Nitrogen fixation by white clover in hill pasture

An experiment is described in which the amount of nitrogen fixed by the white clover component of a surface-sown hill sward was determined using small quantities of ¹⁵N-labelled ammonium sulphate. Between 11 July and 22 August 1974 25 kg ha^?1 N was fixed. No evidence of a transfer of N from clover to the associated perennial ryegrass was detectable over this period although the N content of the grass growing with the clover was higher. The relative advantages of the isotope technique and the acetylene reduction assay are discussed.     

The response of Lotus uliginosus L. grown on hill soils to inoculation with Rhizobium

The response of marsh birdsfoot trefoil ( Lotus uliginosus ) to inoculation with appropriate Rhizobium spp. was compared over 2 years. Inoculated and uninoculated seed was sown at 5 kg ha^-1 on three soil types, brown earth, peaty podsol and deep peat. Perennial ryegrass cv. Perma was sown at 20 kg ha^-1 as a companion species. Plots received either no N or a light starter dressing of 30 kg ha^-1 N. Significant benefits from inoculation were recorded for Lotus germination and nodulation, Lotus and non- Lotus herbage DM harvested and Lotus cover on the deep peat, and to a lesser extent on the other soil-types. Due to particularly uneven early establishment, work on the peaty-podsol site was discontinued at an early stage. There was a trend towards depression of Lotus where N had been applied to inoculated plots.     

The response of mini-swards of perennial ryegrass/white clover to simulated rainfall following slurry application

Perennial ryegrass/white clover mini-swards were used lo determine whether sward responses to slurry were modified by subsequent simulated rainfall (SR). Combinations of two SR volume rates (6.5 and 13 mm), three SR timings (3, 24 and 48 h after slurry application) and two slurry types (cattle, pig) were compared with a fertilizer control (no SR). Scorch, smother and growth of marked stolons and tillers were monitored after slurry application, and yields of both species were recorded. Slurry coverage of plastic squares indicated that smother may be greater on horizontally orientated leaves (clover). Reduced grass leaf growth and numbers of clover growing points in the week following slurry application were attributed mainly to smother, and possibly other phytotoxic effects of slurry. Reasons for the superior clover growth with cattle slurry arc unknown, although its higher P and K content may have contributed. Timing of rainfall is important in influencing sward responses to slurry application. Results suggest that application of slurry may be most beneficial when it immediately precedes rainfall and that longer intervals between slurry application and rainfall reduce grass and clover growth through either greater ammonia loss or slurry negative effects.     

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.     

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.     

The nature of competition between perennial ryegrass and white clover

Perennial ryegrass and white clover were grown in boxes with either no competition, root competition only, shoot competition only, or both root and shoot competition between them, Boxes received either no nitrogen or 200 kg N ha⁻¹ as a split application. The experiment was harvested at 8, 12, 16 and 20 weeks after sowing. Perennial ryegrass was more competitive than white clover throughout the experiment and its overall competitive ability increased with Lime. The effects of root competition were greater than those of shoot competition at the first harvest, with both forms of competition having similar affects at the second harvest, while at the third and final harvests shoot competition had greater effects than root competition. At the later harvests N application increased the overall competitive ability of perennial ryegrass, relative to white clover, mainly owing to the increase in its shoot competitive ability.     

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.     

Thermal time approach to predicting seedling emergence dates of red clover,white clover and lucerne in farm fields

A practical method for predicting seedling emergence dates of red and white clovers, and lucerne is proposed. The germination response at seven different constant temperatures ranging from 5 to 35°C was examined. Germination rate, which is a reciprocal of the duration for 50% germination of seeds, was linearly regressed against temperature to calculate the base temperature and thermal constant of seed germination. The calculated values were 3·9°C and 15·8 degree days for red clover, 4·2°C and 13·6 degree days for white clover, and 2·9°C and 17·7 degree days for lucerne. Using the base temperatures, thermal constants, and the daily mean air temperatures at the study site, the seedling emergence dates of the three forage legumes were predicted. At the same time, in an outdoor pot experiment, seeds of these legumes were sown approximately every 3 weeks and seedling emergence dates were determined. The predicted dates of seedling emergence generally fitted the observed dates. Another prediction was attempted using the base temperatures, thermal constants, and normal daily mean air temperatures recorded over more than 30 years in the study site. This prediction showed that the seedlings of the three forage legumes began to emerge at the beginning of April and could continue to emerge until the beginning of November when their non‐dormant seeds were present in the site, and that when the seeds were sown from mid‐November of one year to late March of the next year, the emergence of seedlings was delayed until the beginning of April.     

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.     

