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.     

Aspects of the productivity of forage legumes in Northern Europe

A database on the productivity of red clover (Trifolium pratense), white clover (Trifolium repens), lucerne (Medicago sativa), lotus (Lotus corniculatus) and galega (Galega orientalis) was compiled. It contains 1852 observations for 330 trials at fifty‐three sites in eight north European countries for the period from 1977 to 1997. The database includes information on climatic and site characteristics, as well as agronomic data, collated into four smaller data sets. This has permitted four key agronomic issues to be examined, namely: (i) how yield for a given variety varies under different environmental conditions; (ii) how the relative performance of different species varies between sites; (iii) how persistence varies between species and sites; and (iv) what the nature of the relationship is between varieties, yield and sites within a species? Cluster analysis revealed that geographical location had a fairly strong influence on yield. Across sites for an individual variety, the cumulative day‐degrees during the regrowth period had the highest correlation with total yield. In terms of the relative performance of different forage legume species across sites, red clover and lucerne were estimated to yield about 2·5 t DM ha^?1 more than white clover and there was a significant impact of cumulative day‐degrees during regrowth and the age of ley on the total yield. It was found that red clover had the lowest persistence index and lucerne the highest. As regards the interaction between yield, varieties and sites, the length of growth period and the age of the ley explained two‐thirds of the variation in total yield in red clover varieties of contrasting maturity types.     

The implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK: 6. The effect of reducing stocking rate and application of fertilizer nitrogen in Wales

Abstract The implications for the agricultural productivity of the UK upland sheep systems of reducing nitrogen fertilizer application and lowering stocking rates on perennial ryegrass/white clover swards were studied over 4 years at a site in Wales. The system involved grazing ewes and lambs from birth to weaning on swards maintained at a constant height with surplus herbage made into silage, thereafter ewes and weaned lambs grazed on separate areas until the onset of winter with adjustments to the size of the areas grazed and utilizing surplus pasture areas for silage. Four stocking rates [SR 18, 15, 12 and 9 ewes ha^?1 on the total area (grazed and ensiled)] and two levels of annual nitrogen fertilizer application (N 200 and 50 kg ha^?1) were studied in five treatments (N200/SR18, N200/SR15, N50/SR15, N50/SR12 and N50/SR9). Average white clover content was negatively correlated with the level of annual nitrogen fertilizer application. White clover content of the swards was maintained over the duration of the experiment with an increasing proportion of clover in the swards receiving 50 kg N ha^?1. Control of sward height and the contribution from white clover resulted in similar levels of lamb liveweight gain from birth to weaning in all treatments but fewer lambs reached the slaughter live weight by September at the higher stocking rates and with the lower level of fertilizer application. Three of the five treatments provided adequate winter fodder as silage (N200/SR15, N50/SR12 and N50/SR9). Because of the failure to make adequate winter fodder and the failure of white clover to fully compensate for reduction in nitrogen fertilizer application, it is concluded that nitrogen fertilizer can only be reduced on upland sheep pastures if accompanied by reduced stocking rates.     

Competition in perennial ryegrass–white clover mixtures under cutting. 1. Dry-matter yield, species composition and nitrogen fixation

