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.     

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.     

Changes in contribution of white clover to canopy structure in perennial ryegrass/white clover swards in response to N fertilizer

Twelve plots were laid down on an existing perennial ryegrass/white clover sward, one plot in each of six replicated blocks receiving 100 kg  N ha⁻¹ (100N) and one plot receiving no N (0N). Biomass, canopy development (stratified cuts and point quadrat records at 2–4-week intervals) and changes in stolon population density were recorded during one 8-week regrowth period (25 July–23 September) to investigate the likely causes of N effects on white clover in mixed swards.
Over the period, N fertilizer resulted in an increase of 74% in perennial ryegrass biomass and a reduction of 24% in white clover biomass. There was also a reduction of 44% in stolon growing point density, mainly due to lower density of younger stolon branches. White clover's contribution to the upper three leaf area index (LAI) units (taken as an estimate of the proportion of photosynthetically active radiation (PAR) intercepted) was, on average, 70% at 0N producing 74% of the sward biomass, compared with 46% contribution to interception and 37% contribution to biomass at 100N.
While there was no evidence of overtopping, it is concluded that N fertilizer application increased the LAI of perennial ryegrass in the upper layers of the canopy thereby reducing the share of available PAR to white clover. This, coupled with a lower radiation use efficiency at high N and lower population density, results in white clover's reduced performance in mixed swards receiving N fertilizer.     

Effects of white clover cultivar and companion grass on winter survival of seedlings in autumn-sown swards

The aim was to study the effects of white clover cultivar and combinations with perennial ryegrass cultivars on seedling establishment in autumn‐sown swards and on winter survival of seedlings. Large‐leaved white clover cv. Alice and small‐leaved white clover cv. Gwenda, and an erect and a prostrate perennial ryegrass cultivar were sown in autumn in pure stands and as four binary grass‐clover mixtures. Mixtures of white clover cv. Huia and Aberherald with perennial ryegrass were also sown. Companion grasses had no significant impact on the establishment of white clover. The number of seedlings of white clover cv. Alice in mixtures (335 m^?2) was higher than cv. Gwenda (183 m^?2) and pure swards had similar white clover population densities as mixed swards. White clover cv. Huia tended to have more seedlings than Aberherald (355 and 205 m^?2 respectively). No stolons were produced prior to a severe winter, because of the late sowing date. Winter survival of clover seedlings was 0·56 in mixtures and 0·69 in pure stands, irrespective of white clover or companion grass cultivar. Stolon development of white clover in autumn is often considered essential for overwintering survival and spring growth. In this study, there was considerable survival of the non‐stoloniferous tap‐rooted seedlings of all four clover cultivars despite a severe winter.     

Simulation of nitrogen uptake, fixation and leaching in a grass/white clover mixture

To represent nitrogen cycling in a low input grass/legume pasture system, a previously developed, weather-driven grass/white clover growth model has been adapted to become the crop growth component of the soil nitrogen dynamics model SOILN. This provides a means of simulating nitrogen uptake by a grass/white clover crop, an important component of the overall nitrogen balance in low-input grassland systems.
Crop growth is represented by a photosynthesis equation adapted to take account of competition between the two crops for resources of light, water and nitrogen in the soil. Water shortage is represented by linked simulations with the soil water and heat model SOIL, and nitrogen shortage by links with the SOILN model. Nitrogen fixation has been introduced according to an equation for potential fixation reduced by environmental factors, particularly temperature. Transfer of nitrogen-rich clover plant material to the soil nitrogen pools of SOILN (from where it becomes available as a nutrient for grass) is also represented. The model is tested by comparing simulated cut crop yields and nitrogen content of cut material with measured data from perennial ryegrass/white clover at a test site. Soil nitrogen processes in the model are tested by comparing simulated and measured nitrate in drainflows. Apart from some discrepancies between simulated and measured results attributable to the inherent instability of a mixed crop system, agreement is reasonable by the standards of biological system models, indicating that the combined model gives a realistic representation of carbon and nitrogen processes in grassland with a grass, legume mixed crop.     

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 acetylene reduction assay as a means of studying nitrogen fixation in white clover under sward and laboratory conditions

The acetylene reduction assay was used to follow seasonal changes in nitrogen fixation activity in a white clover-perennial ryegrass ley in Northern Ireland. The annual estimate for fixation by the ley was 268 kg ha^?1 (239 lb/acre) nitrogen virtually all of which was fixed during March to October. Nitrogen fixation was curtailed drastically after the ley was cut but recovered as new foliage expanded on the clover. Glasshouse experiments described the effects of temperature, shading and defoliation on nitrogen fixation by white clover, and indicated that these factors might be important in modifying symbiosis under field conditions.     

