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.     

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.     

Growth and colonization of a range of white clover cultivars strip-seeded into an upland perennial ryegrass sward

Seven cultivars of white clover (Trifolium repens L.) (Kent, S184, Huia, Menna, Donna, Alice and Nesta) and a commercial mixture, ‘Ensign’, were strip-seeded into an upland perennial rye-grass (Lolium perenne L.) sward in late June 1986. Swards were first grazed by sheep, either on 5/6 August (early) or on 19/20 August (late) and then every 14–21 days (frequently) or 28–42 days (infrequently) during 1986, followed by a common grazing regime in 1987. During April to mid-June 1988 the swards received either a moderate amount of nitrogen or none and were cut frequently or once only in mid-June. Growth of individual seedlings was assessed before and after grazing during 1986 and stolon accumulation and distribution and sward colonization were assessed during 1987 and 1988. All cultivars emerged rapidly and satisfactorily and there were no consistent significant differences in the overall dry matter accumulation per seedling during establishment. During the first autumn the proportion of the aboveground biomass removed during grazing was smallest in Kent (c. 20%) and largest in Nesta (c. 40%). Kent and S184 produced most leaves and stolons and the greatest length of stolons per seedling and per individual stolon, and Nesta and Alice the fewest leaves and stolons and shortest stolons. Seedlings grazed early had heavier and longer stolons than those grazed late; those grazed frequently had more leaves, stolons and growing points than those grazed infrequently, especially following early grazing. During 1987 Kent and S184 had consistently the largest number of stolon growing points, and weight and length of stolons per unit area; these two cultivars and Nesta also colonized the sward more rapidly than the other cultivars. All cultivars contributed substantially and similarly to herbage production in late September. There were no residual effects of the 1986 treatments after the summer of 1987. During 1988 additions of nitrogen fertilizer at 100 kg N ha^-1 or allowing the herbage to remain undefoliated between mid-April and mid-June both independently halved the number of stolon growing points per unit area; together they reduced it by 80%. Nitrogen also, on average, halved stolon weights but less so in Nesta, Alice and Huia and more so in all other cultivars. Infrequent defoliation greatly decreased stolon weights in Kent and S184 but had no significant effects on the other cultivars. Sward colonization was almost complete by June and entirely so by October for all cultivars in all treatments. Implications of the results for the after-management of strip-seeded white clover are discussed.     

Competitive effects between ten cultivars of perennial ryegrass and three cultivars of white clover grown in association

In an experiment of four years duration, the competitive relationships between three cultivars of white clover ( Trifolium repens L.) and ten cultivars of perennial ryegrass ( Lolium perenne L.) were studied under a N fertilizer regime of 200–240 kg ha^-1 a^-1. The clover cultivars were selected to embrace the various leaf sizes from medium large to small and the ryegrasses included early, mid-season and late cultivars with a known range of persistence.
During the experiment there was a progressive decline in the contribution of clover, though the different characteristics of each of the clover and ryegrass cultivars produced substantial deviations from the average trend. The clover cultivar Kersey was significantly more aggressive towards the companion grass cultivars than either S100 or S184. It produced consistently greater clover contribution to total yield than the other two cultivars and significantly depressed the yields of some of the companion grass cultivars. There was evidence that compatibility of the ryegrass cultivars with clover was inversely related to persistence; the non-persistent ryegrass cultivars S321 and Presto consistently produced lower grass yields than the more persistent cultivars and consequently permitted greater clover contribution. In the second and third years yield substitution effects between clover and grass components substantially reduced differences in the total grass-clover yield.
The interactions revealed in the experiment showed that both ryegrass and clover cultivars have the potential to influence each other when in association although, with minor exceptions, total annual yields were similar for all grass and clover mixtures at the moderately high level of N applied.     

Interactions between seedlings of perennial ryegrass and white clover cultivars in establishing swards

Interactions between perennial ryegrass (grass) and white clover (clover) cultivars were investigated at the seedling stage in two experiments: (a) a field experiment in which two clovers, AberHerald and Grasslands Huia, were grown in binary mixture with two grasses, Preference and Ba 10761; (b) a glasshouse experiment in which the same clover/grass combinations were grown in low-N soil either with (+ N) or without (-N) added N. In the field experiment both clovers produced larger and more complex seedlings with Preference, and this was particularly evident in Huia. In the glasshouse experiment grass dry-matter yield was greater in the +N treatment, and this effect increased with time. Clover seedling density and development were suppressed in the +N treatment, and the development of AberHerald was affected more than Huia. Morphological measurements of the clovers showed interactions between clover, grass and N level. In the -N treatment Huia plants were larger and more complex than those of AberHerald, but in +N conditions there was little difference between them. Grass cultivar had an effect on clover via N-level interactions: in +N plants there was no grass effect, but -N plants were significantly larger with Preference. Comparison of the root and shoot morphology of the two grasses revealed no obvious differences that would account for these effects.     

