The effects of frequency of cutting and cultivar on the period of leaf expansion in white clover grown in mixed swards

The period of leaf expansion in white clover ( Trifolium repens ) grown with perennial ryegrass ( Lolium perenne ) in mixed swards and the relationship between the period of leaf expansion and leaf appearance rate were examined in a field experiment in The Netherlands in which two cutting frequencies (low and high) and three white clover cultivars (Alice, Retor and Gwenda) were used. A significant positive correlation existed between the period of leaf expansion and leaf appearance rate under a low frequency of cutting, indicating that a longer cutting interval might allow the expression of cultivar and environmental differences in both leaf expansion and leaf appearance rate. In most cases, frequent cutting tended to shorten the period of leaf expansion, which may be an important cause of lower dry-matter yield in the high frequency of cutting treatment. The large-leaved cv. Alice had a significantly longer period of leaf expansion than the other cultivars under frequent cutting in autumn. This might partly account for the mixtures with Alice having a greater proportion of white clover than the mixtures with Retor and Gwenda in autumn. Some of the variation between low- and high-frequency cutting treatments in the period of leaf expansion could be explained by mean and minimum temperatures (38% and 15% respectively).     

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

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

Effects of 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.     

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.     

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

A 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 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 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.     

Interactions between pesticide treatment, cutting frequency and rate of N fertilizer on white clover grown in mixture with perennial ryegrass

The effects of a combined pesticide and fungicide (P + F) treatment on the proportion of white clover in an established perennial ryegrass/white clover sward were assessed on six occasions during 1988 and 1989, under two cutting frequencies (2- or 6-week intervals) and with or without N fertilizer (120 kg ha^-1 per annum). In 1988 P + F treatment significantly increased the proportion of clover in August and October, with an increase at the latter harvest being much greater where N was also applied. There was no significant interaction between P + F treatment and cutting frequency, although less frequent cutting increased significantly the proportion of clover in October where N was not applied.
In 1989, drought prevented the proportion of clover from exceeding 1% throughout, regardless of management factors. P + F treatment significantly increased the proportion of clover in August, and significantly reduced populations of slugs, foliar-feeding insects (mainly Collembola; Sminthuridae) and the area of clover leaf damaged by pests. No marked effect of P + F treatment on populations of plant parasitic nematodes was detected. No appreciable incidence of fungal diseases was observed in either year.     

Effects of cutting frequency on plant production, N-uptake and N2 fixation in above- and below-ground plant biomass of perennial ryegrass–white clover swards

Nitrogen (N), accumulating in stubble, stolons and roots, is an important component in N balances in perennial ryegrass–white clover swards, and the effects of cutting frequency on the biomass of above‐ and below‐harvest height were studied during two consecutive years. Total dry matter (DM) and total N production, and N₂ fixation, were measured at two cutting frequencies imposed in the summers of two years either by cutting infrequently at monthly intervals to simulate mowing or by frequent cutting at weekly intervals to simulate grazing. Total DM production harvested was in the range of 3000–7000 kg DM ha^?1 with lower DM production associated with the frequent cutting treatment, and it was significantly affected by the different weather conditions in the two years. The higher cutting frequency also reduced the biomass below harvest height but the different weather conditions between years had less effect on stubble and, in particular, biomass of roots. The biomass of roots of white clover was significantly lower than that of roots of perennial ryegrass and remained at a relatively constant level (200–500 kg DM ha^?1) throughout the experiment, whereas the biomass of perennial ryegrass roots increased from 2400 kg DM ha^?1 in the year of establishment to 10 200 kg DM ha^?1 in the infrequent cutting treatment and 6650 kg DM ha^?1 in the frequent cutting treatment by the end of the experiment, giving shoot:root ratios of 4·7–16·6 and 0·5–1·6 for white clover and perennial ryegrass respectively. Annual N₂ fixation was in the range of 28–214 kg N ha^?1, and the proportion of N fixed in stolons and roots was on average 0·28. However, as weather conditions affect the harvested DM production and the shoot:root ratio, care must be taken when estimating total N₂ fixation based on an assumed or fixed shoot:root ratio.     

