Change in the balance of ammonium-N and nitrate-N content in soil under grazed grass swards over 7 years

The pool of nitrate-N (NO₃^–-N) in the soil is more prone to losses than that of ammonium-N (NH₄⁺-N) so any shift towards NO₃^–-N dominance in the soil pools, caused by management intensity, could have environmental implications. The change in the balance of soil NH₄⁺-N and NO₃^–-N content with time was studied using grazed grass swards receiving different fertilizer N inputs. In addition, the effect of past management on net nitrification of 400 μg NH₄⁺-N g^–1 was investigated in a soil incubation study. Mineral N was determined at frequent intervals (at least every 2 weeks) throughout the year in the top 5 or 7·5 cm of a sandy clay-loam soil at the Agricultural Research Institute of Northern Ireland at Hillsborough, County Down, for a 7-year period (1989–90 to 1995–96). The treatments were a perennial ryegrass–white clover sward receiving no fertilizer N, together with perennial ryegrass swards receiving 100, 200, 300, 400 or 500 kg N ha^–1 year^–1 as calcium ammonium nitrate. The plots were continuously grazed by beef steers from April to October to maintain a constant sward height of 7 cm. There was little or no change in average soil NO₃^–-N and NH₄⁺-N content from 1989–90 to 1995–96 on the grass–clover sward and plots receiving 100 and 200 kg N ha^–1 year^–1. However, with the plots receiving 300, 400 and 500 kg N ha^–1 year^–1 NO₃^–-N became progressively more dominant with time. The incubation study confirmed that this was due to an increase in net nitrification rate. There was evidence that rapid microbial assimilation of NO₃^–-N occurred during the soil incubations. Past management history can play an important role in determining soil NO₃^–-N content and hence potential losses of N to the environment.     

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.     

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.     

The effect of strategic use of fertilizer nitrogen in spring and/or autumn on the productivity of a perennial ryegrass/white clover sward

The productivity of a mixed sward, comprising perennial ryegrass cvs Barlano and Bastion and white clover cvs Donna and Aran, was measured under sixteen fertilizer N treatments. These involved 0.25, 50 and 75 kg N ha^-1 in spring only, in autumn only and in all combinations of spring N and autumn N. A simulated grazing regime of six cuts annually at 3- to 6-week intervals was imposed. Increasing rates of total N application increased total herbage DM regardless of application pattern. Yield response was greater with N applied in the spring, and total herbage DM was higher with high spring N-low autumn N than the reverse. Mean yield responses at the first harvest to 25, 50 and 75 kg ha^-1 N in spring were 13.6, 10.8 and 11.6 kg DM per kg N. Corresponding responses at the final harvest to N rates in the autumn were 7.2, 5.8 and 6.8 kg DM per kg N. Responses were similar at these times for treatments receiving combined spring and autumn N. Over all treatments, mean annual production of total herbage was between 7.08 t ha^-1 DM with no N and 8.19 t ha^-1 with 75 kg ha^-1 N in both spring and autumn. Owing to drought, mean production in year 2 fell by 32% compared with year 1. White clover production fell progressively with increasing N application. Treatments with spring-applied N gave the most marked decrease. White clover was more markedly depressed than the associated grass by the drought in the second year. The mean reductions in white clover content were 0.17, 0.07 and 0.12 percentage units per kg applied N for spring N, autumn N and combinations. Autumn N use depressed white clover less than spring N but the yield response of grass was less. It is concluded that any applied N adversely affects white clover performance to some degree. Where management factors are unfavourable to white clover even strategic N use may not be wise. Instead, it is suggested that a ‘dual-sward’ approach be adopted in practice, namely, grass/white clover swards with no N. and complementary grass swards receiving optimum applied N to give better production at times when grass/white clover swards are relatively less productive.     

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.     

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.     

