THE EFFECT OF NITROGEN, STOCKING RATE AND FREQUENCY OF GRAZING BY BEEF CATTLE ON THE OUTPUT OF PASTURE

Two successive grazing experiments were conducted over 12 weeks on perennial ryegrass pastures with 50 and 44 young cattle to study the effect of N fertilizer when applied at a daily rate of 1 or 3 kg N/ha. At each level of N two stocking rates differing hy 20% were imposed. At the liigher N level and stocking rate, three frequencies of grazing were imposed. At the stocking rates imposed N tended to reduce the daily liveweight gain per head, but increased the total liveweight gain per ha by from 0.79 to 1.58 kg/kg N. A 20% increase in stocking rate depressed gain per head in both years. In the first year it did not improve gain/ha, hut in the second year gain/ha was increased by 12–17%. There was a tendency for performance per animal and per ha to increase as the grazing cycle was lengthened. In 1969 the highest yield in the whole season was 1880 grazing days and 1260 kg gain/ha. The overall response to N fertilizer was similar to that recorded in other reports, but it is possible that a lack of K had limited pasture growth.     

FACTORS LIMITING ANIMAL PRODUCTION FROM GRAZED PASTURE*

Animal production from intensively managed pasture (240 units N/ac or 300 kg N/ha applied annually) should in theory reach 11,000 Ib/ac (12,500 kg/ha) of milk or 1550 lb/ac (1750 kg/ha) of liveweight gain per year. The figure for theoretical milk production is achieved in experiments, although rarely approached in commercial practice; the figure for liveweight gain is never approached, either experimentally or commercially. On commereial farms in Britain, the average stocking rate for grazed lowland pasture in 1971 was about 0·74 cow equivalents/ac (l±82/ha), compared with a theoretical target of 1·9/ac; in 1969 fertilizer N application was about 48 units/ae (60 kg/ha). The main purpose of this paper is to suggest reasons for the differences between practical, experimental and theoretical levels of production. Economic factors may deter farmers from intensifying their grassland management, but the deterrent to high stocking rates may be the fear of running out of grass. In practice, ungrazed herbage is used as a buffer, but other feeds might be used for this purpose. Concentrates, pelleted roughages and even long dried grass and silage are too attractive to grazing beef cattle to be offered to appetite; but silage might be used as a long-term buffer. Fouling of pasture reduces efficiency of harvesting, but attempts to overcome this effect, by conditioning of grazing animals, spreading excreta or by alternation of grazing and cutting, have largely been unsuccessful. High production per unit area cannot be achieved without high production per animal. Legumes have often been found to increase production per animal, and a technique has been developed for growing red-clover and grass in adjoining areas and grazing them together. The clover was grazed satisfactorily by dairy cows (and caused no bloat) but failed to increase milk yield per animal. Recent calorimetric studies of grass have shown that the net energy value of digestible organic matter is variable, and in particular is low for late-season herbage.     

THE EFFECT OF NITROGEN AND STOCKING RATE ON THE OUTPUT OF PASTURE GRAZED BY BEEF CATTLE

An experiment was conducted in two successire years to measure the effect of two levels of fertilizer N, 50 and 300 kg/ha (45 and 270 Ib/ac) on the productivity of pastures grazed by young beef cattle. Two stocking rates were imposed at the lower N level and 4 at the higher level. The responses per kg fertilizer N were approximately 1 kg liveweight gain, 20–24 Meal ME and 8–9 kg DM. Maximal yields of about 1000 kg gain/ha (890 Ib/ac) and 19,000 Meal ME/ha (7700 Mcal ME/acre) were recorded. Animal performance was similar on the low and the high N pastures. There was evidence that the chemical quality of pasture was lower on the low N pasture in the first year, but there was no difference in the second year. The numbers of dung pats per ha and the refusal of herbage due to fouling were reduced by Increasing the stocking rate.     

