Variations in herbage mass and digestibility, and the grazing behaviour and herbage intake of adult cattle and weaned calves

Two experiments are described in which groups of lactating cows, non-lactating cattle and weaned calves grazed a sequence of swards varying in maturity and herbage mass under strip grazing management at a daily herbage allowance of 60 g dry matter per kg live weight. Lactating cows ate 43% and 76% more herbage than non-lactating cattle of similar weight in the two experiments but herbage OM intakes per unit live weight by the calves and lactating cows were similar.
Variations in diet digestibility and herbage intake in the lactating and non-lactating cattle with changes in sward conditions were similar. In the first experiment the calves were experienced grazers; the variation in the digestibility of the herbage selected was less in the calves than the adult cattle, but the variation in herbage intake was greater. The calves in the second experiment were younger, and they were inexperienced grazers; their ability to increase herbage intake in response to changing sward conditions was poorer than that of adult cattle, though variations in diet digestibility were similar in all classes.
The practical implications of these results are discussed briefly.     

The effect of daily herbage allowance and sward characteristics upon the ingestive behaviour and herbage intake of calves under strip-grazing management

Measurements of herbage intake and ingestive behaviour were made on British Friesian calves 4–9 months old, which were strip-grazed on swards of S24 perennial ryegrass at three (Experiment A) or four (Experiment B) levels of daily herbage allowance in three (A) or four (B) 12-d periods. Herbage intake declined by approx. 18% as daily herbage allowance was reduced from 90 to 30 g DM per kg LW in both experiments, but the relationship between intake and allowance was not consistent between periods. Bite size and rate of biting were greater in calves on low than on high allowances after entry to a new strip of herbage but the positions were reversed towards the end of grazing. Reduced herbage intakes at low allowance resulted from reductions of approx. equal magnitude in bite size, rate of biting and grazing time, though not all of these effects were significant. It is suggested that these responses result from the increasing difficulty of prehending and ingesting herbage as swards are grazed down. The calves at low herbage allowances did not increase grazing time, probably because their behaviour was conditioned by anticipation of a new allowance of herbage. Bite size and daily herbage intake were greater on a spring sward than on an autumn sward, but within experiments herbage intake was not consistently related to any of the sward characteristics measured.     

The effect of adding surface water to herbage on its digestion by ruminants

In two experiments the effects of adding surface water to herbage on its digestion by ruminants were examined. In experiment 1, with sheep, adding water to silage reduced DM intake and the diet DM digestibility. Offering a high DM alternative forage, hay, increased DM digestibility with the wetted silage, but tended to decrease it with the dry silage. More hay was eaten with the wetted than dry silage. In experiment 2, with steers, adding water to fresh herbage reduced DM intake but had no effect on rumen pH or ammonia levels. Adding water to the herbage reduced the rate of DM disappearance from nylon bags placed in the rumen and the proportion of rumen liquor particles of low specific gravity. The proportion of very short and very long particles in the rumen liquor was also reduced by adding water to the herbage.     

The effect of herbage allowance upon the herbage intake and performance of suckler cows and calves

Twenty-four Hereford × Friesian cows and their South Devon cross calves were allocated to three herbage allowances allotted daily for three periods of 8 weeks in a Latin square design. The daily allowances were 17, 34 and 51 g dry matter per kg cow plus calf live weight. Milk production was depressed by 0·2 and 1·2 kg d^-1 at the medium and low allowances. The corresponding falls in liveweight gain were for cows 0·26 and 0·25, and for calves 0·27 and 0·24 kg d^-1. Residual sward height after grazing gave a better indication of the animals' reaction to sward conditions and the management imposed than actual herbage allowance. The quantity per unit area and the composition of material present were important factors influencing intake. Calves were unable to compete with their dams to maintain herbage intake at the lower allowances and therefore are likely to benefit from additional feeding or creep grazing when residual sward height falls below 6cm for periods in excess of 1–2 weeks.     