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.     

The use of repeated-measurements analysis to describe the overwintering of white clover cultivars

An experiment was set up to investigate the morphological and physiological changes occurring in two white clover (Trifolium repens) (clover) varieties, AberHerald and Huia, growing in binary mixture with two perennial ryegrass (Lolium perenne) (grass) varieties, Preference and Ba 10761. Measurements were made on four occasions during winter/spring 1993–94. The statistical procedure of antedependence analysis was used to check for the existence of serial correlations among data from the different sampling dates. Evidence of antedependence was found in only three of the seventeen variables measured, and an order of antedependence for each of these was estimated. The antedependence structure providing the best fit to the data was then used in an analysis of covariance on the data set for that variable. The rest of the seventeen variables could be safely analysed by a split-plot analysis of variance for each sampling date separately. This result validated the sampling protocol used in the experiment.     

Variation in HCN potential among cultivars of white clover (Trifolium repens)

The hydrogen cyanide potential (HCNp) of 17 cultivars of white clover were compared. With inadequate phosphorus supply, HCNp ranged from 24 mg kg DM⁻¹ for cv Lucky to approximately 640 mg kg DM⁻¹ for cv Huia and from 6 to 322 mg kg DM⁻¹ for the same cultivars, with adequate phosphorus. The four North American cultivars examined had notably lower HCNp than all European or Australasian cultivars except cv Clarence.     

The effects of nitrogen fertilizer applied in spring on swards of ryegrass sown with four cultivars of white clover

Nitrogen fertilizer was applied at 0, 30,60 and 90 kg N ha^-1 in March 1976, 1977 and 1978 to plots containing perennial ryegrass cv. Barlenna sown either alone or with white clover cv. Blanca, Sabeda, Olwen and S100 in four replicated blocks.
Dry matter (DM) yield of all swards responded positively to N with the response being highest in the no-clover swards in two of the three years. Annual DM yields were lower in the no-clover than in the mixed swards at any given N level. The higher levels of N reduced DM yield in mixed swards at some harvests in midsummer.
Swards of S100 were consistently among the lower yielding mixed swards, whereas the other three clover cultivars varied in their relative yields. There were no N x cultivar interactions for white clover yields; irrespective of cultivar, N application reduced annual white clover yield by similar amounts.
It is concluded that it is beneficial to apply moderate amounts of N fertilizer to mixed swards in spring and that under these conditions the yield differences between swards including medium-large and smaller leaved white clover cultivars are similar to differences between the inherent yield potential of these cultivars in mixed swards receiving no N.     

Productivity of red clover grown alone and with companion grasses; further studies

Three diploid—Sabtoron, Violetta and Essex—and three tetraploid—Hungaropoly, Teroba and Red Head—red clover cultivars were sown alone and with each of three companion grasses—timothy (S48), tall fescue (S170) and perennial ryegrass (S24). The productivity and persistence of the red clover cultivars were compared. Results obtained over the first four harvest years were presented in an earlier paper (McBratney, 1981) and this paper presents results from the fifth and sixth (final) harvest years.
The low yield obtained in the fifth and sixth year from Essex under each seeding treatment indicated a deterioration in its persistency or vigour or both. In the fifth year the remaining two diploids, Sabtoron and Violetta, were as highly productive and persistent as the three tetraploids but in the sixth year the decrease in productivity was greater in the diploids than in the tetraploids. In the fifth year Teroba, the highest yielder, produced 9 8 t total DM ha⁻¹ over all seeding treatments of which 73% was clover DM; in the sixth year Teroba was again highest yielding, producing 9.3 t ha^−l with 67% clover content. Tall fescue was the most consistent companion grass in the mixed swards, in the sixth year contributing 53% grass DM to the total 8 8 t DM harvested ha⁻¹.
The trial demonstrates the potential of a number of red clover cultivars grown either pure or with a suitable perennial grass to achieve and maintain a high output of herbage of good quality over a period of 6 years without any application of fertilizer N. The practical agricultural implications of using red clover are discussed.     

Improvement in soil structure associated with white clover

Observations have been made which suggest that clover is more effective than ryegrass in developing rapid improvements in soil structure. To test this hypothesis, grass, grass/clover and clover swards were grown in undisturbed field cores. Significant differences in drainage rates and plant biomass production, and visible changes in soil structure, all supported the hypothesis that clover improves structure rapidly, but there were no significant differences in bulk density, porosity or aggregate stability between the treatments. Soil moisture characteristic curves of soil/peat/grit compost planted with grass, grass/clover or clover and grown for two years indicate that a more free-draining structure develops under clover than under grass due to a higher ratio of macro- to micropores.     

