White Clover In Mixed Swards Research Articles

Overwintering of Trifolium repens L. and Succeeding Growth: Results from a Common Protocol carried out at Twelve European Sites

A common experimental protocol was followed at 12 sites in Europe to study the development of two clover cultivars (AberHerald and Huia) grown in association with a ryegrass cultivar throughout their annual cycle. The duration of the experiment was between 1 and 3 years at each site. Detailed information about the morphogenesis and carbohydrate reserves of white clover during winter and the subsequent productivity of clover plants during the growing season was collected. Yield of the companion ryegrass was also measured at several harvests during the growing season and grass tiller density was recorded several times during winter. There was wide variation among sites and between years in climatic conditions, in the growth characteristics and chemical composition of the two cultivars, and in the tiller density of the ryegrass. The relative performance of the two cultivars varied among sites but AberHerald generally outperformed Huia. Major changes in plant characteristics (morphology, population size, chemical composition, etc.) occurred during overwintering. This paper presents a preliminary analysis of the effects of site, clover cultivar and sward age (years) on various plant characteristics. It is established that the data provide a unique basis for modelling the effects of a wide range of environmental conditions and plant properties on the performance of white clover in mixed swards. A modelling approach that seeks to replace site and year by climatic variables characterizing each site by year combination is developed in two companion papers.

Overwintering of Trifolium repens L. and Succeeding Growth: Results from a Common Protocol carried out at Twelve European Sites

A common experimental protocol was followed at 12 sites in Europe to study the development of two clover cultivars (AberHerald and Huia) grown in association with a ryegrass cultivar throughout their annual cycle. The duration of the experiment was between 1 and 3 years at each site. Detailed information about the morphogenesis and carbohydrate reserves of white clover during winter and the subsequent productivity of clover plants during the growing season was collected. Yield of the companion ryegrass was also measured at several harvests during the growing season and grass tiller density was recorded several times during winter. There was wide variation among sites and between years in climatic conditions, in the growth characteristics and chemical composition of the two cultivars, and in the tiller density of the ryegrass. The relative performance of the two cultivars varied among sites but AberHerald generally outperformed Huia. Major changes in plant characteristics (morphology, population size, chemical composition, etc.) occurred during overwintering. This paper presents a preliminary analysis of the effects of site, clover cultivar and sward age (years) on various plant characteristics. It is established that the data provide a unique basis for modelling the effects of a wide range of environmental conditions and plant properties on the performance of white clover in mixed swards. A modelling approach that seeks to replace site and year by climatic variables characterizing each site by year combination is developed in two companion papers.

Soil N contributes to the oscillations of the white clover content in mixed swards of perennial ryegrass under conditions that simulate grazing over five years

The effect of the initial N‐supplying capacity of soils (SoilN, 90–230 kg N ha–1 year–1) was tested on the dry‐matter and N yields of pure or mixed white clover and perennial ryegrass swards, managed under simulated grazing over a 5‐year period. The cumulated N harvested in the mixed swards was similar, both for white clover and perennial ryegrass, but the proportion of white clover showed oscillations over a 2‐year period. In the first year, the SoilN effect was similar to that of fertilizer N. During the course of the experiment, the effect was always positive on the pure perennial ryegrass sward, alternately negative and nil for the white clover in the mixed sward and alternately positive and nil for the perennial ryegrass in the mixed sward; the period of these oscillations was 2 years. From the third regrowth period after sowing, the ratio between the actual N concentration and the concentration non‐limiting to growth for the perennial ryegrass in the mixed sward, increased above that of the pure perennial ryegrass sward. It was in turn greater in the soils that were initially poor and then greater in those that were initially rich in soil N. The periodic oscillation of the initial SoilN effects implies that the initial SoilN gradient was alternately compensated and restored. It was concluded that N fluxes are partly responsible for the temporal oscillations in the proportion of white clover in mixed swards.