Effect of seed rate of red clover and of companion timothy or tall fescue on herbage production

Tetraploid red clover (cv. Hungaropoly) was sown at seed rates of 6,12 or 18 kg ha^?1 alone and in mixture with timothy (cv. Scots) at 2, 4 or 6 kg ha^?1 or with tall fescue (cv. S170) at 6,12 or 18 kg ha^?1. Two ‘silage’ crops and an ‘aftermath grazing’ crop were harvested in 2 successive years. In harvest years 1 and 2, total herbage production levels of 11.12 and 7.47 t dry matter (DM) ha^?1 respectively were obtained from pure-sown red clover compared with 11.84 and 8.78 t DM ha^?1 for red clover-timothy and 12.23 and 9.64 t DM ha^?1 for red clover-tall fescue. Corresponding red clover production levels were 10.93 and 5.30 t DM ha^?1 (red clover swards), 8.04 and 3.131 ha^?1 (red clover-timothy), and 6.42 and 109 t ha^?1 (red clover-tall fescue). Total herbage organic matter digestibility was improved by the timothy companion grass but not consistently by the tall fescue, whereas crude protein (CP) concentration was decreased by the addition of either grass. Increased seed rate intensified these effects, as well as the general effect of the companion grass in depressing red clover DM, digestible organic matter (DOM) and CP production. Total herbage DM, DOM and CP were not markedly affected by increasing red clover seed rate but red clover DM, DOM and CP were increased as red clover seed rate was raised, due to increases in the red clover component. The potential for silage cropping of red clover swards was confirmed but there was advantage in sowing a companion grass. Taking yield and quality parameters into consideration, timothy proved a better companion than tall fescue. A seed rate of 2 or 4 kg ha^?1 timothy and 12 kg ha^?1 red clover proved the most satisfactory.     

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.     

The effect of strategic nitrogen application and defoliation systems on the productivity of a perennial ryegrass/white clover sward

The productivity of a mixed sward comprising perennial ryegrass cv. Fantoom and white clover cv. Aran was measured under eight defoliation management systems and two fertilizer N rates (0 and 75 kg ha^-1) applied in spring. The defoliations involved a basic six-harvest simulated grazing system together with the interposition of silage cuts once or twice at varying times during the growing season; evaluation was made over three harvest years, 1983-85. Mean annual production of total herbage DM over the three years was 8.351 tha^-1 without N and 9.49 tha^-1 with 75 kg N ha^-1, a mean response of 15.2 kg DM per kg applied N. The responses for individual treatments occurred mainly at the first cuts, whether for simulated grazing (a mean of 12 9 kg DM) or for silage (a mean of 259 kg DM); however, this influence of spring N was not sustained at other cuts over the season. Mean annual white clover DM production was 4.19 t ha^-1 with no N and 3.32 t ha^-1 with 75 kg ha^-1 N, but the reduction due to N was not significant in any year. The mean amount of clover stolon DM present post harvest over all management systems was 1.33 t ha^-1 with no N and 1.03 t ha^-1 with 75 kg ha^-1 N. Mean annual DM production of total herbage from the six-harvest system was 8.11 t ha^-1 Compared with 8 88 t ha^-1 (a 9% increase) from the systems with one silage cut and 9.241 ha^-1 (a 14% increase) from the systems with two silage cuts. Corresponding white clover DM production was 4.02, 3 87 and 3 53 t ha^-1, respectively, and mean stolon DM amounts post harvest, 1 12,1.15 and 1-23 t ha^-1, respectively. It is concluded that grass/white clover swards are suitable for management systems which involve cutting for conservation. Spring N application did not greatly reduce white clover production in this experiment where white clover was at higher levels than are likely in farming practice and the swards were not grazed. More knowledge of spring N rates, and indeed of N application rates generally, would be advantageous in future assessment of silage cutting systems.     

Yield,market quality and petiole nitrate concentration of non-irrigated Russet Burbank and Shepody potatoes in response to sidedressed nitrogen