The effect of surface height of swards continuously stocked with cattle on herbage production and clover content over four years

The long-term influence of sward height from April to July (Phase 1) and from July to early October (Phase 2) on total herbage and white clover production was measured over four years (1988–1991) as herbage accumulation. A subsidiary experiment to determine the influence of leaf area index (LAI) on gross canopy photosynthesis was conducted to aid interpretation of growth from herbage accumulation data. Target sward heights in 0·5 ha plots in two blocks were 5,7 or 9 cm in Phase 1 and 7 or 9 cm in Phase 2, although mean actual heights per phase were slightly higher. Net herbage accumulation (NHA) was measured within mobile exclosure areas over successive two-week intervals. Gross photosynthesis was measured in circular turves removed from the trial area representing a range of LAIs with an at least reasonable clover content. Despite wide differences in mean sward height and herbage mass, NHA and net clover accumulation for a given phase were not generally affected by treatments. Positive effects of grazing at 5 cm in Phase 1 on NHA and clover accumulation later in the year, and of grazing at 7 cm in Phase 2 on NHA in the following spring were sometimes apparent. Gross canopy photosynthesis (g CO₂ m^?2 h^?1) at 1500 μE m^?2 s^?1 and 18–21°C was linearly related to LAI described by 1·003 + 1·165 LAI over the LAI range 0·7 to 4·5. Total herbage and clover growth, interpreted from NHA by a previously described model, was predicted to be marginally lower in shorter swards. Similarity in NHA and clover accumulation between treatments was considered to be because of lower senescene and decomposition, and a higher proportion of new tissue being assigned to lamina growth, despite lower LAI and gross photosynthesis in the shorter swards. It was concluded that stocking intensity in swards continuously stocked with cattle did not have a strong influence on net total herbage and clover growth.     

The effect of herbage allowance on intake and performance of ewes suckling twin lambs

The effect of herbage allowance on the herbage intake and performance of ewes and their twin lambs at pasture was investigated. Daily herbage allowances of 40,80.120 and 160 g organic matter (OM) kg^-1 ewe live weight, based on herbage mass measured to ground level, were offered during the first 12 weeks of lactation.
The sheep were grazed rotationally around four paddocks of a perennial ryegrass pasture for 7-day periods and herbage mass, extended tiller length and digestibility of the herbage consumed by the animals were estimated. Herbage intake by the ewes was estimated during weeks two to twelve and live weights were recorded weekly: during the last two weeks of the experiment grazing behaviour of one ewe on each treatment was recorded continuously.
Herbage intakes by the ewes (164, 1–81, 2–42 and 268 ± 0153 kg d^-1) and live weight gains of the lambs (202, 245, 274 and 300 ± 7–3 g d^-1) increased with increasing herbage allowances. Herbage intakes by the ewes and growth rates of their lambs increased up to a herbage allowance which was over five times the amount of herbage eaten by the ewes.
As the animals reduced herbage mass and sward height, biting rates by the ewes during grazing increased by 4 (± 0·08) bites min^-1 cm^-1 and masticating rates decreased. Mastications reached a maximum of 90 (± 3·5) min^-1 at a sward surface height of 9 cm.     

The effects of level of Molinia utilization on diet selection and herbage intake by cattle grazing Molinia grassland

Groups of mature, non-lactating, cows grazed two Molinia -dominant grassland communities in central and southern Scotland during six consecutive summers. Two treatments, designed to use either 33% or 66% of the estimated annual Molinia leaf production by grazing to different leaf lengths, were imposed at each site. Grazing was restricted to the period of Molinia growth each season. During the first 4 years, diet composition, diet digestibility and herbage organic matter intake were determined during either one or two measurement periods each year. There were differences between sites in the floristic content of the sward and these differences were reflected in the diet selected by the cattle. Cows grazing the taller (33% utilization) plots had higher percentages of Molinia , grass stem, sheath and inflorescences and lower percentages of broad-leaved grasses, sedges, rushes and dead herbage in the diet than those grazing the shorter (66% utilization) plots. Differences between the floristic composition of the sward and the diet were explicable by (a) the height at which cattle grazed in relation to the distribution of components within the sward or (b) the selective grazing of small areas dominated by a particular species. The organic matter digestibility of diets differed between sites but there was no significant difference in digestibility or organic matter intake between the treatments. On average less than 50 d grazing was provided by the experimental sites each year. During this period the liveweight gains of cows grazing the two treatments did not differ significantly. The implications of these results for the management of Molinia -dominant communities are discussed.     

