Effect of sward surface height on the performance of ewes and Iambs continuously grazed on grass/clover and nitrogen-fertilized grass swards

In three successive years, sward height was maintained at 3, 5, 7 or 9 cm on grass swards receiving a total of 300 kg N ha^?1 in six equal monthly dressings from April, and on grass/clover swards receiving 50 kg N ha^?1 as a single dressing in early spring. From turnout in April until weaning in July, 64 ewes and their lambs (mean litter size 1·5) were continuously grazed at the four sward surface heights on the two sward types. White clover content of grass/clover swards remained low throughout the experiment ranging from 0·2 to 7·4% of the herbage mass. During the first two years, lamb gains averaged over sward types were 204, 260, 285 and 308 g d^?1 up to weaning, while in the third year gains were 238, 296, 296 and 260 g d^?1 on 3, 5, 7 and 9 cm swards respectively. Ewes lost live weight on 3 cm swards but apart from this sward height had little effect on performance. During the autumn, weaned lambs gained — 27, 87, 147 and 167 g d^?1 on 3, 5, 7 and 9 cm swards respectively. Sward type had only a small effect on the performance of lambs up to weaning but in the autumn, mean gains of weaned lambs were lower on grass/N swards (73 g d^?1) than on grass/clover swards (115 g d^?1). Relative to 3 cm swards, carrying capacities of 5, 7 and 9 cm swards were 0·76, 0·57 and 0·52 respectively from turnout to weaning and 0·66, 0·52 and 0·44 respectively during autumn. Grass/clover swards carried 0·67 of the ewes carried by grass/N swards from turnout to weaning and 0·51 of the live weight carried by grass/N swards during autumn. The reaction of the two sward types to sward height did not appear to differ but in the third year there was evidence of a reduction in white clover content when swards were grazed at 9 cm. The data suggest that lamb growth rates will increase as sward height increases up to 9 cm and the evidence for this was stronger with weaned lambs in autumn than with suckling lambs in spring.     

A comparison of diploid and tetraploid perennial ryegrass and tetraploid ryegrass/white clover swards under continuous sheep stocking at controlled sward heights. 2. Animal production

Performance of continuously stocked Mule ewes nursing Suffolk-cross twin lambs over three grazing seasons, between April and August, was compared on swards of N-fertilized diploid perennial ryegrass (D), tetraploid perennial ryegrass (T) and tetraploid perennial ryegrass with white clover (TC), the latter receiving no fertilizer N. Sward height was maintained by variable stocking rate close to a target of 4–6 cm (constant treatment) from turnout and compared in July and August with a rising sward height treatment (target 6–8 cm). Lambs on TC swards had significantly higher (P <0·001) liveweight gains compared with lambs on T swards by 41 gd^-1 in April–June and by 68gd^-1 in July-August. Live weight and body condition score of ewes in August were significantly higher (P<0·001) on TC compared with T swards, by 11·3 kg and 0·75 respectively. Rising sward heights in July–August increased live-weight gain of lambs compared with constant sward heights by 102, 39 and 54gd^-1 in consecutive years, associated with sward height increases of 0·9, 0·5 and 0·6cm respectively. Rising sward height increased ewe live weight and body condition score by 5·1 kg and 0·3 respectively, compared with results from constant sward heights. Effects of sward height and sward type were additive. T swards had a significantly (P<0·01) 16% greater overall lamb output than the D swards due mainly to a 10% higher achieved stocking rate. Stocking rates of ewes on TC vs T swards were 40, 13 and 12% lower in April-August in successive years. The higher liveweight gain of lambs on the TC swards resulted in lamb outputs of 76, 105 and 101% of the T swards in successive years, showing that grass/clover swards containing over 20% clover could produce similar lamb output ha^-1 to grass swards given 150–180 kg N ha^-1.     