Sward composition, animal performance and the potential production of grass/white clover swards continuously stocked with sheep

The potential productivity of perennial ryegrass/ white clover swards (GC) under continuous stocking management was assessed by comparing their performance, when grazed by sheep at sward surface heights of 3, 6 and 9 cm, with that of an all–grass sward (G) maintained at 6 cm and fertilized with 420 kg N ha^–1 The grass/clover swards received no nitrogen fertilizer. The different grazing treatments had a marked effect on animal performance. In the first year for example, for treatments GC3, GC6, GC9 and G6–420 respectively, mean stocking rates to weaning were 19–7, 14–3, 8–9 and 18–4 ewes ha^–1 (plus twin lambs); lamb growth rates were 223, 268, 295 and 260 g d^–1and so total lamb live weight gain was 1054, 920, 630 and 1148 kg h a^–1. The relative performance of the treatments was similar in all three years. All three grazing treatments had a similar effect on the composition of the grass/clover swards. Clover content increased in 1985, and was sustained in 1986 and 1987 during the main grazing season, although a marked decline in clover content during the winter led to a progressive long–term decline in both the proportion and the amount of clover.
It is suggested that a management based on maintaining a sward surface height close to 6 cm (as in all–grass swards) leads to optimum performance in grass/white clover swards grazed using continuous stocking with sheep. Despite the presence of a small and declining clover content, the output of the mixed grass/clover sward managed in this way was 80%, 80% and 82% of that of a grass sward supplied with 420 kg N ha^–1 in 1985, 1986, and 1987 respectively and, similarly, 83% of the output in 1987 of a grass sward receiving 210 kg N ha^–1.     

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.     

The response of perennial ryegrass/white clover mini-swards to elevated atmospheric CO2 concentrations: effects on yield and fodder quality

In order to assess the effects of future elevated atmospheric CO₂ concentrations on yield, mineral content and the nutritive value of mixed swards of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), both species were grown as monocultures and as different mixtures and were exposed season-long to ambient (380 p.p.m.) and elevated (670 p.p.m.) CO₂ concentrations in open-top chambers. Mini-swards were cut four times at about monthly intervals at a height of 5 cm, dry-matter yields were determined and content of macroelements (N, P, K, S, Mg, Ca, Na) and crude fibre, crude protein and ash content were measured. The CO₂-related increase in seasonal yield amounted to 16–38% for white clover monocultures, 12–29% for mixed swards and 5–9% for ryegrass monocultures. The white clover content of all swards was significantly enhanced by elevated CO₂. The K and Na content of total yield was decreased by high CO₂ but did not fall below the minimum requirements for ruminants. As the Ca content of total yield was increased by elevated CO₂ and the P content was not changed, the Ca/P ratio of total yield was increased and exceeded values required for animal nutrition. The crude protein content of total yield was reduced by high CO₂ at the beginning of the growing season only and was increased by elevated CO₂ in the course of the experiment, whereas crude fibre content was decreased throughout the season, sometimes falling below the minimum requirement for ruminants. Removal of N, P, S, Mg and Ca by cutting was significantly enhanced because of CO₂ enrichment. The results show that, besides the positive effect of rising atmospheric CO₂ on dry-matter yield of white clover/ryegrass swards, impacts on the nutritive value should be expected. Possible changes in species composition and implications for grassland management are briefly discussed.     

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.     

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

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

The effect of plane of winter nutrition and sward height on the performance of steers grazing grass/white clover swards

An experiment was carried out over 2 years to examine the interactions between two planes of winter nutrition and summer gracing at 5.5 and 7.5 cm compressed sward height on the performance of Limousin X Friesian steers grazing grass/white clover swards. Diets were offered in winter to give liveweight gains of either 0.5 (low) or 0.8 (high) kg d^?1. The experiment was repealed over 2 years. Liveweights gains (low 0.50 vs. high 0.84 kg d^?1 s.e.d. ±0.044) were achieved in winter 1 and (low 0.55 vs. high 0.91 kg d^?1, s.e.d. ±0.041) in winter 2. These differences resulted in animals from the high treatment being 44 and 60 kg head^?1 heavier at turn-out than the low-treatment animals in years I and 2, respectively. There was evidence of compensatory growth with animals from the low treatment subsequently tending to grow faster than those from the high treatment, with liveweight gains during the period from turn-out to 84 d of 1.27 vs. 1.18 s.e.d. ±0.65 kg d^?1; P= 0–213 and 1.11 vs. 0.95 s.e.d. ±0.062 kg d^?1; P=0.015 in summers I and 2 respectively. In general, animals grazing the short sward tended to grow more slowly than those on the tail swards (1.18 vs. 1.27 s.e.d. ±0.065 kg d^?1; P= 0.166 and 0.93 vs. 113 s.e.d. ±0.062 kg d^?1, P = 0.002) for years 1 and 2 respectively. Growth rates in year 1 were significantly higher than those in year 2. However, increased summer growth rates did not compensate for the differences in live weight established during the winter, and more animals reached slaughter weight in a shorter time from the high than the low treatment. Mean stocking rate on treatments 5.5 and 7.5 over the 2 years were 5.2 and 4.3 animals ha^?1: differences were significant in year 2 (P <0.01). The 5.5 cm sward treatment also gave a greater liveweight gain ha^?1 than the 7.5-cm sward treatment in both years with a mean for the two years of 670 vs. 572 kg ha^?1 but differences were not significant.     