The effect of fertilizer nitrogen rate, white clover variety and closeness of cutting on herbage productivity from perennial ryegrass/white clover swards

Four varieties of white clover (small-leaved cv. Aberystwyth S184. medium-leaved cv. Grasslands Huia and large-leaved cvs Linda and Olwen) were sown at 3 kg ha^-1 together with 10 kg ha^-1 perennial ryegrass cv. Talbot. Herbage productivity was measured for three harvest years, 1979-81, over four annual rates of fertilizer N (0,120,240 and 360 kg ha^-1) and two closeness of cutting treatments (80 and 40 mm from ground level). A simulated grazing regime of six cuts per year at 3- to 6-week intervals was used.
Production of total herbage DM was increased by increasing N rate; mean annual DM production ranged from 783 1 ha^-1 with no N to 11701 ha^-1 at 360 kg ha^-1 N. Mean herbage response to N (kg DM per kg N applied) was 73,90 and 108 for the three successive N increments relative to no N. Mean white clover DM production was reduced from 4 14t ha^-1 with no N to 051 t ha^-1 at 360 kg ha^-1 N.
The large-leaved clover varieties were more productive than the small- or medium-leaved varieties at all N rates. Close cutting increased total herbage and white clover by a mean annual 16% and 31%. respectively. White clover varieties did not interact with either N rate or closeness of cutting.
It is concluded that repetitive N application over the growing season is incompatible with white clover persistence and production, even with large-leaved clover varieties or with close cutting, two factors which improved clover performance in the experimental swards.     

The influence of fertilizer nitrogen, white clover content and environmental factors on the nitrate content of perennial ryegrass and ryegrass/white clover swards

In a field experiment carried out over 3 years, the nitrate content of herbage from perennial ryegrass (Lolium perenne) swards increased exponentially with nitrogen application rate, but herbage nitrate content appeared to reach potentially dangerous concentrations only when nitrogen application rates were greater than those needed to stimulate dry-matter production. Thus, on average over all the harvests, maximum yield could be obtained with annual application rates of 400 kg N ha^–1 (six applications of 67 kg N ha^–1) for perennial ryegrass and 300 kg N ha^–1 (six applications of 50 kg N ha^–1) for perennial ryegrass/white clover (Trifolium repens) swards, whereas the mean nitrate concentrations were 3340 and 2929 mg NO₃ kg^–1 dry matter (DM) respectively. Nitrate content, however, varied considerably from harvest to harvest, reaching maxima of 9345 mg NO₃ kg^–1 DM at 400 kg N ha^–1 for perennial ryegrass and 6255 mg NO₃ kg^–1 DM at 300 kg N ha^–1 for perennial ryegrass/white clover. The nitrate content of herbage from perennial ryegrass/white clover swards was always greater than that of perennial ryegrass swards receiving the same rate of nitrogen application, even though in the herbage from the mixed sward the nitrate content of white clover was usually less than half that of the perennial ryegrass component. The physical environment did not have a clearly interpretable effect on nitrate content, although herbage harvested in May had a much lower nitrate content than that harvested at any other time of the season. It was not possible to find a single multiple regression equation relating herbage nitrate content to nitrogen application and to other environmental variables that explained more than 60% of the variance in herbage nitrate, but it is suggested that, by reducing the later-season nitrogen applications from 67 to 50 and finally to 33 kg N ha^–1 for perennial ryegrass and from 50 to 33 kg N ha^–1 for perennial ryegrass/white clover, it would be possible to achieve over 90% of the maximum yield while reducing average nitrate content to <40% of that at maximum yield, with no samples containing more than 2300 mg NO₃ kg ^–1 DM.     

Competition among seedlings of phalaris, subterranean clover and white clover in diallel replacement series mixtures

Phalaris, subterranean clover and white clover were grown together in binary, diallel replacement series mixtures at (day/night) 15/10 or 24/19°C under long-day conditions, and cut at 2- or 4-weekly intervals. Analysis of white clover mixtures was confined to 15/10°C due to losses of seedlings prior to imposition of treatments at 24/19°C.
With white clover at 15/10°X, seedling losses occurred after the first harvest, the greater losses occurring in mixtures where subterranean clover was the major component. The de Wit analyses showed that subterranean clover excluded its companions, whereas phalaris and white clover competed for slightly different niches resulting in over-yielding. Gleeson McGilchrist analyses showed that subterranean clover and phalaris were more aggressive than white clover under infrequent cutting, but that white clover was more aggressive as a major than as a minor component of a mixture under frequent cutting. Between phalaris and subterranean clover [he pattern of competition at 24/19°C differed from that at 15/10°C only in the more rapid development of subterranean clover and consequent swing to phalaris dominance.
We conclude that the seedlings of white clover and subterranean clover are suited to combination with phalaris because phalaris is not too aggressive towards white clover, and is tolerant of the aggressiveness of subterranean clover. The survival of only a few seedlings of white clover in mixtures with subterranean clover may be sufficient for long-term growth provided some perennation occurs after the senescence of subterranean clover.     