A comparison of diploid and tetraploid perennial ryegrass and tetraploid ryegrass/white clover swards under continuous sheep stocking at controlled sward heights. 1. Sward characteristics

Two 1·0 ha plots of a late-heading diploid perennial ryegrass (var. Contender) and a late-heading tetraploid ryegrass (var. Condesa), and two 1·4 ha plots of the tetraploid with Aberystwyth S184 small-leaved white clover, were direct sown in May 1987. Over the three years 1988–90 they were continuously stocked by Mule ewes with Suffolk-cross twin lambs, from early April to the end of August, at a target sward surface height (SSH) of 4–6 cm on one set of plots (constant swards) and, on the other set, al 4–6 cm rising after June to a target 6–8 cm (rising swards). The heights were achieved by variable stocking. Fertilizer N was applied only to the grass plots at the rate of 150- 180kgN ha^-1 annually.
SSH was mainly within the target 4–6 cm, after higher initial heights at turnout in 1988and 1990. Mean heights of the constant swards (April- August) averaged 5·53, 4·43 and 5·04cm in the three years. The rising swards (July-August) increased in height over the constant swards by an average of 0·88, 0·48 and 0·55 cm, in successive years.
Clover content of the herbage mass dry matter in the grass/clover swards increased over each grazing season to average 13·0, 26·5 and 21·2% in the three years, with a high mean stolon density of 130 in m^-2 in August 1990. Ryegrass tiller densities in year 3 were 23% higher in the diploid than in the tetraploid swards, which had 43% more than the 10000 tillers m^-2 of the tetraploid ryegrass/clover swards.
It is concluded that the combination of a densely stoloniferous small-leaved clover with the open growth habit of a tetraploid ryegrass can achieve swards of high clover content under continuous sheep stocking.     

The effect of blending white clover varieties and their contribution to a mixed grass/clover sward under continuous sheep stocking

The effect of blending small- and medium-leaved white clovers together in a mixture of varieties was examined under continuous sheep stocking over a period of three years. Four varieties were used, S184 and Gwenda, small-leaved varieties suitable for intensive sheep grazing, together with medium leaved varieties Menna and Donna, which are mainly used in general purpose seed mixtures for medium term leys. The small-leaved varieties were blended with Menna or Donna, sown with a commercial grass mixture and managed as near as possible to farm practice. Although the leaf size of Gwenda is only slightly greater than that of S184, blends based on these two small-leaved varieties behaved differently. In spring of the first harvest year the yield of Gwenda and of the clover in mixtures containing Gwenda was 27% greater than the yield of the same mixtures which contained S184. As the season progressed this difference decreased. In terms of total annual yields and saving in fertilizer N, the benefits of including white clover in a seed mixture were more pronounced when Menna was mixed with S184 and Gwenda, rather than when Donna was used, although both varieties are in the medium-leaf category. It was concluded that the slower establishment and the lower clover yield of S184, when compared with larger leaved varieties, can be overcome by blending with a variety that is slightly larger in leaf size, such as Menna, although the choice of variety may depend on sward management and its persistency under grazing.     

Effects of continuous sheep stocking and strategic rest periods on the sward characteristics of binary perennial grass/white clover associations

Five binary perennial grass/white clover (Trifolium repens, cv. Menna) mixtures were evaluated over a 3-year period under continuous sheep stocking together with the imposition of a rest period for either an early or a late conservation cut; the experiment with plot sizes of 0·16 ha was replicated three times. The grass species and cultivars used were Merlinda tetraploid and Magella diploid perennial ryegrass (Lolium perenne), Prairial cocksfoot (Dactylis glomerata), Rossa meadow fescue (Festuca pratensis) and Goliath timothy (Phleum pratense). The greatest total lengths of white clover stolon developed in the meadow fescue (171·6 m m^?2) and timothy (151·9 m m^?2) associations compared with those in tetraploid perennial ryegrass (98·6 m m^?2), diploid perennial ryegrass (91·9 m m^?2) and cocksfoot (74·6 m m^?2) (s.e.d. 16·4, P < 0·001). On average, the proportion of white clover stolon that was buried was between 0·86 and 0·89 and this was more abundant in late than early season. Whereas timothy persisted, the persistence of meadow fescue was low under any of the managements tested and this was markedly reduced by the third grazing season. In the diploid perennial ryegrass sward, a late June to early August rest period for conservation enhanced white clover stolon length. An early April to late May rest period greatly reduced total white clover stolon length in both diploid perennial ryegrass and tetraploid perennial ryegrass associations (diploid perennial ryegrass-unrested 89 m m^?2, early rest 56·1 m m^?2, late rest 130·7 m m^?2; tetraploid perennial ryegrass - unrested 125·1 m m^?2, early rest 71 m m^?2, late rest 99·7 m m^?2; s.e.d. 19·19, P < 0·001). The numbers of white clover stolon growing points per unit stolon length were greatest when the sward was rested during late June to early August ?55·9 m^?1 stolon length compared with 45·7 m^?1 for an April to late May rest and 46 m^?1 in the absence of a rest (s.e.d. 2·59, P < 0·001). Likewise, the percentage of stolon above ground was greatest with the late June to early August rest ?15·78% compared with 10·61% for the April to late May rest and 7·69% for no rest (s.e.d. 1·569, P < 0·001). The complementary percentages of buried stolon indicate the important role this fraction has and the need to study stolon behaviour in grazing studies generally. It is concluded that, in relation to perennial ryegrass as a companion grass, meadow fescue and timothy allow better white clover development and cocksfoot less. However, other attributes have to be considered, for example the poor persistence of meadow fescue and the slower regrowth of timothy, both of which allow the invasion of weed grasses, or the lower acceptability of cocksfoot to livestock. The timing of the rest period before the conservation cut can influence white clover development considerably, but the effects differed with different companion grasses.     