Growth and carbon economy of nodulated white clover in the presence and absence of combined nitrogen

An investigation of the carbon economy of single S184 white clover plants nodulated with an effective strain of Rhizobium trifolii growing on N-free nutrient solution and supplied with 150 parts/10⁶ N as NH₄NO₃ has shown that 10% more of the C fixed per day is available for growth in the plants supplied with combined N. The difference between the two groups of plants is a result of higher respiratory activity in the roots of plants growing exclusively on N₂. In terms of shoot growth, however, the difference in growth rate is likely to be less than 10%, because the rate of root growth is greater in the plants supplied with a moderate level of combined N. There is no evidence that NO^-₃ and CO₂ compete for photosynthetically produced reductant in the leaves of plants grown on N₂+NH₄NO₃, since no reduction in net photosynthesis was observed in plants assimilating combined N. An experiment carried out on detached leaflets of white clover grown on N₂ and on N₂+ NH₄NO₃ has shown that NO^-₃ -reducing activity is present in the leaflets of plants grown on N₂+ NH₄NO₃ but not in plants grown on N₂ and that the activity is light-dependent. Measurements of ¹⁴CO₂ respired in the light and dark by leaflets previously allowed to photosynthesize ¹⁴CO₂ showed that the ratio of ¹⁴CO₂ output in the light to ¹⁴CO₂ output in the dark was no higher in plants grown on N₂+ NH₄NO₃ than in plants grown on N₂. This observation is consistent with the finding that N source does not affect net photosynthesis in whole plants.     

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.     

Characteristics of microsites with and without white clover in two field swards

A microsite comparison technique was used to characterize differences between clover and non-clover patches in two field swards, one grazed by cattle and the other sheep-grazed. Within clover patches of both swards, ryegrass tiller weight and the percentage of dead dry matter (DM) was lower than in non-clover patches, while the ratio of other grass DM: ryegrass DM was higher. Leaf senescence rates for ryegrass tillers within clover patch microsites were less than half of those of non-clover patch microsites, and consequently net leaf production was higher. At clover patch microsites, soil carbon dioxide levels were lower, soil oxygen levels higher and levels of N, Na, Ca, Mg and P in the ryegrass component of the herbage DM were elevated, compared with non-clover patch microsites.     

Growth of white clover in eight grass species and the effects of propyzamide, a grass-suppressing herbicide

Seedlings of white clover (cv. Grasslands Huia) were introduced as spaced plants into 3-year-old monoculture plots of eight grass species ( Agrostis capillaris, Agrostis stolonifera, Dactylis glomerata, Festuca rubra, Holcus lanatus, Lolium perenne, Phleum pratense and Poa pratensis ) during June 1984. In mid-April 1985 plots were split for application of propyzamide at the following concentrations: 0, 0·2, 0·4, 0·6 kg a.i. ha^-1.
During 1984 differences between clover seedling growth in the different grass species became apparent within 2 weeks; growth was greatest in F. rubra, P. pratensis and H. lanatus and smallest in D. glomerata, L. perenne and P. pratense. During 1985, when more N fertilizer was given, H. lanatus and D. glomerata, were equally competitive and clover contributed only 16–18% of the total herbage yield of 10·4 t ha^-1 in them, compared with 33–50% of yields ranging from 9·6 to 119 t ha^-1 in the other six species. Propyzamide decreased grass growth in mid-season by more than 50% but there was little overall persistent benefit to clover yield, except for A. stolonifera and P. pratense with 0·4 and 0·6 kg ha_-1 treatments. Dactylis glomerata and perenne were least and H. lanatus, F. rubra and P. pratensis most suppressed by the herbicide.
Possible reasons for the overall large clover contribution from a very sparse seedling population and the relatively small effects of propyzamide are discussed as well as future work required to improve the predictability of effects of grass suppression.     

The effect of pesticide application on the establishment of white clover in a newly-sown ryegrass/white clover sward

In plots of perennial ryegrass and white clover, regular treatment with chlorpyrifos and methiocarb more than trebled the clover content of the sward, compared with untreated plots, in just over 2 years from sowing. Chlorpyrifos alone accounted for most of the effect, which was attributed mainly to the control of Sitona lepidus. Combined treatment increased total herbage dry matter yield by 4·24 t ha⁻¹ during this period but had little effect on grass-only plots. Methiocarb appeared to have contributed by assisting earlier establishment of clover, but otherwise had little effect until slugs, which were initially uncommon, became more abundant in the later stages of the experiment. The effect of slug control was most noticeable in a cultivar of low cyanogenic potential. This factor did not influence the effect of chlorpyrifos.     