Transition through the taprooted growth stage in white clover as related to temperature

Morphology and phenology, including stolon disintegration, of white clover were studied at two sites in the first three years after sowing. The plants reached their maximum size in late summer in the first year after sowing, and the populations contained at that time mainly taprooted plants. The subsequent fragmented plants were smaller in most of the investigated characteristics. A function describing the daily transition rate from taprooted to fragmented plants in relation to daily mean temperature is presented. Deviations between the recorded and the calculated proportion of taprooted plants were small; the overall root mean square error was 0·057. The data indicated that there is a low temperature requirement that must be met prior to the onset of transition. It is also suggested that, in further modelling of the response of white clover to temperature, the effect of daylength should be accounted for, particularly if the data are collected from sites with a considerable latitudinal spread.     

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)     

Digestion in vitro of Italian and perennial ryegrasses, red clover, white clover and lucerne

Herbage at various stages of growth from three field experiments, one with Italian ryegrass, one with perennial ryegrass and one with white clover, red clover and lucerne, was incubated for 0, 24 or 48 h in rumen liquor followed by 48 h in acid-pepsin. The proportion of cell content OM in herbage (measured by pepsin) declined with advancing stage of growth more quickly in the legumes than in Italian ryegrass. At a high level of digestibility the legumes had a higher proportion of cell content OM than Italian ryegrass, but the position tended to be reversed at a low level of digestibility. Decline in proportion of cell content and decline in digestibility of cell wall were both major contributors to the decline in digestibility of Italian ryegrass with advancing maturity, whereas in white clover the decline in proportion of cell content was entirely responsible for the decline in digestibility. Lucerne and red clover occupied intermediate positions. At an early vegetative stage rate of digestion appeared to be higher in Italian than in perennial ryegrass. Rate of digestion was higher in the legumes than in the ryegrasses, the difference being largest at an advanced stage of growth.     

The introduction of red or white clover into a perennial grass sward

Sowing and post-sowing management techniques were assessed for their effectiveness in introducing Trifolium repens and T. pratense into an established Phalaris aquatica sward in spring and autumn.
Percentage emergence, seedling survival, seedling weight and final clover content were greater with T. pratense than with T. repens . Surface sowing gave 10% emergence compared to 62% for tilled treatments. Rotary sowing gave superior emergence in autumn, while tined sowing gave superior emergence in spring. However, seedling survival and weight, and therefore final clover content, were generally greater with rotary sowing than with tined sowing.
The removal of competition from the resident grass by bandspraying with glyphosate prior to sowing, and repeated lenient defoliation postsowing, increased final seedling numbers by 19% and 46%, respectively, and their individual weight by 29% and 82%, respectively.
The results showed that direct sowing in spring or autumn can be used to restore successfully a substantial population of clover plants to an existing perennial grass sward. They also demonstrated the importance of management to manipulate the environment of the developing clover plant.     

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.     

The spatial pattern of vegetation in cut and grazed grass/white clover pastures

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

Contribution of white clover varieties to total sward production under typical farm management

White clover varieties, potentially suitable for inclusion in seed mixtures for mixed stock-rearing farm systems, were evaluated when growing with S23 perennial ryegrass under rotational sheep grazing with a silage cut in late May or early June, as practiced on farms. Monoculture grass swards were also included to enable the direct and indirect contribution of white clover to total sward production to be evaluated.
White clover increased total sward production during all three years of the trial by an average of 50%. Although clover content of swards were similar, large differences occurred in their grass content, especially in the third year, when difference in total yields of swards based on large–leaved clover varieties was 2 t ha ⁻¹ while difference in clover yield was only 0-6 to ha^–1 The indirect contribution of white clover, namely the extra grass resulting from N transfer, was greater in the spring than in the autumn. It was also greater for Nesta than for other varieties. and exceptional for this variety in that the increase in grass yield above that of grass monoculture was maintained over three harvest years.     

Effects of spring defoliation and fertilizer nitrogen on the growth of white clover in ryegrass/clover swards

An examination was made of the effects of different spring treatments on the growth of white clover in a ryegrass/white clover sward. Plots were either cut once (in February, March or April) or twice (in February and April) or left uncut. Nitrogen was applied to half of the plots in each instance. The clover was sampled at intervals of approximately 3 weeks from February to June to determine numbers of leaves and growing points and weights of plant parts. Rates of leaf appearance were also observed and estimates were made of total herbage mass from ground-level cuts.
Percentages of white clover in the herbage were higher in unfertilized than in fertilized plots and in defoliated than in undefoliated plots. The percentage increases that followed defoliation were usually maintained into later regrowth, showing that clover content was not automatically reduced as herbage mass increased. Increases in growing points were recorded after the beginning of April in defoliated unfertilized plots but not in undefoliated fertilized plots or in plots fertilized and defoliated twice during the spring period, in which numbers fell substantially.
Inverse relationships were found between rates of leaf appearance, or the number of green leaves retained per stolon, and herbage mass, whereas heights of clover and grass leaves and the percentage of dry matter allocated to petiole rather than leaf in the clover increased with increasing herbage mass.
We suggest that the observed differences between spring treatments in clover percentage result primarily from their differential effects on the formation and death of tillers and growing points in the early stages of regrowth.