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 sward condition on rates of herbage growth and senescence in mixed swards under continuous stocking management

A study was conducted to determine the influence of herbage mass under continuous stocking management with sheep on rates of herbage growth (G), senescence (S) and net production of green herbage (NP) in a mixed-species sward of Lolium perenne. Poa annua and Trifolium repens. Plots were maintained in as nearly constant herbage mass as possible at 500, 700, 1000 and 1700 kg organic matter (OM) ha⁻¹ by continuous but variable stocking with sheep for the period May to July inclusive. Estimates of G, S and NP were obtained for each species from repeated measurements over two 2-week periods on individually identified grass tillers or clover stolons. Estimates of species population densities enabled estimates of G. S and NP per unit area to be made.
Rates of G were higher and of S were lower for Lolium tillers and Trifolium stolons than for Poa tillers. The laminae of Poa tillers occupied inferior positions in the canopy of the swards maintained at higher herbage masses and were less accessible to the grazing animals, so that Poa tissue was less well utilized.
The total population densities of tillers and stolons declined at herbage masses both above and below 700 kg OM ha⁻¹. The rate of G for the three species combined increased with herbage     

Dietary preference of sheep for perennial ryegrass and white clover at contrasting sward surface heights

Non‐pregnant, non‐lactating ewes grazed adjacent monocultures of white clover and perennial ryegrass with three sward surface height (SSH) combinations [6 cm white clover: 6 cm perennial ryegrass (c6g6), 3 cm white clover: 6 cm perennial ryegrass (c3g6), 3 cm white clover: 9 cm perennial ryegrass (c3g9)] at two stocking densities (21·3 or 29·8 ewes ha^–1). Immediately prior to the experiment, all ewes grazed a c6g6 sward. Grazing time on each plant species was recorded during daylight over two 48 h‐test periods. Subsequently, herbage intake rates for each species at each SSH were measured allowing intakes of each species to be calculated. For the first 24 h of both test periods (D1), ewes on treatment c3g6 spent less time grazing white clover than those on treatment c6g6 (228 vs. 362 min) and more time grazing perennial ryegrass (360 vs. 182 min). Total grazing time on treatment c3g6 was more than on treatment c6g6 (587 vs. 544 min) but the difference was not significant. Perennial ryegrass intake was higher (895 vs. 452 g), and white clover intake (814 vs. 1687 g), total intake (1719 vs. 2140 g) and proportion of white clover in the diet (0·460 vs. 0·794) were lower for treatment c3g6 than treatment c6g6. There were no significant differences in total grazing time, grazing time on either species, proportion of grazing time on white clover or proportion of white clover in the diet between treatment c3g6 and treatment c3g9. However, the higher intake rate of perennial ryegrass in treatment c3g9 led to higher perennial ryegrass and total intakes. For the second 24 h of both test periods (D2), ewes on treatment c3g6 again spent more time grazing perennial ryegrass than on treatment c6g6 (270 vs. 161 min) but time spent grazing white clover was similar (318 vs. 308 min). Total grazing time was significantly higher on treatment c3g6 than on treatment c6g6 (588 vs. 469 min) but proportion of grazing time on white clover was similar (0·554 vs. 0·668). Perennial ryegrass intake was significantly higher for treatment c3g6 than for treatment c6g6 (672 vs. 402 g) while white clover intake was significantly lower (1140 vs. 1435 g) but total intake was similar (1812 vs. 1836 g). The proportion of white clover in the diet was significantly lower for treatment c3g6 (0.628 vs. 0.785) than for treatment c6g6. The only significant differences between treatments c3g6 and c3g9 were in perennial ryegrass intake (672 vs. 906 g) and in total intake (1812 vs. 2287 g). Intake of perennial ryegrass on treatment c3g9 was also significantly greater than on treatment c6g6 (906 vs. 402 g) and total intake was higher (2287 vs. 1836 g). At the higher stocking density, time spent grazing perennial ryegrass and perennial ryegrass intake were significantly lower on D1 and D2 while total grazing time was also significantly lower and proportion of time grazing white clover and proportion of white clover in the diet were significantly higher at the higher stocking rate on D2. The results indicate that behaviour changed over the 48 h observation period for treatments c3g6 and c3g9 but behaviour remained relatively constant for animals on treatment c6g6. Ewes traded off dietary preference against total intake by altering grazing times on perennial ryegrass and white clover to achieve maximum net benefit.     