THE RELATIONSHIP BETWEEN INTENSITY OF GRAZING AND THE HERBAGE CONSUMPTION AND GROWTH OF CALVES

Experiments were conducted in 1967 and 1968 in which HerefordXFriesian (Experiment 1) and Friesian (Experiment 2) steer calves horn in April were turned out to graze at one week or 3 months of age, respectively, and maintained at three stocking densities in the ratio 1:2:3 animals per unit area. The calves grazed paddocks of S23 perennial ryegrass in rotation, and were moved when the height of grazed stubble at the medium stocking density was reduced to 8 cm. The rate of liveweight gain and herbage intake per head declined as stocking rate increased. When the results of the two experiments were compared, the weight gain of the calves was more closely related to the weight of herbage residues than to the height of the grazed sward. The rate of liveweight gain was depressed when the amount of herhage left after grazing fell helow 2000–2500 kg OM/ha (1800–2250 Ib/ac).     

THE EFFECTS OF LEVEL OF NITROGEN FERTILIZER ON BEEF PRODUCTION FROM GRAZED PERENNIAL–RYEGRASS/WHITE–CLOVER PASTURES

The treatments in this summer–grazing experiment were designed as a 2 × 3 factorial, there being two levels of nitrogenous fertilizer (nitro–chalk), supplying 0 and 336kg of N/ha per annum (NO and N3, respectively) and three stocking rates. The fertilizer was applied in 7 dressings at approximately three–weekly intervals. The animals used were Hereford x Friesian steers; these were slaughtered at the end of the grazing season. The stocking rates were calculated on the basis of expected herbage production. The experiment was laid out in 6 randomized blocks which were grazed in rotation. Two blocks were cut for conservation before grazing each year; up to the end of June only 4 blocks were used for grazing.
There was little contamination of the swards with unsown species. With the NO swards the clover contents declined each year, but remained fairly high at over 20% in 1965. Clover contents also declined each year in the N3 swards and by 1965 were only 2 to 4%.
The quantities of herbage cut for conservation were higher from the swards receiving N. The live–weight gains of the cattle (per head) were higher at the lower stocking rates, the effect of stocking rate being more noticeable in the July to Oct. period. Live–weight gains per ha increased at the higher stocking rates. In 1963 and 1965, as stocking rates increased carcass weights per animal decreased and carcass quality, as indicated by the commercial grades, was poorer. In 1964, a dry spell from July to Oct. necessitated the removal of cattle from the experiment and the effects of treatment on carcass weight and quality were not apparent.     

THE EFFECT OF NITROGEN, STOCKING RATE AND GRAZING METHOD ON THE OUTPUT OF PASTURE GRAZED BY BEEF CATTLE

An experiment was conducted in 1971 and 1972 to study the effects of two levels of fertilizer N (50 and 504 kg/ha) on the productivity of pastures grazed by young beef cattle. Two stocking rates were imposed at the lower N level and three at the higher. In addition to rotational grazing, set-stocking and an integrated grazing-conservation system were included. The responses/kg N were approximately 1 kg live-weight gain, 19–24 Meal ME and 8–9 kg DM. Maximum yields of 1200–1300 kg live-weight gain/ha and 25,000–27,000 Meal ME/ha were recorded. Productivity of set-stocked pasture was similar to rotational grazing at the high N level, but lower at low N. The integrated system yielded comparable results to high-N rotational grazing. Performances and herbage intakes per animal reflected the stocking rates imposed.     

The effects of stocking rate and nitrogen fertilizer on a perennial ryegrass-white clover sward

The effects of stocking rate and N fertilizer on a mixed sward of perennial ryegrass ( Lolium perenne) and white clover ( Trifolium repens ) set-stocked with sheep were examined. Sward production and composition, and sheep production were studied.
Increasing the stocking rate over the range 25–55 yearling sheep ha⁻¹ reduced herbage accumulation by about 40%, whether or not N fertilizer was applied. Increasing the stocking rate increased the density of ryegrass tillers, but reduced the density of clover stolons and the clover content of the swards. Applications of N fertilizer (200 kg N ha⁻¹ a⁻¹) increased herbage accumulation by about 20% but substantially reduced the clover content.
Liveweight gain per animal and per unit area were greater at the lower stocking rates where the clover content and nutritive value of the diet were greatest. Wool growth per unit area was greater at the higher stocking rates. Applications of 1M fertilizer increased liveweight gain at stocking rates above 25 sheep ha⁻¹, but had no effect on wool production at any stocking rate.
The results demonstrate that a stable and productive grass-clover association was maintained under conditions of set-stocking at around 23 yearling sheep ha⁻¹and that at this stocking rate, which appears to be about the biological optimum, there was no advantage in using N fertilizer.     