Acoustic measurement of intake and grazing behaviour of cattle

Acoustic analysis of grazing behaviour was found to allow accurate identification of chewing and biting, and estimation of intake by cattle. Four steers grazed six types of turves, three were short (14, 16 and 20 cm tall), leafy turves from mowed Setaria lutescens with high application of N fertilizer, and three were tall (36, 49 and 62 cm tall), mature turves from uncut areas with low application of N fertilizer. Each animal grazed ten bites from each turf type while behaviour was videotaped and grazing sounds were recorded with a wireless microphone taped to the animal's forehead. The intake was measured by the difference in pre- and post-grazing turf weight, corrected for water loss. Chews and bites differed in acoustic characteristics, with bites producing more output in the 3–8 kHz frequency band. Discriminant analysis of the acoustic characteristics correctly classified chews and bites with 94% accuracy. Intake was predicted with a high degree of accuracy by total energy flux density (fJ m^–2) in chewing sounds (EFDC), EFDC per chew and average intensity of chewing sound in the 1–2 and 2–3 kHz bands ( R ²=0·90, CV=3·6%). Although bite mass ranged from 1·37 to 4·50 g, EFDC per g of intake (CV=0·015) was not affected by treatments or animals. Chewing sounds carried more information about intake than biting sounds and chews per bite. Analysis of grazing sounds has the potential to overcome many of the problems associated with the measurement of grazing intake.     

Effect of restriction of silage intake and the provision of an alternative forage on the performance of dairy cows

In three experiments the effects of restricting the silage ration of dairy cows and of offering alternative forages as buffer feeds were investigated.
In the first experiment of changeover design with 3-week periods, restricting the intake of low-quality silage to 0.62 of ad libitum had no significant effect on milk yield in spite of the large energy deficit. Offering high-quality hay as a buffer feed increased total DM intake and milk production of cows receiving either restricted or ad libitum silage.
In the second experiment of similar design, restricting the DM intake of high-quality silage to 0·58 of ad libitum significantly reduced milk yield and was associated with a large negative energy balance. Offering straw or ammonia-treated straw with the restricted silage diet did not restore milk production to the level achieved with ad libitum silage but these supplements slightly reduced the energy deficit of the cows. In the third experiment of continuous design lasting 7 weeks, restricting the silage ration to 0.85 of ad libitum had no significant effect on milk production. Offering a strawmix based on straw, barley, molasses and soya with the restricted silage ration restored total DM intake to the ad libitum silage level.
In all three experiments milk protein content was reduced by restricting the silage ration and partially restored by offering alternative forages. There were no significant effects on milk fat content and milk lactose content was only reduced by the severest restriction in Experiment 2. It is concluded that short-term minor restrictions of silage intake can be partly sustained by body fat mobilization but more severe restrictions will result in loss of milk yield. Good-quality hay and a strawmix were of benefit in restoring DM and metabolizable energy intakes, whereas straw and ammonia-treated straw were of little value.     

Estimation of herbage consumption by grazing cattle using measurements of eating behaviour

Feed consumption (I). of a group of eight non-lactating Jersey cows strip-grazing oats at three stages of growth was estimated from number of eating bites (N) and bite size (5) measurements using the expression, I= N × S. N was measured with four non-fistulated animals while S was determined independently with four oesophageal fistulated animals. These estimates were compared with herhage consumption estimates using a cutting technique (fourteen samples of both herbage offered and refused). Eating behaviour estimates of intake based on N and S were 29% higher than cutting estimates due to fistulated animals having a high number of mastication bites with the head up during sampling and a large diurnal variation in bite size. After allowing for these sources of error eating behaviour estimates were only 2% higher than cutting estimates. Sampling variability was similar for both cutting and eating behaviour techniques; on an individual plot basis the percentage coefficients of variation were 11.2 and 10.1 for cutting and eating behaviour techniques, respectively. Methods of reducing sample variability are discussed and tables of numbers of animals required to detect true differences in intake at various levels of precision are presented. It is concluded that herbage consumption by grazing ariimals can be obtained from measurements of eating behaviour.     

The effect of fasting on short-term cattle grazing behaviour

The effects of fasting on feeding behaviour, intake rates, and diet quality were studied with steers grazing vegetative Lolium multiflorum swards. Fasting interrupted the normal alternation of grazing and rumination and led to longer grazing times (0·65 vs. 0·45 of observed time), shorter rumination times (0·15 vs. 0·30 of observed time), and higher total dry matter (DM) intakes (6200 g DM vs. 3750 g DM d⁻¹)-Fasting did not affect prehended bite size which averaged 047 g DM bite⁻¹. Declining bite size within a day was explicable by changes in herbage mass. Biting rate (38.9 bites min⁻¹) and instantaneous intake rate (2675 g DM min⁻¹) of fasted animals were considerably higher than non-fasting animals in the mornings, but neutral detergent fibre (NDF) concentration of the diet (0 742 of organic matter) was generally not depressed by fasting. Consequently, non-fasted animals appeared to be feeding suboptimally since their nutrient intake rates were considerably lower than that which they are capable of exhibiting. Mastication rates per unit NDF intake were considerably lower in the morning for fasted animals (0·78 chews g⁻¹ NDF intake) which suggested that their intake rates may have led to larger ingested particle size. Therefore, on Lolium multifiorum swards, the steers attained a higher intake rate by masticating the forage less while maintaining diet quality.     