Frequency and severity of defoliation of grass and clover by sheep at different stocking rates

The frequency and severity of defoliation of individual grass tillers and clover plant units was studied in Lolium perenne-Trifolium repens swards grazed by sheep at stocking rates ranging from 25 to 55 sheep ha^-1 and either receiving no N fertilizer or 200 kg N ha^-1. On average, sheep at the highest stocking rate defoliated individual tillers once every 4·2 d compared with once every 9·2 d at the lowest stocking rate with the removal of 58% and 47% of the leaf length of each tiller leaf at these stocking rates. Clover plant units were defoliated once every 4·2 d at the highest stocking rate and once every 7·2 d at the lowest stocking rate with the removal of 51% of its leaves and 12% of its stolon at the high stocking rate and 42% and 4% respectively at the low stocking rate. Differences in frequency and severity of defoliation between N fertilizer treatments were smaller than between stocking rates. Grass tillers and clover plant units were both defoliated less frequently and less severely in swards fertilized with N, though the difference in defoliation frequency between fertilizer treatments decreased as stocking rate increased. Defoliation frequency was related to the length of grass leaf per tiller or number of clover leaves per plant unit, and to the number of these tillers and the herbage on offer.     

The effects of clover variety, cutting interval and nitrogen application on herbage yields, proportions and heights in perennial ryegrass-white clover swards

Six varieties of white clover, each grown with perennial ryegrass, four intervals between cuts and two levels of applied nitrogen in all combinations, were compared in a field experiment during the first 27 months after sowing.
Increasing the interval between harvests from 3 or 4 to 8–12 weeks increased the yield of white clover and generally did not reduce the proportion of clover in total herbage. Increasing the interval between harvests reduced the number of grass tillers but increased grass yield and the size of grass leaves and increased grass height more than clover height; it also increased the proportion of petiole relative to leaflet in the clover. Differences between varieties in response to interval between harvests were small but supported the view that medium large-leaved varieties can with advantage be defoliated rather less frequently than small-leaved ones. The adverse effect of applied N on clover appeared almost equally great with all four intervals between harvests and further research on this topic is suggested. Applied N increased grass height more than clover height and increased the number of grass tillers, the size of grass leaves and grass yield. The medium large-leaved varieties seemed more tolerant of applied N than the smaller varieties.     

