses in seed yield (difference between actual and potential) up to and during harvesting, threshing and cleaning were lower in the tetraploid cultivar (27–39%) than the diploid cultivars (35–l55%). Each cultivar had an optimum harvest time before which yield was affected by immature seeds and beyond which it was adversely affected by shedding of inflorescences and seeds and sprouting of seeds on the inflorescences. The optimum time to harvest Sabtoron was early September, Hungaropoly early to mid-September and the late flowering cultivar S123 mid-September when less than 4% of the inflorescences were still unripe. Seed yield and inflorescences per unit area were lower in the tetraploid cv, Hungaropoly (maximum 542 kg ha⁻¹) than diploid cv. Sabtoron and S123 (864 and 897 kg ha⁻¹ respectively) although the tetraptoid had heavier seeds. It is concluded that the optimum time to harvest red clover for seed production is about three or four weeks after the end of the period of rapid inflorescence production and that this coincides with the time when only a small proportion of unripe inflorescences remain.     

Peduncle characteristics, inflorescence survival and reproductive growth of white clover (Trifolium repens L.)

Field experiments were carried out in four harvest years to compare the peduncle characteristics and reproductive growth of the small-leaved white clover ( Trifolium repens L.) cv. AberDale, selected for long and strong peduncles, with other varieties bred in the Institute of Grassland and Environmental Research (IGER). The peduncles of cv. AberDale were of larger diameter and had a greater fresh and dry weight than those of other varieties. There were also differences in structure as the peduncles of cv. AberDale had more pronounced ridges and furrows than those of cv, Menna.
Inflorescences of cv. AberDale and other varieties were tagged when in full bloom throughout the flowering period in each of the four harvest years. Generally, more inflorescences of cv. AberDale remained intact until harvest, particularly from the early tagging dates. Although there were seasonal differences, at harvest in each of the 4 years, cv. AberDale had more inflorescences and seeds per floret and a greater potential seed yield than the other varieties.
The basis of peduncle strength and effect on reproductive growth is discussed, and the implications for improving the seed yield potential of white clover varieties is considered.     

Evaluation of seed production potential in white clover (Trifolium repens L. varietal improvement programmes

The components of actual and potential seed yield were examined in field experiments on a wide range of varieties of white clover (Trifolium repens L.). The factors affecting seed yield under conditions typical of management regimes used in the production of commercial seed crops were assessed in two experiments. In the first, carried out on spaced plants, considerable diferences are shown between six varieties across the range of leaf sizes in the distribution and profuseness of inflorescence production through the flowering season. In the second, carried out in plots, typical on-farm criteria were used to choose a single harvest date. Significant variation was found between varieties, including representatives of different leaf size categories, for seed yield components that include number of inflorescences m^?2, number of florets per inflorescence and harvestable seed weight. Large-leaved varieties tended to produce more seeds per floret and higher seed weights per inflorescence, whereas small-leaved varieties gave the highest number of inflorescences ^?2. However, the small-leaved variety AderDale, selected for strong peduncles, was exceptional, giving high values for all seed yield components. The impact of weather conditions on many seed yield components (e.g. total number of inflorescences) was demonstrated by the differences between the 2 years of the experiment. However, other characterstics, e.g. number of florets per inflorescence and number of seeds per floret, did not vary between years. Deviations from potential seed yield were assessed fromthe perspective of commerical seed production. The implications of these results for the production of white clover varieties with increased seed yields under UK conditions are discussed.     

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.     

Effect of three clover varieties on growth, 15N uptake and fixation by ryegrass/white clover mixtures at three sites in Wales

Grass and clover production and nitrogen cycling were compared in 1983 and 1984 at three sites: an upland peaty gley and upland and lowland brown earths. The clover varieties Olwen and S184 were compared in 1983 and S100 and S184 in 1984. Ammonium and nitrate sources of ¹⁵N were used to measure nitrogen recovery from fertilizer and soil, nitrogen fixation and nitrogen transfer from clover to grass. Acetylene reduction was measured once, in 1983, but isotope dilution was used in both years.
Olwen clover produced more dry matter and took up more ¹⁵N than S184. Olwen fixed more nitrogen than S184 over the whole season, as measured by ¹⁵N isotope dilution. Companion grass took up more soil nitrogen when growing with S184 than with Olwen. The clover variety S100, tested at the lowland site in 1984, caused no significant variations in dry matter accumulation or N fixation.
In the dry 1984 season, grass dry matter accumulation and ¹⁵N uptake were less than in 1983, and plants actually lost total nitrogen from their roots to the soil. Fixation rate varied more in 1984 than in 1983. Nitrogen transfer from clover to grass was detected by isotope ratio differences in 1983, and by total N differences in 1984.
Site differences were dominated by the greater dry matter accumulation of Olwen in the lowland in 1983, but in that year there was also increased dry matter accumulation and increased ¹⁵N fertilizer uptake, but less nitrogen fixed on the mineral upland site than on the peat soil.
Whether ¹⁵N was given as ammonium or nitrate made little difference in these experiments.     