Russet Burbank and Shepody potatoes were grown with the following four nitrogen treatments: 1) 90 kg ha^?1 at planting; 2) 180 kg ha^?1 at planting; 3) 90 kg ha^?1 at planting followed by an additional 90 kg ha^?1 side-dressed after tuber initiation; or 4) 90 kg ha^?1 at planting followed by an additional 45 kg ha^?1 sidedressing. When compared to the 90 kg ha^?1 at-planting treatment, petiole NO₃-N concentrations increased rapidly after sidedressing and were relatively constant through mid-season. Sidedressed N significantly increased total yields relative to the 90 kg N ha^?1 at-planting treatment by an average of 5.0 t ha^?1 in three of nine experiments. Three of the experiments, where yields did not significantly increase, were on sites which were not expected to respond to supplemental N based on petiole NO₃-N testing. A red clover green manure crop was the previous crop for two of these experimental sites. Petiole NO₃-N testing criteria were only partially effective in detecting sites where response to sidedressed N occurred. When compared to a single application of 180 kg N ha^?1 at planting, split application of 90 kg N ha^?1 at planting followed by a 90 kg N ha^?1 sidedressing significantly reduced total yields in one of nine experiments and did not affect yields in the remaining eight experiments. Tuber uniformity was improved in three of nine experiments by the split-N treatment. Specific gravity was not significantly affected. Use of 45 kg N ha^?1 at side-dressing resulted in similar yield as the 90 kg N ha^?1 sidedressing, although yield of large-sized tubers was often decreased with the lower N rate. Use of reduced at-planting N rates followed by sidedressed N does not appear to increase yields of non-irrigated Russet Burbank and Shepody potatoes when compared to the at-planting N rates that are currently recommended. This management approach can maintain yields at levels comparable to at-planting N programs and does provide an opportunity to reduce N application rates on sites where soil N reserves and soil amendments may make a substantial N contribution to the potato crop. Side-dressed N application can frequently improve yields and tuber size when potatoes have been underfertilized at planting; however, some inconsistency in response can be expected in regions that rely on unpredictable natural rainfall.     

The influence of date of sowing and seed rate on the production of pure-sown red clover

Hungaropoly tetraploid broad red clover was sown at seed rates of 6, 12 or 18 kg ha^-1 on six dates from April to September 1971. Three crops were harvested in 1972 and one in June 1973. In 1972, total herbage dry matter yields ranged from 5.22 to 12.22 t ha^-1 and red clover dry matter yields from 3.61 to 11.92 t ha^-1 when meaned over all seed rates. April to July sowing dates gave significantly higher yields than later sowings. In general, August and September sowings gave the lowest red clover contents in a range from 63.2 to 96.5%, the highest digestibilities within a range 61.9 to 65.0% and the lowest crude protein contents in a range 15.5 to 17.3%. The influence of seed rate was less marked than sowing date. Mean annual yields of total herbage dry matter increased from 9.88 to 10.85 t ha^?1 as seed rate was increased from 6 to 18 kg ha^?1. Red clover dry matter yields and contents of red clover and crude protein in the total herbage also followed this trend. The sowing date effects on total herbage yield and content of red clover did not persist into the second harvest year but the seed rate effects were still noticeable. Plant numbers in spring the first harvest year and hence percentage survival from sowing were depressed by late sowing; plant numbers rose but percentage survival declined as seed rate was increased. The seed rate effects on plant population persisted until spring of the second harvest year but sowing date effects did not. Better stands of red clover were obtained from sowings made between April and June, when a seed rate of 12 kg ha^?1 was adequate. The adverse effects of late sowing cannot be fully compensated by raising seed rates of clover. There was a significant interaction between seed rate and date of sowing. For April-May sowings, seed rate was not critical. Thereafter, a linear effect of seed rate on yield was discernible.     

The effect of sowing date and nitrogen on the dry‐matter yield and nitrogen content of forage rape (Brassica napus L.) and stubble turnips (Brassica rapa L.) in Ireland

Two field experiments were conducted at Teagasc, Moorepark, Ireland, to determine the effect of sowing date and nitrogen application on the dry‐matter (DM) yield and crude protein (CP) content of forage rape and stubble turnips. The first experiment consisted of three sowing dates (1 August, 15 August and 31 August) with four rates of fertilizer N (0, 40, 80 and 120 kg N ha^?1) on forage rape DM yields. The second experiment consisted of three sowing dates (1 August, 15 August and 31 August) with four rates of fertilizer N (0, 40, 80 and 120 kg N ha^?1) over two soil sites (fertile or nitrogen depleted) on forage rape and stubble turnip DM yields. A delay in sowing from 1 to 31 August characterized a 74·5% decrease in forage rape DM yield, while stubble turnip DM yield decreased by 55·5%. Forage rape DM yields increased positively up to 120 kg N ha^?1 at the first two sowing dates over both sites. In contrast, stubble turnips showed less response beyond 40 kg N ha^?1 on site 1 in the first two sowing dates, while DM yield increased positively up to 120 kg N ha^?1 on the less fertile site. The results indicate that the optimal sowing time for forage rape and a stubble turnip in Ireland was early August.     