Influence of sward condition on leaf tissue turnover in tall fescue and tall wheatgrass swards under continuous grazing

Two experiments were carried out on a tall fescue sward in two periods of spring 1994 and on a tall wheatgrass sward in autumn 2001 and spring 2003 to analyse the effect of sward surface height on herbage mass, leaf area index and leaf tissue flows under continuous grazing. The experiment on tall fescue was conducted without the application of fertilizer and the experiment with tall wheatgrass received 20 kg P ha^?1 and a total of 100 kg N ha^?1 in two equal dressings applied in March (autumn) and end of July (mid‐winter). Growth and senescence rates per unit area increased with increasing sward surface height of swards of both species. Maximum estimated lamina growth rates were 28 and 23 kg DM ha^?1 d^?1 for the tall fescue in early and late spring, respectively, and 25 and 36 kg DM ha^?1 d^?1 for tall wheatgrass in autumn and spring respectively. In the tall fescue sward, predicted average proportions of the current growth that were lost to senescence in early and late spring were around 0·40 for the sward surface heights of 30–80 mm, and increased to around 0·60 for sward surface heights over 130 mm. In the tall wheatgrass sward the corresponding values during spring increased from around 0·40 to 0·70 for sward surface heights between 80 and 130 mm. During autumn, senescence losses exceeded growth at sward surface heights above 90 mm. These results show the low efficiency of extensively managed grazing systems when compared with the high‐input systems based on perennial ryegrass.     

Effect of fertilizer nitrogen on six-year-old red clover/perennial grass swards

Three diploid red clover cultivars—Sabtoron, Violetta and Essex—and three tetraploid, Hungaropoly, Teroba and Red Head, were sown separately in pure culture and with each of three companion grasses: timothy (Aberystwyth S48), tall fescue (Aberystwyth S170) and perennial ryegrass (Aberystwyth S24).
The effects of fertilizer N on yield and on clover/grass ratio over a 2-year period (seventh and eighth harvest years) subsequent to 6 harvest years during which no N fertilizer was applied were investigated. The data for productivity and persistence have already been published (McBratney, 1981; 1984).
Application of fertilizer N increased DM yields in the eighth year. In this year, the highest yield, 11·9t ha^-1, averaged over the six clover cultivars, was given in association with tall fescue. Tall fescue contributed 90% of this yield. Clover content continued to decrease in all swards but the decrease was greatest in the swards receiving fertilizer N. The yield of clover DM averaged over the six cultivars under N treatment declined from 5·6t ha^-1 in the seventh year to only 0·4t ha^-1 in the eighth year.
The results from this trial demonstrate the potential of red clover sown either pure or in mixture with a suitable perennial grass, to maintain high output of quality herbage over a 6-year period without the aid of fertilizer N. They further demonstrate that following decline in red clover content, both herbage yield and quality may be restored by the application of N fertilizer, particularly where the clover was seeded with a highly productive companion grass.     

Persistence and colonization of gaps in sown swards of grass and clover under different sward managements

The effects of extensive sward management and patch size on the persistence and colonization of gaps in sown swards was examined by creating gaps of five different sizes (2·3, 7, 10, 14 and 19 cm in diameter) in four different sward treatments: a fertilized sward grazed to 4 cm, i.e. relatively intensive management, and three extensively managed unfertilized swards, which were not grazed or grazed to 4 cm or 8 cm. The swards were originally sown with ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), but had developed differences in species composition as a result of the management treatments imposed 2 years before and during the experiment. Light quality measurements, i.e. red-far red (R/FR) ratio, were used to determine when the light environment in the gaps no longer differed from that in control, uncut patches and this was used as an estimate of gap persistence. Persistence of gaps depended on both sward management and gap size. Gaps disappeared most rapidly in the ungrazed sward and fertilized 4-cm sward, and most slowly in the unfertilized 8-cm sward. Small gaps persisted for up to 2 weeks in all but 8-cm swards, whereas larger gaps were estimated to persist for up to 20–25 weeks in unfertilized, grazed swards. There was no evidence that the number of grass or dicotyledonous species increased in the gaps compared with the control areas. There were significant positive linear relationships between the vegetation that developed in gaps and that in the control, uncut patches, reflecting the different species composition of the established sward of the grazed (grass-dominant) and ungrazed (Ranunculus repens-dominant) treatments. For total grass dry matter and tiller numbers, as well as L. perenne tiller numbers, there was a small, but significant, effect of both patch size and sward management on the slopes of the regressions between the controls and gaps. The results are discussed in relation to the potential for species composition of sown swards to change as a result of gap creation.     