The effects of ensiling on dry-matter intake and milk production by lactating dairy cattle given forage as the sole feed

An experiment involving forty-five British Friesian cows in mid-late lactation was carried out to evaluate the effects of ensiling on herbage digestibility, dry-matter intake and milk production. The herbage from the primary growth of a predominantly perennial ryegrass sward that had received 123 kg N ha^?1 was zero grazed (ZG) from 27 April to 24 May 1992. Before going onto the experimental diets all animals were offered a common silage as the sole diet and dry-matter intakes and milk yields were recorded. The herbage was mown, picked-up with a precision-chop harvester and offered as the sole diet, twice daily, 10 fifteen cows that were on average 176 days into lactation. On May 20 herbage from the same sward was harvested identically to the ZG herbage and ensiled, alternative loads being untreated (UT) or treated with formic acid (2.4 l^?1) (FA). For silages UT and FA respectively, pH values were 3.94 and 3.92 and ammonia nitrogen concentrations were 95 and 75 [g(kg total N^?1)]. Forty-two days after ensiling, the silages were fed twice daily as the sole diet to thirty cows which were on average 166 days into lactation. The feeding interval was 28 days and the last 7 days was the main recording period for each treatment. For diets ZG, UT and FA, dry-matter intakes (DMI) (kg d^?1), milk yields (kg d^?1), fat plus protein yields (kg d^?1). milk fat concentration (g kg^?1), milk protein concentration (g kg^?1)and fat plus protein yields [kg (kg DMI)^?1] were 12.70, 11.51 and 12.07 (Av s.e.d. = 0.458); 12.79, 10.01 and 10.18 (Av s.e.d. = 0.346); 0.900, 0.649 and 0.682 (Av s.e.d. = 0.026); 39.8, 33.7 and 36.7 (Av s.e.d. = 0.174); 32.9, 29.9 and 30.0 (Av s.e.d. = 0.83); 0.073, 0.055 and 0.055 (Av s.e.d. = 0.003). UT treatment significantly increased dry matter (DM), organic matter (OM). energy and crude fibre digestibilities and the digestible OM concentration. UT and FA compared with ZG altered rumen fermentation patterns, significantly decreasing butyrate and increasing valerate concentrations. FA treatment significantly decreased the non-glucogenic ratio. It is concluded that ensiling using formic acid had no effect on forage DMI relative to the parent fresh herbage. Ensiling either untreated or with formic acid significantly decreased milk yield and milk fat plus protein yield, resulting in a lower efficiency of conversion of DMI to milk fat plus protein.     

The effect of herbage allowance on daily intake by Creole heifers tethered on natural Dichanthium spp. pasture

Two experiments were carried out in Guadeloupe to estimate the organic matter intake (OMI) and digestibility (OMD) of a Dichanthium spp. sward, grazed by tethered Creole heifers [mean live weight (LW) 202 ± 2·0 kg], at three daily herbage allowances. Experiment 1 examined herbage allowances of 16, 25 and 31 kg of dry matter (DM) d^–1 on a fertilized sward at 21 days of regrowth whereas, in experiment 2, lower allowances of 11, 15 and 19 kg DM d^–1 were examined on the same sward, which was unfertilized and grazed at 14 days of regrowth. In each experiment, the herbage was grazed with three groups of two heifers in a 3 × 3 Latin square design. Sward characteristics were described before grazing. OMI was calculated from total faecal output, and OMD was predicted from the crude protein (CP) content of the faeces. The amount of herbage defoliated by the heifers was also estimated on tillers selected at random.
Organic matter intakes were on average 26 g and 19 g OM kg^–1 LW, and OMD values were 0·740 and 0·665 for Experiments 1 and 2, respectively, and were not affected by allowance. In Experiment 1, the herbage quality was high [0·50 of leaf and 116 g CP kg^–1 organic matter (OM)] for a tropical forage, whereas in Experiment 2, the quality of the herbage (0·27 of leaf and 73 g CP kg^–1 OM) was lower. These differences were reflected in differences in intake and digestibility in the two experiments.
The experimental tropical Dichanthium spp. swards can have intake characteristics similar to those of a temperate sward.     