The influence of autumn management and companion grass on the development of white clover over winter in mixed swards

Three experiments designed to investigate different facets of autumn management on white clover stolon development are described. The effects of defoliation interval (2, 4, 6 and 8 weeks during 16 weeks from 27 July) were investigated. The shortest interval resulted in the shortest length of stolon material per unit area but cutting interval had no effect on growing point density nor on hardiness of stolon tips evaluated in October, December and January.
Chemical grass suppressants were employed to reduce grass biomass during winter in two experiments to evaluate the influence of grass on white clover development. One experiment involved varying grass tiller density by spraying a perennial ryegrass/white clover sward in October with three rates of three chemical suppressants (Clout, Kerb and Checkmate). Although tiller and clover growing point density were inversely related in January, the overall relationship was not strong.
Clout at l·5kg a.i. ha⁻¹ was sprayed in October on one of two subplots in each of twelve grazed grass/white clover plots that had been maintained at 7 or 9 cm from July to October then grazed to 3–4 cm with sheep. Sward height had no effect on clover population density but the shorter sward had a greater mean node number per secondary stolon branch. By March, suppressing grass resulted in more than double the stolon population density, a higher proportion of plants with tertiary and quaternary branches, and on marked stolons, five times more branches and 60% higher dry matter (DM) produced during winter but with shorter petioles compared with clover in untreated plots.
It is concluded that white clover has the capacity to branch during a mild winter and as stolon branch numbers can suffer a net loss as a result of the presence of the grass canopy, management that controls grass growth during winter should aid over-wintering and improve persistence of white clover.     

A comparison of grass/white clover and grass silages offered to dairy cows as the sole feed

Two diets were compared: perennial ryegrass ( Lolium perenne L.) silage and perennial ryegrass/white clover ( Trifolium repens L.) silage, in each case fed ad libitum , without supplementation, to lactating dairy cows. The comparison was made with silages cut on each of four dates. The crops were wilted to a dry matter content of at least 256g kg^-1 and no additives were used.
The grass/clover silages were well preserved at all four cuts. The grass was well preserved at three cuts, but, at a May cut, the grass silage was less well preserved and less digestible than the grass/clover silage. When the May silages were fed, milk yield was higher with grass/clover than with grass. Taking the experiment as a whole, however, milk yield and composition were similar on the two diets. The grass/clover silages had a lower proportion of cell wall and their intake by dairy cows was consistently higher than that of the grass silages.
It is concluded that perennial ryegrass/white clover crops can be ensiled successfully and fed successfully, with high intake, to lactating dairy cows, but it should not be assumed that cows will give more milk than when fed equivalent all-grass silage.     

Turnover of grass laminae and white clover leaves in mixed swards continuously grazed with steers at a high-and low-N fertilizer level