Intake and behaviour responses by sheep to changes in sward characteristics under continuous stocking

Perennial ryegrass pastures were maintained at sward surface heights (SSH) of 30, 60, 90 and 120 mm by continuous variable stocking using lactating ewes and their twin lambs in spring and non-lactating (dry) ewes in autumn.
The effects of SSH on ingestive behaviour, herbage intake, animal performance and sward structure and production were measured in spring, and again in autumn, when an additional SSH of 20 mm was also established.
In spring, differences in sward structure were quickly established with mean tiller numbers of 41 000, 30 000, 21 000 and 19 000 m⁻² and leaf area indices (LAI) of 1·5, 2·2, 3·3 and 4·1 at SSH of 30 to 120, respectively. By autumn the mean tiller numbers had fallen to 26 000, 26 000, 23 000, 18 000 and 13 000 m⁻² and LAI to 1·0, 1·1, 1·5. 2·0 and 1·5 for treatments 20 to 120.
The mean stocking rates for the ewes in spring were 27, 20, 22, and 19 ewes ha⁻¹ and growth rates of their lambs were 208, 275, 250 and 263 g d⁻¹ for treatments 30 to 120. In autumn the stocking rates maintained for dry ewes were 22, 15, 9, 4 and 0 for treatments 20 to 120.
In spring, both grazing times (GT) and prehension biting rate (BR) were negatively correlated with SSH, while bite mass (BM) was positively correlated with SSH. However, mean daily intakes of organic matter (OM), were reduced only for animals grazing the 30 mm sward.
In autumn, intake rate was lower only for the animals grazing the 20mm sward and GT and BR were not significantly affected by SSH.
Relationships between SSH and, components of ingestive behaviour and intake, are presented and the control mechanisms involved are discussed. The results showed that the optimum sward surface height for continuously stocked swards, grazed by sheep, Was between 30 mm and 60 mm.     

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.     

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.     

The effects of sward height and nitrogen fertilizer application on changes in sward composition, white clover growth and the stock carrying capacity of an upland perennial ryegrass/white clover sward grazed by sheep for four years

Perennial ryegrass/white clover pastures grazed by sheep and receiving either no fertilizer N (No) or 120 kg N ha^?1 year^?1 (N₀) were maintained with surface heights of 2·5, 3·5 and 5·0 cm for over four years. The treatments were replicated. The white clover (WC) population was greatest in the N₀treatment, and declined during the study. Between-year variation in WC was negatively related to rainfall and positively related to temperature, WC as a proportion of the total plant population decreased during the summer in the Nl treatment. The perennial ryegrass (PRG) population was greater in the Nl treatment, declined during the study and both within and between years was positively related to temperature. The population density of the unsown grasses was highest in the N-fertilized treatment and in the swards maintained at the lowest heights (these treatments also had the highest stocking rate); it increased during the study, within-years being positively related to temperature and between-years being positively related to rainfall. The WC stolon extension rate was largely unaffected by N fertilizer application and was greatest in the taller swards. Leaf appearance rate was unaffected by N fertilizer application and sward height; it was positively related to temperature and negatively related to rainfall. Branching rate was greater in the N₀ treatment with significant sward height effects confined to a negative relationship with local sward height within treatment plots on one occasion; it was negatively related to rainfall. The ground level red:far red light ratio was negatively related to local sward height. The total live weight of sheep carried in the No treatments was 0·7 of that in the N₁ treatments. Expected photomorphogenic responses by we were confined to stolon extension. It was concluded that on the poorly drained clay-loam soil used in this study the effects of sheep, in interaction with climatic factors, had an overriding effect on clover branching rate and the ultimate species composition.     