Effect of VA mycorrhizas on phosphorus uptake and growth of white clover (Trifolium repensL.) growing in association with ryegrass (Lolium perenneL)

Abstract In a phosphorus‐deficient soil, vesicular‐arbuscular mycorrhizas (VAM) helped white clover to compete more effectively with ryegrass for phosphate, resulting in better phosphate acquisition by mycorrhizal clover than non‐mycorrhizal clover. The benefit was greatest when competition from ryegrass was severe. Mycorrhizal clover did not grow much faster than non‐mycorrhizal clover, probably because of competition for light from neighbouring ryegrasses. When the soil was fertilised with phosphate, clover and ryegrass yields increased, and non‐mycorrhizal clover plants outyielded mycorrhizal plants. Similar parasitic effects of VAM have been recorded before. It is concluded that the presence of VAM linkages between clover and grass roots will probably negate any benefits to clover of the introduction of elite VAM fungi to pasture soils. Genetic manipulation of a host plant's response to VAM, may be a more profitable way of improving the phosphate nutrition of white clover in mixed swards.

The effect of time of sowing and sowing method on production of white clover in mixed swards

Two field trials were carried out in successive years in which (1) perennial ryegrass and white clover seeds were drilled together, or (2) clover was broadcast and grass drilled, or (3) both clover and grass were broadcast. The last two treatments were followed by harrowing or not of the seed bed. Sowing took place in early May or August. Four harvests were taken in each full harvest year. In establishment years, sowing in May resulted in a total dry matter (DM) production at least ten times higher than that resulting from sowing in August. No sowing method treatment effects on total DM or clover yield were significant in the establishment year (measured only in spring-sown plots). In the first harvest year the August-sown treatments produced 15% less DM than those sown in May and clover yield was, on average, 40% lower than the earlier sown treatments; clover proportion followed a similar pattern to yield. Drilling of both grass and clover without harrowing produced swards with a significantly lower proportion of clover in the first harvest year in the first trial than in treatments in which clover was broadcast but not harrowed. In the second trial at harvest 1, clover proportion just failed to be significantly lower in the treatment in which grass and clover were drilled than in the treatment when clover was broadcast and the seed bed harrowed. In the second harvest year (first trial only), annual clover yield and proportion were not affected but drilled grass and clover had lower DM yield than when grass was drilled and clover broadcast without harrowing and when both were broadcast, without harrowing. At one harvest, the yield of clover in treatments sown in May was actually lower than that in the later-sown plots. In a subsidiary controlled environment experiment to investigate the effect of stage of clover development on cold hardiness (a factor in autumn-sown swards), plants which were about to initiate stolons (58 days old) had an LD50 of −5·1° compared with −9·3°C for plants 60 days older. It is concluded that autumn sowing delays the time at which optimum clover production is achieved (late in the first full harvest year) and method of sowing does not compensate for this. However, sowing in August under Northern Ireland conditions does not seem to jeopardize the chances of a successful establishment of white clover, and plants should be sufficiently winter hardy to withstand relatively hard freezing conditions.

Impact of the field slug Deroceras reticulatum on establishment of ryegrass and white clover in mixed swards

SummaryThe relationship between numbers of Deroceras reticulatum and the establishment of ryegrass cv. Melle and white clover cv. Milkanova was studied on a mini‐plot experiment. Assessments 12 and 33 days after sowing showed that Deroceras reticulatum had a proportionally greater impact on survival and growth of clover than on ryegrass. Slugs had no apparent effect on numbers or growth of three species of broad‐leaved weeds (Senecio vulgaris, Cardamine hirsuta and Epilobium sp.) which occurred in the field experiment. In pot experiments where seedlings of ryegrass together with one of three clover cultivars, with or without chickweed (Stellaria media), were exposed to Deroceras reticulatum, seedlings of white clover cv. Aran were less affected by slug grazing than cvs S184 or Milkanova. The presence of different clover cultivars did not significantly influence the impact of slugs on growth of ryegrass or chickweed, nor did the presence or absence of chickweed influence the effect of slugs on ryegrass and clover seedlings. Slugs had less impact on the growth of chickweed than on ryegrass or clover. Thus slug grazing would make clover less able to compete with chickweed as well as the other weeds found in the mini‐plot experiment.