The herbage intake and performance of set-stocked suckler cows and calves

Groups of eight Hereford × Friesian cows and their South Devon cross calves were set stocked over a 24-week grazing season at 3·23 (low), 3·21 (medium) or 4·24 (high) cows ha^-1 together with their calves. For the first 8 weeks only two-thirds of the total area was grazed. Increasing the stocking rate from low to medium reduced daily milk yield and cow and calf liveweight gains by 1·2, 0·24 and 0·29 kg d^-1 respectively, and from medium to high by 1·2, 0·24 and 0·23 kg d^-1. The main sward factor influencing faecal output and herbage intake was the quantity of organic matter on the pastures but the digestibility of the herbage selected also exerted a significant effect on the intake of cows. Major depressions in the herbage intake of cows occurred once the average sward height fell below 7 cm. Output of calf live weight was 628, 658 and 743 kg ha^-1 for the 3 stocking rates from low to high, and for cows 246, 179 and 30 kg ha^-1. It was concluded that decisions on pasture management should be taken in relation to the cow rather than the calf on set-stocked swards.     

The effect of silage chop length on the voluntary intake and rumination behaviour of sheep

Two experiments were conducted with sheep to investigate the effect of silage chop length on voluntary intake and eating and ruminating behaviour. The silages, made from Italian rye-grass, either received no additive or formic acid at a rate of 4 g kg^?1 and were chopped to lengths of 5.3 cm (long), 1.8 cm (short) before ensiling or 1.8 cm (long/short) before feeding. The addition of the additive and short chopping improved the fermentation characteristics of the silage. Voluntary intake of silage dry matter and organic matter was higher with the short than with the long silage. Differences in silage digestibilities and cellulolytic activity did not explain the increase in silage intake. Sheep on the long silage had a lower rate of intake and a shorter ruminating time than on short silage, and thus the effective breakdown of the silage in the rumen was delayed, i.e. an increased latency time. Pseudo-rumination was significantly higher on the long than on the short silage.     

Effect of maize grain and herbage allowance on estimated metabolizable energy intake and animal performance in beef cattle finishing systems

This study investigated the effects of levels of supplementation with maize grain and herbage allowance (HA) on grass herbage and maize intake, animal performance and grazing behaviour in two replicated grazing experiments with Angus beef cattle in Argentina. In Experiment 1, the response to increasing HA (2·5, 5·0 and 7·5 kg DM herbage 100 kg^?1 live weight (LW) d^?1 with and without 0·5 kg DM maize grain 100 kg^?1 LW d^?1) was investigated. In Experiment 2, the responses to level of maize grain offered (0, 0·5 and 1·0 kg DM maize grain 100 kg^?1 LW d^?1) at an HA of 2·5 kg DM herbage 100 kg^?1 LW d^?1 and an HA of 5·0 kg 100 kg^?1 LW d^?1 without maize grain were assessed. In Experiment 2, soyabean meal was added to control the crude protein concentration in the diet. Two methods were used for intake estimations: pre‐ and post‐feeding herbage mass difference, and the use of the n‐alkane and ¹³C technique. The latter predicted most accurately the metabolizable energy requirements calculated from live weights and liveweight gain of beef cattle attained in each treatment in both experiments. Increasing HA significantly increased herbage intake and liveweight gain (P < 0·01), and general quadratic relationships between these variables could be fitted across experiments despite differences in animal and pasture characteristics. Increasing the amount of maize grain offered significantly reduced herbage intake and grazing time, but increased liveweight gain and digestibility of the diet. Substitution rate increased with increasing HA in Experiment 1 but was not affected by level of maize supplementation in Experiment 2. These relationships will aid the development of grazing management models for Argentinean conditions.     

The effect of the density and physical properties of grass stems on the foraging behaviour and instantaneous intake rate by cattle grazing an artificial reproductive tropical sward

In reproductive swards, stems appear to act as vertical or horizontal barriers to bite formation, influencing instantaneous intake rate (IIR). The hypothesis was tested that the stems’ barrier effect is determined by the physical properties and density of stems. Artificial microswards, consisting of 20‐cm leaves and 15‐cm stems of Panicum maximum, were offered to three steers (362 kg) in a factorial combination of three stem densities (0, 100 and 400 stems m^?2) and two levels of stem tensile resistance [low (LTRS) and high tensile‐resisting stems (HTRS)]. LTRS were not a barrier to defoliation and did not affect bite depth and bite mass. HTRS acted as both a horizontal barrier and a vertical barrier depressing bite depth (13·4, 13·6 and 5·1 cm for 0, 100 and 400 stems m^?2, respectively), bite area (89·3, 50·8 and 47·6 cm² for 0, 100 and 400 stems m^?2, respectively), bite mass (0·51, 0·29 and 0·11 g for 0, 100 and 400 stems m^?2, respectively) and IIR (23·8, 10·5 and 3·6 g sec^?2 for 0, 100 and 400 stems m^?2, respectively). The results confirmed the importance of the density and physical properties of stems as determinants of the stems’ barrier effect on bite dimensions and IIR.     