Comparison of white clover varieties under cutting and grazing

Seven varieties of white clover ( Trifolium repens L.), of varying leaf size from large to small, were sown with perennial ryegrass ( Lolium perenne L.) in 1985, at an upland site, and compared over three harvest years under a cutting management (5-6 cuts), and under continuous grazing by sheep. Fertilizer N input totalled 80 kg ha⁻¹ in both the cutting and the grazing trial. The same varieties were included in two official National List (NL) trials sown the same year at a nearby site; these comprised a yield trial with 6-7 cuts and a persistency trial mown very frequently (17 cuts) to simulate intensive grazing.
At the upland site the large-leaved varieties, Milkanova and Blanca, were the highest yielding under cutting (mean 3·3 t DM ha⁻¹), and the small-leaved S184 and Kent the lowest yielding (mean 2·3 t DM ha⁻¹). The rankings were reversed under grazing (corresponding yields 1·1 and 2·1 t DM ha⁻¹). The medium-leaved varieties Donna, Menna and Grasslands Huia behaved similarly to the large-leaved varieties. Ranking order in the cutting trial was similar to that in the NL trial.
Ground covers after three harvest years differed significantly only under grazing, when the small-leaved varieties had a 2-3 times greater cover than the larger-leaved varieties. A poor separation of the varieties in the NL persistency trial suggests that defoliation was not sufficiently severe to simulate intensive grazing.
These comparisons indicate that the performance of clover varieties under the cutting regimes used should not be extrapolated to continuous sheep grazing.     

Evaluation of white clover varieties under grazing and their role in farm systems

Four experiments were established in 1981–84 to investigate the effect of defoliation treatments on white clover varieties when grown with S23 perennial ryegrass. Treatments included a cutting only regime, as used in National List trials, and grazing systems simulating as near as possible those used on farms.
Differential effects of cutting and continuous sheep stocking on white clover varieties, together with significant variety × defoliation interactions, illustrated the importance of the grazing animal in the evaluation of white clover. Differential effects of cattle and sheep grazing were also evident. Cattle grazing was less detrimental to white clover than was sheep grazing, i.e. cattle were not selective, and less stolon was removed. In general, with both cattle and sheep grazing the larger the clover leaf size the greater the loss in stolons, which in turn decreased persistency. The results illustrate how alternating cutting, sheep grazing and cattle grazing managements can be used to maintain optimum clover/grass balance.
Successful clover/grass swards depend on the retention of clover, yet avoiding clover dominance. The difference in N transfer between clover varieties, especially those within the same leaf category, and the extra grass produced without fertilizer N, emphasized the importance of varietal choice. Breeding programmes have been concerned with the selection of larger-leaved, long-petioled varieties for growing in competition with grass in the presence of fertilizer N. However, the present results showed that, under continuous sheep stocking, increase in leaf size does not increase clover yield or persistency.
The results presented emphasize the importance of the grazing animal in the evaluation of white clover varieties and indicate that yield of clover dry matter should not be the major criterion for selection of varieties for farm systems.     

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.     

Breeding and evaluation of new white clover varieties for persistency and higher yields under grazing

Two experiments were established in 1989 and 1990 to evaluate the performance of white clover ( Trifolium repens ) varieties under grazing, using sheep and cattle, and also under a 'cutting-only' regime. The three potential medium/large-leaved varieties (AberVantage, AberCrown and AberDai) were selected and evaluated in a breeding programme that emphasized improving yield and persistency under grazing.
In Experiment 1, the potential varieties were sown with two different grass companions, and evaluated under a continuous cattle-grazing system, representative of their future role in actual farm practice, i.e. a set stocking system. Clover yield of the potential varieties in the second harvest year averaged 1–5 t ha⁻¹ more than the control variety Olwen. Total sward production of AberVantage, AberCrown and AberDai was 2.0–1.4 t ha⁻¹ higher than that of Olwen.
In Experiment 2, clover yields of the three potential varieties averaged 2.1 t more than Olwen in the second harvest year under continuous sheep grazing, but all three were lower yielding than Olwen in the cutting regime.
Both experiments illustrate how the use of grazing treatments during breeding and evaluation can improve the role of white clover varieties.