Comparison between isotope dilution and acetylene reduction methods to estimate N2 fixation rate of white clover in grass/clover swards

AbstractIn two field experiments acetylene (C2H2) reduction by white clover in mixed swards was compared to N2‐fixed measured by 15N dilution. In both experiments, samples for C2H2 reduction were 7.5 cm diameter turves taken from plots within which microplots of 24 cm diameter were delimited and to which 15N was applied as ammonium sulphate (15NH4)2SO4). C2H2 reduction was assayed every 6–7 d. The rate of C2H2 reduction per unit length of stolon was applied to the estimated stolon length within the appropriate microplot at the time of assay, and the amount of C2H2 that would have been reduced within the microplot was estimated by integration.In experiment 1, turves taken from grass/clover swards to which 0, 1·5, 3·0, 4·5 or 6·0 g N m−2 had been applied were incubated in sealed chambers (10% C2H2, 90% air). The mean ratio of C2H2 reduced to N2 fixed during 5 weeks was 0.74:1. Application of N fertilizer lowered the proportion of assimilated N derived from N2 fixation from 95% in unfertilized swards to 83% in those receiving 6 g N m−2 (60 kg N ha−1). In experiment 2, clover roots and stolons from plots that previously had been grazed were dissected from turves and incubated in a stream of C2H2 and air (i.e. the open system). The maximum rate of ethylene (C2H4) produced during the first 12 min was taken as a measure of true nitrogenase activity. The relationship between C2H2 reduced and N2 fixed was significant (r=0.80**). The mole ratio was 0.55:1 for the 6 weeks duration of the experiment, the low ratio possibly being due to disturbance of the nodules adversely affecting acetylene reduction. Mole ratios from both experiments were well short of the theoretical 4·3:1. Using the open system does not, therefore, overcome the shortcomings of the acetylene reduction technique for measuring N2 fixation of white clover in mixed swards.

Frequency distribution of shoot weight of plants in populations ofTrifolium repenspersisting by clonal growth in grazed pastures

SUMMARYOver two years (1984/85 and 1986/87), monthly sampling of shoots of white clover plants compared the populations of white clover in mixed swards at Palmerston North, New Zealand, under set stocking, rotational grazing and a combination of both systems, at a common stocking rate of 22·5 ewe equivalents/ha.The frequency distributions of shoot (or stolon) dry weight per plant in each population over the study period was described by a log-normal model, which indicated that the populations consisted of many small individuals and few large individuals. Such inequality of shoot dry weight within populations is commonly termed size hierarchy; a statistic giving a measure of such size hierarchy is the Gini coefficient. The populations under different managements had similar Gini coefficients which differed little among seasons or between years. Lack of significant correlation between the Gini coefficient and mean shoot dry weight per plant of each population indicated that, in these white clover populations, size hierarchy was independent of mean plant size.These results were considered in relation to the clonal growth of white clover in grazed swards and it is suggested that the variable nature of death of older basal stolons makes an important contribution to the variability in size of individual plants and hence to size hierarchy. As size hierarchy, as assessed by Gini coefficients, was relatively stable in these populations over 3½ years, it appears that clonal growth of white clover incorporates sufficient variability within the growth and death processes at the individual plant level to maintain the size hierarchy, irrespective of variations in mean plant size of populations.

Morphology of white clover (Trifolium repensL.) plants in pastures under intensive sheep grazing