Liveweight gains of lambs from Caucasian clover/ryegrass and white clover/ryegrass swards on soils of high and low fertility

The high nutritive value and persistence under a wide range of climatic and soil fertility conditions make Caucasian clover a potentially useful forage legume but there is little information about the performance of livestock grazing Caucasian clover/grass swards. This study compared liveweight gains of lambs grazing Caucasian clover/perennial ryegrass and white clover/perennial ryegrass swards on high fertility (Olsen P 20 mg L^?1, SO₄‐S 12 mg kg^?1) and low fertility (Olsen P 11 mg L^?1, SO₄‐S 7 mg kg^?1) soils from 1998 to 2001 in the South Island of New Zealand. Mean annual liveweight gains were 1178 kg ha^?1 for Caucasian clover/perennial ryegrass and 1069 kg ha^?1 for white clover/perennial ryegrass swards at high fertility compared with 1094 kg ha^?1 and 1015 kg ha^?1, respectively, at low fertility. There was a higher mean proportion of clover in Caucasian clover/perennial ryegrass (0·19) than white clover/perennial ryegrass (0·11) swards, but there were no differences in total herbage production between the two clover/perennial ryegrass swards. The mean concentration of crude protein in the herbage of Caucasian clover (302 g kg DM^?1) was higher than that in white clover (287 g kg DM^?1) and grass herbage (227 g kg DM^?1). Estimated mean metabolizable energy concentrations in the herbage were 12·5 MJ kg DM^?1 for the two clovers and 11·6 MJ kg DM^?1 for grass herbage. The difference in liveweight gain between swards on soils of high and low fertility was associated with an increase in total herbage production of similar composition and nutritive value, giving a greater number of grazing days for the swards on soils of high than low fertility.     

A comparison of the herbage productivity of Bromus wildenowii cv. Grasslands Matua with four cultivars of Lolium perenne when grown in association with Trifolium repens

In a small-plot trial five grass varieties bromegrass cv. Grasslands Matua, perennial ryegrass CVS. Melle (diploid) and Bastion, Condesa and Meltra (tetraploid) were established as grass/white clover swards with white clover cv. Menna. Productivity was measured under 6-weekIy cutting both without N fertilizer (N_o) and with 100 kg N ha^?1 applied in spring (N₁₀₀) Evaluation was made over 2 harvest years, 1986–87. Total mean annual production of herbage dry matter (DM) in the first harvest year at No and N_o was 5·07 t ha^?1 and 6·93 t ha^?1 respectively. In year 2, corresponding values were 11·81 and 12·67 t ha^?1. In year 1, Matua swards at N_o and N₁₀₀ yielded 5·08 and 6·65 t DM ha^?1 compared with 507 and 70 t DM ha^?1 for the mean of the four ryegrass varieties. In year 2, corresponding values were 12·90 and 12·29 for Matua and 11·54 and 12·78 for the four ryegrasses. In year 1, the digestable organic matter in the dry matter (DOMD) of the Matua swards was lower than that of Melle, Bastion and Condesa at N_O, particularly at the first cut. In year 2, differences in DOMD between treatments and varieties were not significant. The proportion of white clover was found to be higher in the N_o than the N₁₀₀ treatment, and also higher in year 2 in most treatments. For the _No treatment Matua swards had the highest proportion of white clover in year 1 (32% compared with 24% for the mean of the ryegrass varieties) but the lowest proportion in year 2 (27% compared with 60% for the ryegrasses). For the N_o treatment in year 1 clover production was also 43% higher, on average, from the tetraploid treatments than with Melle as the companion grass; for this comparison in year 2 the differences were not significant. It is concluded that Matua bromegrass/white clover swards receiving no N fertilizer may have a good potential under cutting management. However, the evidence from this trial is that in the second year the proportion of white clover is lower with Matua swards than with perennial ryegrass as the companion grass.     