Persistence of white clover (Trifolium repens) in mixtures was studied in a long-term experiment. Mixtures of two cultivars of perennial ryegrass (Lolium perenne) with contrasting growth habits and three white clover cultivars differing in leaf size were sown in 1991 and evaluated at two cutting frequencies. During 1995 and 1996 mixtures with large-leaved white clover cv. Alice had the highest dry-matter (DM) content, clover and N yield, and the highest white clover content, and mixtures with medium-leaved Retor the lowest, whereas mixtures with small-leaved Gwenda yielded most grass DM. In 1995 averaged over cutting treatments and mixtures, the mixtures yielded 11·8 t DM ha^?1 with a white clover content of 0·6; the apparent N fixation was 393 kg N ha^?1. In 1996 these values declined to 8·5 t DM ha^?1, 0·48 white clover and 236 kg N ha^?1. There was no significant effect of cutting frequency on DM yield or white clover content, whereas the effects of grass cultivar were not consistent. In spring there was a peak in the DM production of the mixtures, coinciding with a peak in production of the grass component. However, in summer and autumn the seasonal pattern of DM production of the mixtures was similar to that of the white clover component. Both cultivars of perennial ryegrass showed the same seasonal response, but the seasonal growth pattern of white clover differed slightly between clover cultivars and cutting treatments. In later years only one cutting frequency was imposed, and no yield measurements were taken. White clover was judged to have performed well during 1997; the clover content in September was very high (0·76), whereas in October 1998 it was 0·45. Mixtures with Alice contained most white clover. Despite fluctuations in white clover content during 1991–98, all clover cultivars had persisted 7 years after sowing, irrespective of companion grass cultivar, at both cutting treatments.     

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.     

Effects of rates and times of application of starter dressings of nitrogen fertilizer to surface sown perennial ryegrass-white clover on hill peat

The effects on clover and grass growth of five levels of ammonium nitrogen (NH₄-N) applied before sowing, at sowing and after nodule initiation have been investigated. A pot experiment in which S184 white clover was grown in a peat soil showed that NH₄-N up to 688 mg N per pot (approx. equivalent to a field rate of 120 kg ha^-1 N) applied before sowing and at sowing did not affect clover growth. N-fixing activity (C₂H₂-reduction), on the other hand, was reduced progressively up to the highest level (688 mg N per pot). Application after nodule initiation increased growth relative to the zero-N treatment at all levels of application. Maximum growth and N fixation occurred at 516 mg N per pot where the DM yield was 70% higher than in the absence of added N. A field trial in which S184 clover and S24 perennial ryegrass were surface sown on to a peat soil showed an increase in grass and clover growth in the first year in response to 120 kg ha^-1 N applied at sowing. Grass growth alone was increased at 120 kg ha ^-1 N applied 40 d before sowing. Lower rates of application before sowing and at sowing did not affect clover or grass growth. The effect of the delayed application of NH₄-N on legume growth was less marked than that in the pot experiment, 90 kg ha^-1 N stimulating clover growth by 40% in the first year. The effect was however different from that in the pot experiment, in that, whilst 30kg ha^-1 N increased N fixation relative to the zero-N treatment, plants exposed to higher levels showed a depression in N-fixing capacity. N-fixation was correlated with nodule numbers in the delayed NH₄-N application, the closest correlation being with the number of multilobed nodules which was highest at 30 kg ha^-1 N and lowest at 120 kg ha^-1 N. It is suggested that circumstances exist when the use of a relatively low starter N dressing (20–60 kg ha^-1 N) at sowing would not increase clover or grass growth in the early stages of the establishment of a hill reseed. Under such circumstances higher rates of application (100 kg ha ^-1 N), preferably delayed until the seedlings are in a position to take up the nitrogen rapidly, would have the greatest effect.     

Effects of an autumn defoliation on overwintering, spring growth and yield of a white clover/grass sward