Variations in the surface characteristics of the sward and the short-term rate of herbage intake by calves and lambs

The results are reported of an examination of detailed measurements on the ingestive behaviour of calves and lambs in response to variations in the surface characteristics of swards under strip-grazing and continuous stocking managements. It is shown that intake per bite and the short-term rate of herbage intake were both sensitive to the height of the surface horizon above ground level but, at least under strip-grazing management, not to variations in herbage density in the grazed horizon. The rate of biting was less sensitive to variations in sward conditions, particularly under strip-grazing.
Both intake per bite and rate of intake were more sensitive to variations in grazing height under strip-grazing, where sward changes were rapid, than under continuous stocking, where they were slow. Under continuous stocking, ingestive behaviour was more sensitive to changes in sward conditions in lambs than in calves.     

The effect of frequency of harvesting grass for silage on the intake and performance of beef cattle

Two randomized-block experiments were conducted to examine the effects of frequency of harvesting grass for silage on the intake and performance of beef cattle. In both experiments swards of S24 perennial ryegrass were harvested at 63-, 49- and 38-d intervals throughout the growing season. Grass harvested before and after 10 July was ensiled separately and termed spring and autumn silage respectively. In Experiment 1 the silages were offered ad libitum either unsupplemented or supplemented with 2 kg barley per head daily to eighty-four Hereford-cross weaned, single-suckled steer calves of mean initial live weight 284 kg, in a 3 × 2 × 2 factorial design. Silage dry matter (DM) intakes and liveweight gains for the silages harvested at 63-, 49- and 38-d intervals were 4.80, 0.72; 4.49,0.76; and 4.62,0.78 kg d⁻¹ for the spring silages and 4.69, 0.67; 4.59, 0.85; and 4.55,0.86 kg d⁻¹ for the autumn silages respectively. There was no significant interaction between frequency of harvesting and concentrate supplementation. In Experiment 2 the silages were offered ad libitum and unsupplemented to forty-two Hereford-cross weaned, single-suckled steer calves of mean initial live weight 240 kg, and forty-two Hereford-cross store cattle of mean initial live weight 356 kg, in a 3 × 2 × 2 factorial design. Silage DM intakes and liveweight gains for the silages harvested at 63-, 49- and 38-d intervals were 4.94,0.49; 5.69,0.80; and 619,0.93 kg d⁻¹ for the spring silages and 5.50, 0.61; 5.57, 0.72; and 505, 0.65 kg d⁻¹ for the autumn silages respectively. There were no significant interactions between frequency of harvesting and type of animal. It is concluded that a cutting interval of 49 d commencing on 21 May is likely to be optimum for this type of sward except under exceptional weather conditions.     

The effects of variation in sward characteristics upon the ingestive behaviour and herbage intake of calves and lambs under a continuous stocking management

Groups of calves and lambs grazed on plots of S23 perennial ryegrass, in successive periods of 2, 2 and 3 weeks, at stocking rates which resulted in a slow change in herbage mass on each plot and measurements were made of ingestive behaviour (biting rate, bite size and grazing time) and herbage intake. As the green herbage mass was progressively reduced from approx. 3000 to 1000 kg OM per ha between periods, biting rate and grazing time increased, but insufficiently to offset the rapid fall in bite size so that herbage intake declined by 24% and 39% for calves and lambs respectively. Patterns of response were similar in calves and lambs, though in all parameters except biting rate the lambs appeared to be more sensitive than the calves to changes in sward conditions and there was no evidence that lambs were better adapted to grazing on short swards than calves. Intake increased in both calves and lambs up to levels of 2500–3000 kg green OM per ha, though this response may have been attributable in part to the reinforcing effects of increasing herbage mass and diet digestibility. Lambs consistently selected a more digestible diet than calves. Both calves and lambs increased grazing time substantially in response to limiting sward conditions, in contrast to results in earlier strip-grazing trials with calves, thus supporting the suggestion that the lack of response under strip-grazing is a characteristic of the management imposed.