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.     

A comparison of diploid and tetraploid perennial ryegrass and tetraploid ryegrass/white clover swards under continuous sheep stocking at controlled sward heights. 3. Sward characteristics and animal output, years 4–8

Results for years 4–8 of a long-term grazing experiment on swards of a diploid perennial ryegrass (Lolium perenne), var. Contender (D swards), a tetraploid ryegrass, var. Condesa (T swards) and Condesa with S184 white clover (Trifolium repens) (TC swards), direct sown in May 1987, are presented. The swards were continuously stocked with sheep from 1988 to 1990, as previously reported, and for a further 5 years, 1991–95, at a target sward surface height (SSH) of 4–6 cm. Control of sward height was successfully achieved by variable stocking, except in 1993 when paddocks were set stocked and the resulting mean SSH was 9·3 cm. Grass swards received on average 160 kg N ha^?1 year^?1; grass/clover swards were mainly not fertilized with N with the exception that they were given 30 kg N ha^?1 as a remedial mid-summer application during a period of low herbage mass on offer in 1994 and 1995. Mean white clover content of the swards fell from 18·2% of herbage dry-matter (DM) in 1992 to 8·5% in 1993, whereas stolon lengths fell from 120 to 58 m m^?2. A return to lower sward heights in 1994–95 resulted in an increase in white clover content to 12·8% by the final sampling in August 1995. Perennial ryegrass content of the grass swards remained high throughout (mean 96·7% in 1995). Perennial ryegrass tiller densities recorded in August 1991, 1993 and 1994 showed consistently significant (P < 0·001) sward differences (3-year mean 16 600, 13 700 and 10 100 perennial ryegrass tillers m^?2 for the D, T and TC swards). In 1994, the year after lax grazing, a low perennial ryegrass tiller density (9100 m^?2) and low white clover content (mean 4·3%) in the TC swards resulted in a much lower herbage bulk density than in the grass swards (April–July means 72, 94 and 44 kg OM ha^?1 cm^?1 for the D, T and TC swards). There was a consistent 40 g d^?1 increase in lamb liveweight gain on the TC swards over the T swards, except in 1994. In that year there was a reduction in lamb liveweight gain of 33 g d^?1 on the TC swards and a significant increase in ewe liveweight loss (117 g d^?1) associated with low herbage bulk density despite optimal sward height. Lamb output (kg liveweight ha^?1) on TC swards reflected white clover content, falling from a similar output to that produced from grass given 160 kg N ha^?1, at 18% white clover DM content, down to 60% of grass + N swards with around 5% clover. A 6% greater output from the T than the D swards was achieved mainly through higher stocking rate. The experiment demonstrated a rapid, loss in white clover under lax grazing, and showed that the relationship between performance and sward height is also dependent on herbage density. High lamb output from a grass/clover sward was only achieved when the clover content was maintained at 15–20% of the herbage DM.     

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.     

A comparison of the output from permanent swards containing clover or receiving nitrogen fertilizer when continuously grazed by ewes and lambs