Diet selection by sheep and goats and sward composition changes in a ryegrass/white clover sward previously grazed by cattle, sheep or goats

An experiment was conducted to examine how variation in the composition and structure of mixed grass/white clover swards affected diet selection by sheep and goats. Sward composition in a mixed perennial ryegrass/white clover sward was manipulated by continuous grazing from 28 May to 28 July (pre-experimental phase) with cattle, sheep or goats, and then from 29 July to 2 September (experimental phase) with sheep or goats in a factorial design replicated twice. Sward surface height was maintained at 6 cm by regular adjustment of stocking density. Grazing by different sequences of animal species resulted in significant differences in the proportions of white clover in the sward, and especially in the proportion of clover lamina and petiole. Grazing by goats in the pre-experimental phase led to greater proportions of clover lamina and petiole in the whole sward and the sward surface. The proportion of white clover in the diet selected by sheep in the experimental phase was consistently higher than that in the sward as a whole, but was closely related to that near to the sward surface (approximately the top 2 cm). For goats there was no significant relationship between the proportion of clover in the diet and in the whole sward, and they generally selected a diet with a lower proportion of white clover than was present in approximately the top 2 cm of the sward. It is concluded that on mixed grass/white clover swards goats do not graze as deeply into the sward as sheep and that this results in a lower proportion of white clover in their diet and therefore allows higher proportions of white clover to develop under grazing by goats than by sheep.     

A comparison of quadrat, capacitance meter, HFRO sward stick, and rising plate for estimating herbage mass in a smooth-stalked, meadowgrass-dominant white clover sward

A single-probe capacitance meter (Pasture Probe), the Hill Farm Research Organization (HFRO) sward stick, a rising-plate meter, and cut quadrats were used to estimate herbage mass of swards that were rotationally grazed by cattle, cattle followed by topping, cattle followed by sheep, or sheep alone during 1989 and 1990 grazing seasons. Mean target pre- and post-grazing herbage masses were 2200 and 1100 kg dry matter (DM) ha⁻¹ respectively. Linear regressions, correlations and scatterplots were calculated relating meter and sward stick readings to herbage mass on an ash-free organic matter basis measured by cutting quadrats of herbage at ground level.
Mean coefficients of variation for quadrat, capacitance meter, sward stick and rising plate were 28·8, 15·5, 27·2 and 27·9% respectively for pre-grazing herbage mass measurements, and 20·2, 10·1, 21·4, and 18·4% respectively for post-grazing measurements. These coefficients indicate that the capacitance meter varied less in estimating pre- and postgrazing herbage mass than the other three methods.
Correlation coefficients relating cut quadrats to capacitance meter, sward stick and rising plate readings were 0·65, 0·70 and 0·72 for pre-grazing, and 0·36, 0·31 and 0·05 for post-grazing herbage mass measurements respectively.
The non-destructive methods provided quick herbage mass estimates at a level of precision adequate for making day-to-day grazing management decisions on farms.     

Dry-matter yield and herbage quality of a perennial ryegrass/white clover sward in a rotational grazing and cutting system

The expected reduction in the use of fertilizer nitrogen (N) on grassland in the Netherlands has led to renewed interest in white clover. Therefore, the performance of a newly sown perennial ryegrass/white clover sward on clay soil was assessed during 4 consecutive years. The experiment consisted of all combinations of two defoliation systems, i.e. one or two silage cuts per year (S1, S2), spring N application rate, i.e. 0 or 50 kg ha⁻¹ year⁻¹ (N₀, N₅₀), and the management system, i.e. rotational grazing and cutting, or cutting only (RGC, CO). The overall mean white clover cover was 30%. All treatments affected white clover cover, which was 8% higher with S2 than with S1, 6% higher with N₀ than with N₅₀ and 12% higher with CO than with RGC. The overall mean annual dry-matter (DM) yield (13·1 t ha⁻¹ year⁻¹) was significantly affected only by the management system: in two relatively wetter years, the annual DM yield was 1·19 t ha⁻¹ higher with RGC than with CO, whereas there was no difference in two relatively drier years. Nitrogen application increased the DM yield in the first cut by 7·0 kg kg⁻¹ N applied, but had no significant effect on the annual DM yield. Herbage quality was not affected by the experimental treatments. The average in vitro organic matter digestibility was 0.801, and the average crude protein content was 193 g kg⁻¹ DM. With the expected reduction in the use of fertilizer N, perennial ryegrass/white clover swards should be seriously considered as an alternative option to perennial ryegrass swards on these clay soils.