Effects of defoliation intensity on white clover seedling growth

White clover seedlings were defoliated when either two, four or six leaves had opened. There were four defoliation treatments, involving removal of younger laminae, older laminae, all laminae or all laminae and petioles, plus a control (no defoliation).
Leaf removal reduced area of subsequently emerging leaves when measured as they became fully opened, though some compensatory expansion occurred after this. Petiole length was also reduced considerably. The magnitude of these effects varied according to severity of the treatment: removal or older laminae had little effect, removal of younger laminae had a similar effect to removing all laminae, but the effect was considerably increased by removing petioles in addition to laminae. The youngest plants showed the greatest reductions in leaf size following leaf removal. Defoliation had little effect on the rate of development of subsequent leaves.
The most severe defoliation treatments reduced plant dry matter but younger seedlings appeared to have a capacity for recovery equal to or greater than that of older seedlings.
It is concluded that white clover seedlings have a considerable ability to recover from leaf removal, especially if only laminae of old leaves are removed, but the growth reduction following removal of petioles as well as laminae appears to be especially severe.     

The pattern of growth in swards of two contrasting varieties of white clover in winter and spring

In a study of growth rates and developmental morphology of simulated swards of two contrasting white clover varieties, Katrina and Kent, it was found that both varieties continued to produce dry matter throughout the winter at Aberystwyth. The gross crop growth rate in the coldest periods was about 7 kg ha^-1 d^-1. The rates of formation and loss of new leaves were approximately equal so that there was no net increase in weight until the latter part of March. This initial net weight increase in both varieties was observed in the same harvest interval, the most important factor being the difference in weight between the new leaf plus petiole unit and the weight of the unit which it replaced. The weight of laminae and petioles throughout the experiment was somewhat higher in Katrina which has come to be regarded as the 'earlier' variety.
The prospects for improving spring production in white clover by variety improvement are discussed.     

Effect of cultivar and seed rate of perennial ryegrass and strategic fertilizer nitrogen on the productivity of grass/white clover swards

Four cultivars of perennial ryegrass (intermediate diploid cv. Talbot and tetraploid cv. Barlatra, and late diploid cv. Parcour and tetraploid cv. Petra) were each sown at 10,20 and 30 kg ha^-1, all with 3 kg ha^-1 of white clover cv. Donna. Herbage productivity was measured over 3 harvest years, 1982–84. under two annual rates of fertilizer N (0 and 150 kg ha^-1); the 150 kg ha^-1 rate was split equally between March and August applications. Fertilizer N increased total herbage DM production; the 3-year means for the 0 and 150 kg ha^-1 N rates were 8·04 and 8·91 t ha^-1, respectively. In successive years, total herbage responses to N (kg DM (kg N applied)^-1) were 6·6, 35 and 72 (overall mean, 58). Mean white clover DM production over the 3 years was reduced from 4·48 t ha^-1 at nil N to 2·82 t ha^-1 at the 150 kg ha^-1 rate, a fall of 37%. Grass seed rate did not influence total herbage production or white clover performance. The two intermediate perennial ryegrass cultivars had a marginal advantage in total herbage production over the two late cultivars, but white clover content and production were higher with tetraploids than diploids. It is concluded that the value of increased herbage production from strategic use of fertilizer N has to be weighed against its depressive effect on white clover performance; application of 75 kg ha ha^-1 N in both spring and autumn was excessively high if maintenance of a good white clover content in the sward is an objective. There is considerable flexibility in the grass: clover seed ratio in seeds mixtures. Modern highly-productive perennial ryegrass varieties do not differ substantially in compatibility with white clover but tetraploids permit better clover performance than diploids.     

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.     

Some effects of added phosphorus on perennial ryegrass—white clover swards

The effect of applying 100kg P ha^-1 per year in the form of triple superphosphate to mixed swards of perennial ryegrass ( Lolium perenne ) and white clover ( Trifolium repens ) was examined on a soil of low P status.
The dry matter yield of total herbage was increased by 10'/i by added P. In the year of sowing the ryegrass benefited more than the clover from added P: in the subsequent four years the two species benefited equally. Both species responded 10 added P to a similar extent in terms of leaf size; the clover responded less well than the ryegrass in terms of rate of leaf emergence. However, clover responded positively to added P in terms of stolon internode length, length of stolon per m² and number of growing points per m². It is suggested that the application of P may promote the spread of white clover within an open sward, but that its application may not enhance the competitive power of white clover when growing with vigorous grasses.     