THE EFFECT OF HIGH NITROGEN LEVELS AND STOCKING RATES ON MILK OUTPUT FROM PASTURE

An experiment using 40 lactating dairy cows was conducted for 3 successive years to examine the effects of fertilizer N applied at 400 and 700 kg/ha (358 and 627 lb/ac) at stocking rates of 4.94 and 7.41 cows/ba (2 and 3 cows/ac). The animals were grazed in groups of 10 per treatment, using a fixed rotational grazing system, for 22 weeks in each year. Milk ontput/cow and/ha was significantly affected by stocking rate, the mean milk outputs being 2499 and 2218 kg/cow (5498 and 4880 lb/ cow) and 12313 and 16396 kg/ha (11032 and 14691 lb/ac) at the lower and higher stocking rates, respectively. Milk yield/cow at the lower stocking rate was not affected by N level. At tbe higher stocking rate, increasing the amount of N resulted in a 7.4% increase in milk output/cow and/ha. This response to N declined from 1.7% in the first year of the trial to 2.4% in the third year. The response in milk yield to N appeared to he greatest during the first part of the grazing season. Milk composition, liveweight change, and herbage availahilities and intakes were also recorded.     

THE EFFECTS OF LEVEL OF NITROGEN FERTILIZER ON BEEF PRODUCTION FROM GRAZED PERENNIAL RYEGRASS/WHITE–CLOVER PASTURES

The treatments in this summer–grazing experiment were designed as a 3X3 factorial; 3 levels of nitrogenous fertilizer, supplying 0, 336 and 672 kg N/ha per annum, and 3 stocking rates. The fertilizer was applied in 6 to 8 dressings at intervals of 3 to 4 weeks. The animals used were fattening Hereford X Friesian steers. The stocking rates were calculated on the basis of data available on herbage production under N and irrigation treatments and varied according to the level of N input. The experiment was laid out in 6 randomized blocks, which were grazed in rotation. Blocks were removed from the grazing cycle in spring and early summer of each of the 3 years (1962—-4) over which the experiment was conducted and cut for conservation: yields were recorded. Details are given of: grazing management; results of studies of soil–N levels; the quantities of herbage removed for conservation: the quantity, chemical and botanical composition of the herbage available for grazing; and herbage consumption.     

The use of conserved forage as a supplement for grazing dairy cows

The difficulty in matching the herbage requirements of grazing dairy cows to herbage production, due mainly to the unpredictability of the latter., causes stocking rates to be too low for maximum per hectare production and, thus, cows to be underfed at certain times in the grazing season. Conserved forage may be used as a supplement for grazing dairy cows in order to reduce variation in forage intake by the cow, to allow pasture stocking rates to be increased and to increase the efficiency of land use. The effect of offering conserved forage with herbage on intakes and production is reviewed in comparison to both ad libitum and restricted herbage. Total nutrient intakes and milk fat + protein yields are reduced for cows offered herbage and supplementary forage compared with cows offered ad libitum herbage, but increased compared with cows offered a restricted herbage level. Increasing pasture stocking rates may allow increases in utilized metabolizable energy levels from grassland but further research is needed in this area. Both grass and maize silage supplements offer potential for increasing the efficiency of land use, but in the case of grass silage this is only achieved in the best management practices.     