An established sward of binary mixtures of meadow fescue (Festuca pratensis) and white clover (Trifolium repens) (either AberHerald, Grasslands Huia or Sandra) was subjected to (A) no further defoliation, (B) a defoliation in late September or (C) a defoliation in late October after four harvests had been taken during the grazing season. About a tonne of dry matter (DM) was removed by the autumn defoliations. There were two levels of nitrogen application in spring, either 0 or 90 kg ha^?1. The development of grass and clover morphology and population sizes from early autumn until the first harvest the following year was followed by regular sampling of the above-ground material. Stolons were analysed for total non-structural carbohydrates (TNCs), and the temperature at stolon level was continuously recorded. There were no interactions between autumn defoliation, clover cultivar or nitrogen treatments on any of the parameters studied. White clover growing-point numbers and stolon morphological characteristics were reduced in size during the winter and did not recover during the spring. A defoliation in late September resulted in the greatest reduction, whereas there were no differences between the other two treatments. The grass tiller population increased from early autumn until the last sampling occasion in May, but both autumn defoliations resulted in a smaller increase. Defoliation in late September had the greatest impact. The TNC content of white clover stolons fell from about 350 g kg^?1 to 150 g kg^?1 DM from late autumn until late April. There were small differences between the treatments, but a defoliation in late September resulted in a significantly lower level in late autumn. The temperature amplitude at stolon level was consistently greater in plots defoliated in late September. Total DM harvested in spring was 4367, 2564 and 3536 kg ha^?1, of which 388, 352 and 460 kg ha^?1 was white clover, from treatments A, B and C respectively. It is concluded that an autumn defoliation may affect the overwintering of white clover negatively, but that the effect on the grass may be even more detrimental.     

Improving the effectiveness of wood‐ash fertilization in mixed mountain pastures

Mountain pastures can be improved by the application of wood ash, but its effectiveness as a source of N, P and other elements requires improved management practices. Two application rates of wood ash (6 and 12 t ha^?1) were tested in a 4‐year trial on a low‐input fertilized pasture. Ash was complemented with mineral P, and N fixation was encouraged by sowing white clover. Wood‐ash application led to increased pasture yield (by 100%) and to increased proportions of white clover and ryegrass (60%), thus improving pasture feed value. N fixation by white clover, and possibly increased mineralization because of higher pH, may have compensated for lack of N in the wood ash. Addition of mineral fertilizer improved the supply of P, although incorporation of the wood ash into the soil may also have enhanced release of P from non‐extractable components. High amounts of compounds containing Ca and Mg contributed to the long‐term maintenance of soil reserves of these elements. Despite high inputs of K (175–350 mg kg^?1) from wood ash, annual additions of K are required to counteract depletion via plant uptake and leaching. Increased Mn availability after the treatment (up to 400 mg kg^?1) may limit repeated applications of wood ash, at least on poorly aerated soils.     

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 implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK. 5. The effect of stocking rate and reduced levels of nitrogen fertilizer

The implications for UK upland sheep systems of reducing nitrogen fertilizer application to perennial ryegrass/white clover swards were studied over 3 years. Sward height (3·5–5·5 cm) was controlled for ewes with lambs until weaning using surplus pasture areas for silage; thereafter, ewes and weaned lambs were grazed on separate areas, and sward height was controlled by adjusting the size of the areas grazed and using surplus pasture areas for silage if necessary. Combinations from three stocking rates [10, 6 and 4 ewes ha⁻¹ on the total area (grazed and ensiled)] and four nitrogen fertilizer levels (150, 100, 50 and 0 kg ha⁻¹) provided six treatments that were replicated three times. Average white clover content was negatively correlated with level of nitrogen fertilizer. The proportion of white clover in the swards increased over the duration of the experiment. Control of sward height and the contribution from white clover resulted in similar levels of lamb liveweight gain on all treatments. All treatments provided adequate winter fodder as silage. It is concluded that the application of nitrogen fertilizer can be reduced or removed from upland sheep pastures without compromising individual animal performance provided that white clover content and sward height are maintained. Resting pastures from grazing by changing ensiled and grazed areas from year to year sustained white clover content over a 3-year period.     

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.     

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.     