Turnover rates of grass laminae and clover leaf tissue were estimated over a range of intervals within three periods each year in the second to fourth years (1983-85) of a trial involving swards continuously grazed by steers and receiving either 60 kg N ha-1 in spring (60N) or 360 kg N ha^?1 throughout the year (360N). Within the 60N swards initial stocking rates at turnout were low (60N LS) at 7-2 steers ha^?1 and high (60N HS) at 90 steers ha^?1 in 1983, and in 1984 and 1985 corresponding rates were 10-8 and 13-5 ha^?1. The 360N swards were initially stocked at turnout at 96 (360N LS) and 120 (360N HS) steers ha^?1. Stocking rates were reduced by 33% in midsummer except for 60N in 1984 and 1985 when they were reduced by 50%. Meaned over 3 years, 360N HS had lower herbage mass than 60N LS. Tiller density in 360N was almost 50% higher than in 60N and clover growing point density was only one quarter that of 60N with the 60N LS having lower clover densities than 60N HS in 1985. Generally, leaf extension rate per tiller was higher in 360N than 60N and, when significant, 60N LS had higher senescence rates per tiller than 360N HS. Rate of increase in new clover lamina tissue per stolon was not affected by treatments, whereas in 1983 LS had higher senescence rates of clover laminae than HS. Petiole growth per stolon was higher in LS than HS in 1983 and 1984, the mean over these years for 360N HS being 77% that of 60N LS. Petiole senescence per stolon was lower in 360N HS than 60N LS only in 1983. When comparing 60N HS and 360N LS (representing similar levels of grazing intensity, having similar herbage mass) the gross growth of leaf material in the former was 75% of the latter, in contrast to 57% for net growth. Clover contributed 18% to the estimated growth of leaves compared to a mean of 7% in herbage mass. Taking inflorescence and pseudostem into account in 1984 and 1985,60N HS had 7% clover in standing herbage and 14% in net growth. Therefore, the contribution of clover to growth is considerably higher than its presence in herbage mass would suggest in continuously grazed swards. It is concluded that low-N swards, owing to their lower tiller density and slower grass leaf extension rate, will be less efficiently grazed than swards at higher N levels at a given herbage mass, but the presence of clover will partly offset that disadvantage.     

Herbage and nitrogen yields,fixation and transfer by white clover to companion grasses in grazed swards under different rates of nitrogen fertilization

In grass–legume swards, biologically fixed nitrogen (N) from the legume can support the N requirements of the grass, but legume N fixation is suppressed by additional fertilizer N application. This study sought to identify a fertilizer N application rate that maximizes herbage and N yields, N fixation and apparent N transfer from white clover to companion grasses under intensive grazing at a site with high soil‐N status. During a 3‐year period (2011–2013), swards of perennial ryegrass and of perennial ryegrass–white clover, receiving up to 240 kg N ha^?1 year^?1, were compared using isotope dilution and N‐difference methods. The presence of white clover increased herbage and N yields by 12–44% and 26–72%, respectively. Applications of N fertilizer reduced sward white clover content, but the effect was less at below 120 kg N ha^?1. The proportion of N derived from the atmospheric N fixation was 25–70%. Nitrogen fixation ranged from 25 to 142 kg N ha^?1 measured using the isotope dilution method in 2012 and from 52 to 291 kg N ha^?1 using the N‐difference method across all years. Fertilizer N application reduced the percentage and yield of fixed N. Transfer of N from white clover to grass was not confirmed, but there was an increased N content in grass and soil‐N levels. Under intensive grazing, the maximum applied N rate that optimized herbage and N yields with minimal effect on white clover content and fixation rates was 60–120 kg N ha^?1.     

The influence of defoliation height on dry-matter partitioning and CO2 exchange of perennial ryegrass miniature swards

This study investigated the effects of defoliation intensity on the above- and below-ground plant mass, rates of CO₂ exchange and leaf appearance rate of ryegrass miniature swards maintained at constant cutting height ranging from 20 mm to 160 mm for 5 months. Total plant mass, above-ground herbage mass and root mass increased as cutting height increased from 20 to 120 mm. Further increase in cutting height did not increase total plant mass or its components. Leaf appearance rate and photosynthesis per unit of leaf dry matter (DM) decreased as defoliation height increased from 20 to 160 mm. Gross and net CO₂ uptake per unit soil surface area increased with cutting height to 120 mm. Further increase in cutting height to 160 mm decreased gross and net CO₂ uptake and herbage harvested. A multivariate canonical discriminant analysis indicated different responses of root and shoot mass to cutting height and a reduction in CO₂ uptake rate at the 160 mm cutting height. The implications of those responses to defoliation management of forage plants are discussed.     

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.     

Variation in rate of phenological development and morphology between red clover varieties: Implications for clover proportion and feed quality in mixed swards

Red clover (Trifolium pratense) grown in mixtures with grasses often constitutes a lower proportion of total yield in spring than in summer growth. A more even red clover proportion between the harvests would benefit forage quality and management at feeding. We investigated whether inclusion of early versus late‐maturing red clover varieties could reduce this disproportionality. In a two‐year field trial harvested three times per season, each of six red clover varieties was grown in two grass mixtures. Rate of phenological development did not differ during spring growth, but did so in regrowth after first and second cuts. Here, the earliest varieties constituted the highest proportion. At all harvests, the early varieties had lower crude protein concentrations and a higher content of neutral detergent fibre (NDF) and indigestible NDF than the late varieties. Clover proportion was higher in swards with a mixture of timothy and meadow fescue than in swards with perennial ryegrass during the first year and lower in the second year. It is concluded that developmental rate should be explored further as a key character for red clover competiveness in spring growth of rapidly elongating grasses.