Growing point dynamics and spring growth of white clover in a mixed sward and the effects of nitrogen application

The effects of N applied in spring on the growth and development of white clover ( Trifolium repens L.) in a mixed sward were investigated at Uppsala, Sweden. In early spring, the plots received 0 or 90 kg N ha⁻¹ in the form of nitrochalk. The formation and loss of stolon branches were followed on marked stolons. The proportion of white clover in the total above-ground dry matter (DM) of the sward fell substantially in both treatments during the 8-week growth period, but more so in N fertilized than in unfertilized plots; fertilizer N increased the accumulation of grass DM, whereas it reduced the accumulation of white clover DM. Over time, there was a loss of white clover growing points and this loss was greatest in plots where N was applied. Branching occurred predominantly during the first half of the growth period and was unaffected by N application, whereas the loss of branches was more confined to the latter period and was increased in plots receiving N. Irrespective of node position, the size of the branches that died was comparatively small, and more branches were lost from young than from old node positions. It is concluded that the decline in the number of growing points in the N fertilized sward was mainly due to an increased loss of recently formed branches. As a consequence of its stimulating effect on grass growth, the N fertilized treatment initially led to a reduction in the R:FR ratio of light and subsequently also in the availability of photosynthetically active radiation within the canopy. These changes in the light environment of the sward caused morphological adjustments to be made by the white clover and may have contributed to the observed loss of branches.     

The combined use of fertilizer nitrogen and white clover on grazed ryegrass swards

A 1-year grazing experiment with dairy cows is described in which the milk yield and herbage intake from a sward of S23 perennial ryegrass alone receiving fertilizer N at an annual rate of 360 kg ha⁻¹ were compared with those from a ryegrass-white clover var. Blanca sward given a total N application of 180 kg ha⁻¹. Both treatments gave virtually the same total milk yield of about 12500 kg ha⁻¹ over a 20-week grazing season.     

Dynamics of perennial ryegrass and white clover in sown swards in NW Greece

Seasonal dynamics of white clover and perennial ryegrass were examined in sown perennial ryegrass/white clover swards subject to a 2 × 2 factorial treatment combination of defoliation (rotational grazing by sheep and cutting) and nitrogen fertilizer application (0 and 40 kg N ha^–1 year^–1) in NW Greece. Sward surface height and percentage cover were measured before and after five defoliation periods in 1996 within permanent microplots (30 × 30 cm, divided into nine cells) in which white clover was either initially present or absent. Both white clover and perennial ryegrass achieved maximum height and cover in April–May. Defoliation treatment and whether white clover was present initially significantly affected height and cover of both species. Total plant cover was similar prior to all defoliation periods except in July, a time of drought. Cover of perennial ryegrass was greater where white clover was initially absent, but total plant cover was greater in microplots containing white clover and the extent of the differences varied during the year. In contrast, N fertilizer application had little effect on species cover, other than small reductions in white clover cover. When white clover was present in April, it was found in virtually every microplot cell until July, but if it was absent in April there was little colonization of the microplot.     

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.     

Lactation performance of spring-calving dairy cows grazing mixed perennial ryegrass/white clover swards of differing composition and height

An experiment was designed to examine the changes in clover content of three mixed perennial ryegrass/white cover swards of differing initial clover contents subjected to different grazing height management regimes and their effect on lactation performance of 48 Friesian dairy cows and heifers. Two paddocks were established for each treatment and grazed on alternate days. Treatments T17 and Tl3 consisted of swards with initial clover contents of 0·17 and 0·13 of the dry matter (DM) mass, respectively, grazed to maintain compressed sward heights of 6 cm throughout the season. A third treatment, SI5, consisted of a sward with an initial clover content of 0·15 grazed to maintain a compressed sward height of 4·5 cm for the first 78 days of the grazing season (period 1). Throughout period 1, half the animals on each treatment each received 4 kg of a concentrate supplement daily, while the others remained unsupplemented. From days 79 to 90, the cattle on treatment S15 grazed a similar sward, while the compressed sward height of the S15 paddocks was allowed to increase to 6 cm before re introduction of the animals. The three swards were then grazed for a further 47 days (period 2) before the animals were housed and milk yield recorded for a further 63 days (period 3). While sward T17 showed little change in clover content over the first 29 days of grazing, remaining at just below 0·18 of DM mass, swards T13 and S15 showed a marked decline in clover content to 0·05 and 0·07 of DM mass respectively. However, by the end of period 1 the clover content of all three swards had increased markedly (0·25, 0·15 and 0·15 of DM mass respectively). By the end of period 2, clover proportions were slightly higher than initial values (0·19. 0·15 and 0·15 of DM mass for treatments T17, T13 and S15, respectively). Owing to the relatively small differences in clover content of swards TI7 and T13, there were no significant effects of these two treatments on milk yield or composition in any period. Supplementation had no effect on milk composition and had little effect on milk yield, except when sward height was maintained at 4·5 cm. There was no carryover effect of supplementation on milk yield or composition in periods 2 or 3.