A PORTABLE-CORRAL TECHNIQUE FOR MEASURING THE EFFECT OF GRAZING INTENSITY ON YIELD, QUALITY AND INTAKE OF HERBAGE

A grazing experiment using four stocking rates of sheep, equivalent to 34, 45, 57 and 68 sheep/ha on an assumed 200-day grazing season, was conducted using a portable grazing corral technique. Plots were subjected to fotir grazing periods between mid May and early September. The effect of stocking rate on herbage yield and quality and the influence of these factors and feed intake on liveweights of the sheep were recorded. The mean daily herbage organic matter available over the trial was 9.6, 5.8, 3.3 and 23% of the total liveweight of the sheep at each of the four stocking rates. The highest grazing pressure led to the production of high-quality herbage, but also led to reduced productivity, low feed intake and liveweight losses. The most lenient grazing pressure failed to provide adequate herbage utilization. Even at the most intensive stocking pressure, only 66% of the herbage available to ground level was utilized in grazings after July. Intake results suggested that sheep of 45 kg liveweight required 1000–1200 g digestible organic matter per day to maintain body weight. Despite the higher in vitro digestibility of herbage on offer at the higher stocking rates, intake was limited through lack of herbage; a high degree of correlation existed between herbage availability and herbage intake over the four stocking rates and at all grazings. It is concluded that the portable corral technique is well suited for grazing studies and the assessment of sward response to varying stocking rates and is particularly useful where facilities for more extensive studies are limited.     

THE USE OF LIVEWEIGHT–GAIN TRIALS FOR PASTURE EVALUATION IN THE TROPICS

A grazing experiment is described in which the liveweight gains of small East African Zebu heifers were measured from Hyparrhenia rufa/Stylosanthes gracilis and H. rufa/Centrosema pubescens swards. The design of this trial enabled animal production to be assessed from variations in liveweight gain since stocking rate was maintained at the same level on both pastures, but the overall stocking rate was varied according to seasonal changes in herbage availability.
Production was measured both by using individual animals within a group as independent units, and by true animal replication. Gains from individual animal blocks were shown to be of value for comparing pasture species, provided that the composition of the grazing group was varied. Problems of conducting and interpreting small–scale grazing trials as means for evaluating pastures are discussed.     

A COMPARISON BETWEEN A PADDOCK SYSTEM AND A 'WYE COLLEGE' SYSTEM OF GRAZING FOR MILK PRODUCTION

Two grazing systems, a paddock system and a ‘Wye College’ system, were compared in two 20-week experiments nsing 16 and 20 spring-calved Ayrshire cows in 1970 and 1971, respectively. A sward of S23 perennial ryegrass was used, and an average of 342 kg N/ha (305 lb N/ ac) was applied per annum. The paddock system had 28 separate paddocks; one paddock was grazed each day in rotation and occasionally topped in mid-season. The Wye College system had 4 plots of equal size and the cows were offered 1/7 of one plot each day; no back fence was used and surplus herbage was neither cut nor topped. The stocking rate on hoth systems was 1 cow per 0.20 ha (0.49 ac) in 1970 and per 0.16 ha (0.40 ac) in 1971. The mean daily milk yields per cow on the paddock and the Wye College systems were 16.2 and 15.3 kg (35.7 and 33.7 lb), respectively, in 1970; and 18.1 and 18.4 kg (39.9 and 40.6 lb) in 1971. The average annual output of milk was 13,500 and 13,150 kg/ha (1200 and 1170 gal/ac) on the paddock and Wye College systems, respectively. The fat and solids-not-fat (S.N.F.) contents of the milk were not significantly affected by the grazing system. It is concluded that the Wye College system was as effective as the paddock system in producing a high output of mUk per ha, but at a lower cost and with far fewer management decisions.     

A FURTHER INVESTIGATION OF THE EFFECT OF NITROGEN AND STOCKING RATE ON THE PRODUCTIVITY OF PASTURE FOR BEEF CATTLE

The liveweight gain was measured of beef cattle on pastures stocked at 5.9 to 11.8 cattle/ha and receiving 50, 182 or 329 kg N/ha in a 12-week season. N levels and stocking rates were so adjusted that similar yields of herbage per animal were expected. Results showed that similar liveweight gains per animal, averaging 1.03 kg/day, occurred at all N levels. Liveweight gain/ha ranged from 547 to 1037 kg in 12 weeks and the corresponding quantities of metabolizable energy utilized were 9890 and 17590 Meal. The results confinn earlier work (4, 7).     