Performance of white clover/perennial ryegrass mixtures under cutting

Clover persistence in mixtures of two varieties of perennial ryegrass (Lolium perenne) with contrasting growth habits and three white clover (Trifolium repens) varieties differing in leaf sizes was evaluated at two cutting frequencies. An experiment was sown in 1991 on a clay soil. The plots received no nitrogen fertilizer. In 1992, 1993 and 1994, mixtures containing the large-leaved clover cv. Alice yielded significantly more herbage dry matter (DM) and had a higher clover content than mixtures containing cvs Gwenda and Retor. Companion grass variety did not consistently affect yield or botanical composition. Cutting at 2 t DM ha^?1 resulted in slightly higher total annual yields than cutting at 1.2 t DM ha^?1, but did not affect clover content. In 1992 the mixtures yielded, depending on cutting frequency and variety, 10·6–14·6 t DM ha^?1 and 446–599 kg ha^?1 N, whereas grass monocultures yielded only 1·2–2·0 t DM ha^?1 and 25–46 kg ha^?1 N. From 1992 to 1994 the annual mean total herbage yield of DM in the mixtures declined from 12·2 to 10·5 to 8·7 t ha^?1, the white clover yield declined from 8·7 to 6·5 to 4·1 t ha^?1 and the average clover content during the growing season declined from 71% to 61% to 46%, whereas the grass yield increased from 3·4 to 4·0 to 4·5 t ha^?1. The N yield decreased from 507 to 406 to 265 kg N ha^?1 and the apparent N fixation from 470 to 380 to 238 kg N ha^?1. Nitrate leaching losses during the winters of 1992–93 and 1994–95 were highest under mixtures with cv. Alice, but did not exceed 10 kg N ha^?1. The in vitro digestible organic matter (IVDOM) was generally higher in clover than in grass, particularly in the summer months. No differences in IVDOM were found among clover or grass varieties. The experiment will be continued to study clover persistence and the mechanisms that affect the grass/clover balance.     

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.     

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.     

The implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK: 7. Sustainability of white clover in grass/clover swards with reduced levels of fertilizer nitrogen

The sustainability of white clover in grass/clover swards of an upland sheep system, which included silage making, was studied over 5 years for four nitrogen fertilizer rates [0 (N₀), 50 (N₅₀), 100 (N₁₀₀) and 150 (N₁₅₀) kg N ha^?1]. A common stocking rate of 6 ewes ha^?1 was used at all rates of N fertilizer with additional stocking rates at the N₀ fertilizer rate of 4 ewes ha^?1 and at the N₁₅₀ fertilizer rate of 10 ewes ha^?1. Grazed sward height was controlled, for ewes with their lambs, from spring until weaning in late summer by adjusting the proportions of the total area to be grazed in response to changes in herbage growth; surplus pasture areas were harvested for silage. Thereafter sward height was controlled on separate areas for ewes and weaned lambs. Areas of pasture continuously grazed in one year were used to make silage in the next year. For treatments N₀ and N₁₅₀, white clover stolon densities (s.e.m.) were 7670 (205·4) and 2296 (99·8) cm m^?2, growing point densities were 4459 (148·9) and 1584 (76·0) m^?2 and growing point densities per unit length of stolon were 0·71 (0·015) and 0·67 (0·026) cm^?1 respectively, while grass tiller densities were 13 765 (209·1) and 18 825 (269·9) m^?2 for treatments N₀ and N₁₅₀ respectively. White clover stolon density increased over the first year from 780 (91·7) cm m^?2 and was maintained thereafter until year 5, reaching 8234 (814·3) and 2787 (570·8) cm m^?2 for treatments N₀ and N₁₅₀ respectively. Growing point density of white clover increased on treatment N₀ from 705 (123·1) m^?2 to 2734 (260·7) m^?2 in year 5 and it returned to the initial level on treatment N₁₅₀ having peaked in the intermediate years. Stolon density of white clover was maintained when the management involved the annual interchange of continuously grazed and ensiled areas. The non‐grazing period during ensiling reduced grass tiller density during the late spring and summer, when white clover has the most competitive advantage in relation to grass. The increase in stolon length of white clover in this period appears to compensate for the loss of stolon during periods when the sward is grazed and over winter when white clover is at a competitive disadvantage in relation to grass. The implications for the management of sheep systems and the sustainability of white clover are discussed.     

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.     