A 2-year experiment was designed to compare the output from a permanent grass sward, either containing clover (GC) or receiving 200kg N ha^-1 (GN), when continuously grazed by sheep stocked at 12 and 15 ewes ha^-1 on GC and 15 and 18 ewes ha^-1 on GN. Sward surface height (SSH) was controlled within designated guidelines by adjusting the size of the grazing area with an electrified ‘buffer’ fence; the herbage surplus to grazing requirement was cut and conserved. A dried grass supplement was offered to the sheep during periods of low herbage availability. The experiment was carried out during 1989 and 1990. No silage was made on GC15 in either year compared with 28 and 90kg DM ewe-¹ on GN15 in 1989 and 1990 respectively; supplementation was consistently and significantly greater on GC15 than on GN15. Clover proportion was generally higher on GC12 than GC15 (significant (P <0·01) in September 1989) and reached a maximum level of 14%on GC12 in August 1990. Herbage organic matter digestibility was little affected by the inclusion of clover in the sward. Differences in lamb growth rate were not significant and, at the common stocking rate, there was no difference in lamb output between GC and GN. In 1990, GC12 and GN15 treatments proved to be successful after weaning in balancing the increasing nutritional requirements of the ewes during the period prior to mating, the requirements of a declining lamb population as the lambs satisfied the criteria for slaughter and were sold, and a continuing need to conserve surplus herbage. The metabolizable energy requirements of the ewes and lambs over 2 years were 75·8 and 74· 7 GJ ha-¹ for GC15 and GN15 respectively, and the differences in total utilized metabolizable energy output of the two systems were due to the amounts of herbage conserved and supplement consumed. The 2-year mean total UME output on GC15 was 80% of that on GN 15 (67·8 and 84·4 GJ ha^-1 for GC15 and GN15 respectively) and the experiment provided further evidence that grass/white clover swards with no fertilizer N applied are capable of producing about 80% of the total output of grass swards receiving 200 kg N ha^-1.     

Herbage intake and N excretion by sheep grazing monocultures or a mixture of grass and white clover

In 1988 and 1989, swards of grass (G0), while clover (C0) and grass/white clover (GC0) receiving no N fertilizer, and a grass sward supplied with 420 kg N ha^?1 (G420), were grazed by non-lactating sheep to maintain a sward surface height of 6 cm. Herbage organic matter (OM) intakes averaged between 1200 and 1700 g OM ewe^?1 d^?1. For treatments G0, C0, GC0 and G420 respectively, the ewes' live weight gain was 102, 112, 100 and 110 g d^?1 and changes in body condition scores were +0·28, +0·52, +0·36 and +0·44 units season^?1. However, the effect of treatment was not significant for either variable. There were similar levels of output of faecal N ewe^?1 but significantly more urinary N ewe^?1 was excreted on treatments C0 and G420, where the concentrations of N in herbage laminae were also higher. For example, in 1989, total daily N excreted was 39·7, 64·4, 44·0 and 63·3 g N ewe^?1 for G0, C0, GC0 and G420 respectively. Taking into account the mean daily stocking rates, which were 19·4, 26·6, 27·2 and 36·5 ewe ha^?1, the total faeces and urine returns over the season were 161, 358, 249 and 484 kg N ha^?1 for each treatment respectively. The herbage OM intakes ewes^?1 d^?1 measured in September and October were similar for C0 and G420, and so the intake of herbage OM ha^?1 d^?1 was related to stocking rate, i. e. the estimated herbage intake ha^?1 over the growing season for the white clover monoculture was 73% of that for N-fertilized grass. Excretal nitrogen returns to the pasture from grazed mono-cultures of clover were high, and similar to those from a grass sward receiving 420 kg fertilizer N ha^?1. Consequently potential losses of N to the environment are high under these management systems.     

Optimizing legume content and forage yield of mown white clover–Italian ryegrass mixtures through nitrogen fertilization and grass row spacing

Mixed swards of white clover–grass mixtures in highly productive environments often fail to reach the minimum recommended annual clover proportion of about 0·30. This study assessed the effect on clover content and total dry matter (DM) yield of two spring N applications (0 and 45 kg N ha^?1) and two distances between drilled grass‐rows (0·18 and 0·36 m) over 3 years for mown swards of white clover–Italian ryegrass (Trifolium repens–Lolium multiflorum) in binary mixtures in northern Italy. An additional aim was to determine the advantage of association of grass–clover compared with grass and clover monocultures. On average, N fertilization of mixtures resulted in almost 9% higher total yield (P < 0·01; mean response = 18·1 kg of total DM per kg of N) but decreased the clover proportion (0·250 vs. 0·312). Wider grass‐row spacing increased clover proportion (0·327 vs. 0·234; P < 0·01) with no reduction of total DM yield. N fertilization × grass‐row spacing interaction occurred only for clover content (P < 0·01). Without N fertilization, mixtures out‐yielded clover and grass pure stands. With N fertilization, at double rate to pure grass, yields from mixtures were greater than from clover and comparable to Italian ryegrass.