A COMPARISON BETWEEN SPRING AND AUTUMN PASTURE FOR BEEF CATTLE AT EQUAL GRAZING PRESSURES

Two comparisons between spring and autumn pasture for beef cattle were made. Animals used in all comparisons were of similar breed and weight and were subjected to the same feeding regime for 6–8 weeks before turn-out. The pastures were grazed on an equal grazing pressure basis between season comparisons. The pastures received similar rates of fertilizer N between seasons and had similar lengths of rest period for regrowth. Intakes of digestible OM were greater per unit of metabolic liveweight in spring than in autumn. Daily liveweight gains in spring were high, being 1·09 and 1·37 kg (2·4 lb and 3·0 lb), but were lower from autumn pasture at 0·98 and 0·71 kg (2·2 lb and 1·6 lb). Weather was implicated as a factor affecting daily gain from autumn pasture. Greater herbage yields in spring supported 42 and 204 more grazing days per ha which together with the greater gains per animal supported 80–120% more liveweight gain/ha.     

Comparison of herbage production under continuous stocking and intermittent grazing

The possibility of increasing the herbage utilized over a grazing season was investigated in a study comparing continuously stocked steady-state swards maintained at optimum height (3.5 cm) with intermittently grazed swards. The intermittent systems were designed (a) to allow periodic increase in leaf area and hence growth rate, (b)to ensure that the accumulated herbage was eaten before it senesced, and (c) lo retain high tiller density by alternating periods of herbage accumulation with periods of continuous stocking. Two treatments (no animals or animal numbers reduced to half those on the 3.5 cm steady-state treatment) were used during the 17-18-d periods of herbage accumulation. Grazing down was completed in 3–4 d, after which two treatments (14 d or 28 d) were used for the intervening periods of continuous stocking when sward height was maintained at 3.5 cm. Herbage production was estimated using the tissue turnover technique, with tiller population densities and rates of growth, senescence and net production per tiller measured at frequent intervals. Intermittent grazing treatments where animals were removed during herbage accumulation resulted in changes in tiller size and number, and in growth rates, but not senescence rates, per tiller such that short-term deviations in the net rate of herbage production occurred compared with the continuously stocked control. The periods of advantage during phases of herbage accumulation were counterbalanced by those of disadvantage during the subsequent steady-state phases. Where animal numbers were reduced during herbage accumulation, sward conditions differed little from those of the continuously stocked control, implying that intake per individual animal was increased. It was concluded that intermittent grazing systems offered no advantage over simpler continuous stocking systems, provided that a flexible approach to conservation was incorporated to allow control of sward conditions on the grazed area.     

THE EFFECT OF LEVEL OF NITROGEN FERTILIZER ON BEEF PRODUCTION FROM GRAZED PERENNIAL RYEGRASS/WHITE CLOVER PASTURES

In a 3 × 3 factorial experiment, the effects of three levels of nitrogenous fertilizer (0, 336 and 672 kg/N per ha) and three grazing intensities on live-weight gains and carcass attributes were measured.
The live-weight gains of the animals (per head) over the season were greater at the low and moderate grazing intensities than at the high; this effect occurred mainly from July to October each year. Gains were low on the no-nitrogen swards from July 1962 to the end of the grazing season and throughout 1963: this was related to the very high clover content of the swards.
In all years, as grazing intensities increased, the carcass weights per animal became lower. Carcass balance, carcass length, depth of longissimus dorsi muscle, fat thickness over the longissimus dorsi, weight of kidney suet and channel fat, and weight of the alimentary tract also reflected the effect of treatment on weight gains.
The results are discussed in relation to the botanical composition and consumption of the herbage, presented in Part I.     