Nutrient efficiency and cellular contents of phosphorus and nitrogen in Trifolium repens and a diploid and tetraploid Lotus uliginosus

Two cultivars of white clover (Ladino and Kent Wild White) and two cultivars of lotus (Grasslands Maku and G4703) were grown in pots of soil at low to moderate levels of phosphorus (P) supply. Nitrogen supply was by fixation. Cell size and nitrogen, phosphorus and chlorophyll content per cell were estimated from samples of leaf tissue. Maku lotus had larger cells and higher cellular contents of metabolites than the clovers. These measurements support earlier suggestions that, compared with white clover, the higher P-efficiency (dry matter per unit of internal P) of Maku lotus may be explained by cytological factors without recourse to explanations based on differences in nutrient metabolism. The relatively large cells and high metabolite levels per cell of Maku lotus appear to arise from its creation as an artificial tetraploid.     

Selection by sheep and goats for perennial ryegrass and white clover offered over a range of sward height contrasts

The selection by sheep (six Coopworth ewe hoggets, 44·3 ± 4·6 kg live weight) and goats (six Saanen/Anglo‐Nubian yearling males, 38·1 ± 3·8 kg live weight) for perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) and for sward height was measured in two experiments involving paired turves. Pairs of turves with herbage of differing height and of either the same or different plant species were offered. One sward (fixed height species, FHS) was always offered at 130 mm and the other (variable height species, VHS) at 130, 90 or 50 mm. Turves (450 mm × 220 mm) were cut to a soil depth of 100–150 mm from areas of perennial ryegrass and white clover regrown to the desired height after previously being cut to 30 mm. Each turf in a pair was weighed (±1 g) before and after grazing by penned animals maintained on a barley‐based pelleted diet. The number of prehending bites taken from each turf was recorded over a grazing period (128 ± 12 s). Bite mass, bite rate and intake rate were calculated. As the sward height of the VHS turf declined, an increasing proportion of the diet was selected from the 130 mm turf. When averaged over all height contrasts, both animal species selected a higher proportion (0·776 ± 0·026) of their diet from 130‐mm white clover than from 130‐mm perennial ryegrass (0·591 ± 0·018) turves. On average, goats selected a higher proportion (0·721 ± 0·022) of their dry‐matter (DM) intake from the 130‐mm turf than sheep (0·646 ± 0·019), but the effect was not consistent. In contrasts with perennial ryegrass as the VHS (and both perennial ryegrass and white clover as FHS), the proportion of the diet selected from the 130‐mm turf was very similar for both animal species. However, with white clover as the VHS (and both perennial ryegrass and white clover as FHS), goats selected a higher proportion of their intake from the 130‐mm turf to the extent that in the 130‐mm perennial ryegrass/50‐mm white clover contrast sheep showed as strong selection for 50‐mm white clover as goats did for 130‐mm perennial ryegrass. This lesser selection of goats for white clover as its height in a sward declines is likely to contribute to the higher white clover content observed in swards grazed by goats. Bite mass was greater on white clover (246 ± 5 mg DM bite^–1) than on perennial ryegrass (173 ± 5 mg DM bite^–1) and was greater for goats (255 ± 6 mg DM bite^–1) than for sheep (195 ± 5 mg DM bite^–1). Bite rate was greater on perennial ryegrass (45·9 ± 1·0 bites min^–1) than on white clover (39·9 ± 1·0 bites min^–1) and was greater for sheep (45·5 ± 1·1 bites min^–1) than for goats (42·5 ± 1·1 bites min^–1). Apparent intake rate by both sheep and goats was lower (mean, 5·0 ± 0·29 g DM min^–1) on 130 mm perennial ryegrass/white clover than on 130 mm perennial ryegrass/perennial ryegrass (7·0 ± 0·27 g DM min^–1), but was higher (9·62 ± 0·29 g DM min^–1) on 130‐mm white clover/perennial ryegrass than on 130‐mm white clover/white clover (8·2 ± 0·29 g DM min^–1) combinations.     

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.