SummaryThere has been little study on the growth and morphology of individual plants constituting the population of white clover in mixed swards under grazing. Such information is required if the mechanisms governing plant productivity and persistence are to be understood.Intact white clover plants were sampled from intensively sheep-grazed pastures under set stocking, rotational grazing, and a combination of both systems, by taking turves (250 × 250 mm), and washing out the plants, every month for a year. Characters measured for every stolon of each plant were: presence of a growing point; numbers of leaves, roots and axillary buds; stolon length. Total plant leaf and stolon dry weight were also recorded. Plants were classified according to degree of branching, and the contribution of each branching order to the population determined.There were strong seasonal variations in plant size (leaf and stolon dry weight, stolon length, and numbers of stolons and leaves per plant) which showed a significant decrease in spring with recovery over the following summer. This was paralleled by a rapid increase in the proportion of less branched plants (1st and 2nd branching order) in the population from 60 to 80% in spring, as higher-order plants broke up into smaller- and lower-ordered plants at this time. Numbers of roots per plant increased over winter to peak in early spring then declined in the following summer-autumn. While system of grazing management had no significant effect on branching structure of plants, it had a large effect on plant dry weight; rotationally grazed plants were 2·5 times larger than set stocked plants (0·182 cf. 0·073 g respectively).Other general features of plant morphology were that each successive order of branch stolons was shorter and length before branching was less than that of their preceding parent stolon. The highest branching order observed was 6th order. There was no relationship between branching and numbers of roots; in branched plants only 55% of stolons were rooted regardless of plant order, but rooted stolons accounted for 85% of total stolon length and carried 62, 48 and 90% of the leaves, growing points and axillary buds per plant, respectively.Comparison with other studies suggests that the processes outlined in this report may be common to white clover growth under grazing over a wide range of favourable environments.

Effect of grazing management and season on nitrogen and phosphorus content of leaves and stolons of white clover in mixed swards

Abstract The nitrogen (N) and phosphorus (P) contents of leaf, surface stolon, and buried stolon of white clover (Trifolium repens L.) were measured in samples taken every 2 months, from pasture under rotational grazing by cattle (RGC) and set stocking by sheep (SSS), for 3 years. The effect of the system of grazing management of sheep on N and P content of 3 herbage classes of white clover was examined during a 1-year period by analysing samples taken monthly from pastures with 3 different management backgrounds; set stocked (SS), rotationally grazed (RS) or set stocked from lambing to drafting, then rotationally grazed for the rest of the year (CS). In the 3-year study, the content of N did not differ between the RGC and SSS treatments but the mean values of 4.59% in leaf, 2.75% in surface stolon, and 2.22% in buried stolon all differed. However, P contents differed between treatments: with RGC treatment, mean P levels decreased (0.43, 0.29, and 0.23% for leaf, surface stolon, and buried stolon respecti...

Regulating the content of white clover in mixed swards using grass‐suppressing herbicides

AbstractA preliminary investigation evaluated six grass‐suppressing herbicides applied on two occasions in late winter to a predominantly ryegrass ley containing only 15% ground cover of white clover. Substantial increases in clover growth, estimated visually, and flower head numbers per unit area were recorded in the first summer after treatment with 2·8 kg ha‐1 carbetamide, 0·8 kg ha‐1 propyzamide and 0·6 kg ha‐1 paraquat. To achieve these increases, visual estimates suggested that spring growth of grass was reduced by 40–80%. However, grass growth recovered fully by mid‐summer on the majority of the treatments.The following year five of the herbicides were compared in a field experiment. Dry matter (DM) and nitrogen (N) assessments of the grass and legume components were made at three harvests in the first growing season and a single harvest in the second year. Carbetamide, paraquat and, especially, propyzamide increased the proportion of clover in the DM (to 89% in the case of 1·2 kg ha‐1 propyzamide); in general, using herbicides to raise clover contents above 20% lead to reductions in spring grass growth of about 70%. However, such reduction was offset by subsequent increased growth so that total annual yields were largely unaffected. The increased legume content resulted in an increased N concentration in both grass and legume components, measured in the second summer. At this time, the greatest increase in total N yield (up to 35%) was recorded from 0·6 kg ha‐1 propyzamide. Potential uses to achieve legume dominance by grass‐suppression are suggested and the needs for further research are outlined.

THE EFFECT OF NITROGENOUS FERTILIZER ON AN ESTABLISHED WHITE CLOVER SWARD

The effect of nitrogenous fertilizer on a white clover sward was studied to obtain information to be used in the evaluation of white clover in grassland. There was no evidence of a yield response. The weight of nodules was depressed by the addition of fertilizer.Where management is adjusted to maintain white clover in swards receiving large quantities of nitrogenous fertilizer the clover may use part of this fertilizer at the expense of symbiotic fixation.The relation of the results obtained to the assessment of the benefit derived from white clover in mixed swards is briefly discussed.