Effects of field drainage on the growth of herbage and the liveweight gain of grazing beef cattle

A long-term field grazing experiment was begun in 1982 to examine the impact of efficient field drainage on herbage and animal production from swards on an impermeable clay loam in the south-west of England. Drained and undrained lysimeter plots (each of 1 ha) were established on the existing permanent sward and received annual applications of fertilizer N of 200 or 400 kg ha⁻¹. Similar plots were initially ploughed and reseeded with Lolium perenne (cv. Melle), and received fertilizer N at an annual rate of 400 kg ha⁻¹. All plots were continuously stocked by beef cattle and stock numbers were adjusted to maintain a constant sward height and to avoid poaching damage. Results for the first 5 years show that the benefits from drainage were modest and, for beef production, unlikely to pay for its costs over the shorter term. The main benefit was in spring when herbage dry matter yield was 11% greater on the drained plots, but with no significant interaction with fertilizer N level or sward type. This benefit was reduced to 3% on an annual basis, due to the effect of the larger soil water deficits sustained by the drained swards in mid-season. Drainage increased the annual liveweight gain per ha by the grazing cattle by 11%. Possible mechanisms accounting for these effects are discussed in relation to the influences of seasonal patterns of weather.     

Sodium fertilizer application to pasture. 1. Direct and residual effects on pasture production and composition

Results are given on the effect of sodium (Na) fertilizer applied as NaNO₃ in five application sat 0 (Nil), 32 (Low) or 64 (High) kg Na ha ^-1yr^-1 and the residual effect of sodium chloride(NaCl) fertilizer applied in the previous year on the growth rate and the chemical composition of herbage cut at fortnightly intervals over the grazing season. Na fertilizer at Low and High rates increased herbage Na contents by 20 and 40% respectively. Magnesium (Mg) content in herbage was increased at Low Na fertilizer application rate but there was no difference between the Nil and the High Na treatments. Calcium (Ca) content was increased in proportion to Na fertilizer level. Potassium (K) content of Nil Na herbage exceeded that of Low and High, but there was no effect of treatment on total harvested K yields. The contents of phosphorus (P), sulphur (S), true protein (TP) and modified acid detergent fibre (MADF) in herbage were not affected, but the dry matter digestibility (DMD) was increased with increasing Na fertilizer level. Applying Na fertilizer at the Low and High levels gave similar increases in water-soluble carbohydrates (WSC) and ash contents and decreased crude protein (CP) and non-protein nitrogen (NPN) contents. Net herbage accumulation(fresh and dry weight) was increased by 15 and 26% at the Low and High Na fertilizer application rates respectively. NaCl fertilizer residues from the previous year increased herbage Na, DM, DMD and WSC contents, reduced K contents and did not affect Ca, P, S, CP, TP and NPN contents. Residues from Na fertilizer decreased MADF and ash contents at the Low and High Na application rates. The main and residual effects of Na fertilizer were generally maximal in early season and diminished as the season progressed, with many of the effects eliminated by late season. Proportionally 0·70 of Na applied was recovered in the first two grazing seasons after application. The effects of Na fertilizer on mineral ratios are also described, together with the implications for animal health.     

Effects of grazing and precipitation on herbage production,herbage nutritive value and performance of sheep in continental steppe

The present study highlights the effects of sheep grazing and precipitation on herbage and animal performance in a grazed steppe of Inner Mongolia. Experimental data were collected during grazing periods of four consecutive years (2005–2008), and effects were analysed across a gradient of seven grazing intensities. Variation in annual precipitation, reflected by the effect of ‘year’, was the major factor affecting herbage; i.e., the production and nutritive value of herbage increased with increasing precipitation. Herbage parameters were also affected by grazing intensity, as herbage production (HP) and herbage nutritive yields decreased, while herbage nutritive values increased with increasing grazing intensity. The grazing‐induced decrease in herbage nutritive yields suggests that decreases in HP offset the positive effect of grazing on the nutritive value. Liveweight gain (LWG) was predominantly affected by grazing intensity, as LWG per sheep and per ha decreased and increased, respectively, with increasing grazing intensity. However, responses varied among years: LWG per sheep was maximized by light grazing in the drought year and by moderate grazing the wet year. Our results showed that herbage shortage at high grazing intensities reduces LWG per sheep and thus diminishes responses in LWG per ha. Nevertheless, the highest grazing intensity provides highest animal production per ha in the short term; however, this is not sustainable in the mid‐ and long term because decreasing HP induces degradation processes. Based on our results, a reduction in grazing intensity that still provides 78% of the maximum LWG per ha meets the requirements of a sustainable grazing management.