Herbage growth, white clover content and lamb production on grazed ryegrass-white clover swards

Two contrasting Gremie perennial ryegrass and Blanca white clover seeds mixtures were established. Each sward type was either continuously or rotationally (four paddocks) grazed at two stocking rates by lambs of 26–28 kg initial mean live weight in two 12-week experiments. Dry matter production, assessed by the cage method, was lower on the high-clover sward during the first experiment but overall was similar between seeds mixtures. Clover content, and differences between sward types, declined with time and was lowered by continuous grazing in both experiments and by the higher stocking rate in the first experiment. Animal performance was related to intake and both were increased by lowering stocking rate, increasing clover content and adopting a continuous grazing system. The results are discussed in relation to the experimental methods used and to other published findings.     

Yield of white clover populations in mixture with contrasting perennial ryegrasses

A field experiment was established in May 1986 in which four white-clover populations originating in Switzerland, and a bred variety, Menna, were grown in binary mixtures with their 'co-existing' perennial ryegrasses and three other bred ryegrasses— Aurora, Talbot and Melle. The productivity of both components of the mixtures was measured by cutting samples in October 1986 (after establishment) and on five occasions during the growing season in 1987, Substantial differences were observed between clover yields in different mixtures, and the yield ranking of clover was found to vary with companion grass. It was concluded that the interaction between grass and clover populations originated at an early stage in sward development, being evident in both the 1986 establishment cut and the spring cut of the following year. The implications of these results for the formulation of breeding objectives and the evaluation of breeding material are briefly discussed.     

Effects of sward type and rest periods from sheep grazing on white clover presence in perennial ryegrass/white clover associations

The effects of the imposition and timing of a rest period from continuous sheep stocking, for a conservation cut, on white clover presence in forty perennial ryegrass/white clover associations were studied over two full grazing seasons. Each association consisted of one grass variety along with one white clover variety, the grasses being diploid and tetraploid ryegrasses from each of five maturity groups and the white clovers from each of four leaf size categories. The presence of white clover within each association was assessed at the beginning and end of both seasons by means of a 0–64-m² quadrat subdivided into 100 squares, each 80 mm × 80 mm, the number of squares in which any part of a white clover plant was visible being recorded. Complementary point quadrat data were also collected. Although continuous sheep stocking did not necessarily have an adverse effect on white clover presence, a July to mid-August rest period increased white clover proportions in the swards (means: unrested, 48–1; April to late May rest, 32.7; July to mid-August rest, 67.3 - s.e.d. 7.59; P < 0.05) the benefit increasing with increasing white clover leaf size. The early rest period (April to late May) reduced white clover presence and the late rest period (July to mid-August) increased white clover presence, these effects being intensified with increasing white clover leaf size (very large-leaved clover: unrested, 20.6; April to late May rest, 8.3; July to mid-August rest, 41.1 and small-leaved clover: unrested, 96.3; April to late May rest, 84.8; July to mid-August rest, 97 - s.e.d. 9.2; P <0001). Tetraploid ryegrass/white clover associations had consistently and significantly more white clover than diploid ryegrass/white clover associations of similar ryegrass maturity group (tetraploid, 53.4; diploid 44.8 - s.e.d. 2.12; P <0.001) and associations with early maturing ryegrass contained more white clover than those with late maturing ryegrasses, the effect of maturity group being greater than that of ploidy. Overall, white clover presence increased with increasing openness of grass growth habit.     

Silage and milk production: studies with diets containing white clover silage

In three separate feeding experiments using a total of twenty-six individually-housed Ayrshire cows, three wilted silages made from Blanca white clover were offered ad libitum with either different supplements or different proportions of grass silage. The clover silages contained 680 g white clover kg⁻¹ on a DM basis, and had a mean DM concentration of 263 g kg⁻¹ with 231 g CP kg⁻¹ DM and 91 g ammonia-N kg⁻¹ N. The pH values averaged 4·16 and the DOMD concentrations 611 g kg⁻¹. In experiment 1 the daily intake of clover silage given alone was 15.2 kg DM per cow, i.e. 30·1 g kg⁻¹ live weight, and decreased by 0·76 kg DM kg⁻¹ barley DM and by 0·66 kg DM kg⁻¹barley plus soybean meal DM when these feeds were offered as supplements. Milk yield and fat concentration were higher on the supplement treatments than on the clover silage-only treatment. In experiments 2 and 3 the intakes of silage and total DM increased as the weight of clover in the diet increased from 0 to 700 g kg⁻¹ with parallel increases in milk yield. The effects on milk composition were small and generally non-significant. Although white clover silages with excellent fermentations were made, it is concluded that the main role of white clover in a silage system will be in mixed swards with grass to reduce the input of fertilizer N and to increase the voluntary intake of silage.