Bite Mass Research Articles

Changes in ingestive behaviour of yearling dairy heifers due to changes in sward state during grazing down of rotationally stocked ryegrass or white clover pastures

A group of 12 Holstein×Friesian heifers were rotationally stocked in sequence on uninterrupted spring growths and regrowths following cutting of perennial ryegrass and then on regrowths of white clover. Measurements of ingestive behaviour and plant factors were made every 2–3 days during four grazing periods, which lasted between 12 and 20 days. The sward surface height in the ryegrass paddocks at the start ranged between 20 and 38 cm and contained 2.7–2.9 t DM ha −1 of green leaf mass. At the end of the grazing periods, the swards had a surface height of 8–9 cm, most of the leaf had been consumed and bite mass was considerably reduced compared with levels measured at the start. There was no change in eating time but ruminating time increased as the leaf:stem ratios declined. Total jaw movement rate during grazing declined, possibly because the heifers spent more time searching for leaves. Correlations between bite mass and plant factors suggested that total herbage mass was less useful for assessing likely levels of intake, than green leaf mass or sward surface height when managing grazed ryegrass swards, but total mass may have a place in predicting intake of white clover. Further, more detailed plant measurements indicated that the lengths of the 2nd and 3rd oldest fully expanded grass live leaves were significantly correlated with bite mass and this, along with the role of sheath tube lengths in defining the grazed horizon in ryegrass swards, warrants further investigation.

The effect of bulk density on bite dimensions of cattle grazing microswards in the field

The effect of sward bulk density on bite mass and bite dimensions was examined in two experiments using oesphageally fisulated cows to graze microswards under field conditions. In Experiment 1 microswards of uniform sward height were hand thinned to create differences in sward bulk density. In Experiment 2 plots were sown with monocultures of a range of ryegrass cultivars selected on the basis of their contrasting sward structures to provide a range of sward bulk densities and grazed down in two consecutive strata. Both methods were successful in providing a range of microswards of different sward bulk densities. In Experiment 1 bite mass increased significantly as sward bulk density increased. In Experiment 2 bites taken from the first stratum did not differ significantly in mass due to bites being deeper in swards with low bulk density, compensating for the lower bite bulk density. However, mass of bites taken mainly from the second stratum were lower in some swards due to lower bite bulk density which was related to tiller density rather than sward bulk density. The results emphasize the importance of sward bulk density in determining bite mass and dimensions, especially for the first stratum, and of tiller density in the bulk density of bites in the lower stratum in sward depletion, the latter requiring to be studied further.

An improved method for quantifying intake rate and ingestive behaviour of ruminants in diverse and variable habitats using direct observation

Using heterogeneous vegetation through grazing is a necessary component of de-intensification of livestock systems and conservation of natural environments. Better understanding of the dynamics of animal feeding behaviour is therefore needed, in particular in response to feed diversity and variability. On the basis of three on-farm experiments with ewes and goats, we improved a direct observation method that links intake behavioural response with variations in plant structure. This method makes it possible to continuously and closely monitor an individual animal’s ingestive behaviour during the daily meals, by recording the mass and nutritive quality of its ingestive bites in situ and within its group. “Language” stemming from a coding grid for “bite categories” is proposed for real-time recording of all the bites taken from the full range of available plant parts. Bite categories are defined in order to be applicable to diverse and numerous plant species which allowed real-time recording in very diversified environments, while the impact of grazing alters the plant structure. The continuous presence of an observer does not appear to disturb the animals being monitored, since the variations of their total daily intake time and their diet quality are close to the ones of the groups. The dynamics of ingestive behaviour is explored at different time scales: daily intakes, meal kinetics and biting process. Daily intakes were similar for dry ewes grazing swards invaded by shrubs (77.9±10.2 and 75.1±16.9 g DM/kg LW 0.75). They were much higher for lactating goats browsing within a shrub encroached oak coppice (125.8±7.5 g DM/kg LW 0.75). Cumulative meal kinetics were quite regular and their slopes are of descending form. Our method allowed characterising the huge diversity of bite types that compose such a regular meal. Bite mass ranged between 0.01 and 1.72 g DM. Instantaneous DM intake rate and instantaneous quality of intake were highly variable (i.e. about 20% of variation in OM digestibility during a single meal). At the scale of biting, our method allows to explore the effect of plant structure alteration on the ingestive response and the resulting intake rate. We confirm that intake rate is more influenced by the mass than by the frequency of bites. The main advantage of this method lies in its capacity to characterise bite intake dynamics in very diversified vegetation, from the very short term, i.e. the meal components, to the long term, i.e. several days in a paddock.

A mechanistic model of intake and grazing behaviour in sheep integrating sward architecture and animal decisions

The grazing process determines not only the nutrient intake of ruminants at pasture but also the intensity of their impact on vegetation. Grazing dynamics are the result of complex interactions between animal and sward characteristics. While sward is being depleted, a decline in intake rate is partly offset by longer grazing time. This behaviour may be controlled by nutritional feedback from digestion and nutrient absorption as the quality of the ingested herbage decreases. To simulate the dynamics of feeding behaviour and intake during sward exploitation, we developed a mechanistic model of intake rate that combines sward architecture and foraging decisions, and we linked this model with another focusing on control of intake. The sward was divided into horizons characterised by bulk density and nutritive value (NDF content and digestibility), enabling prediction of bite mass and potential intake rate for each grazed horizon. Animal decisions were simulated at two levels: (i) animal activity (eating, ruminating or resting) was self-regulated every minute by comparing a motivation-to-eat function with a satiation function based on a digestion and metabolic sub-model; (ii) while eating, the horizon to be grazed was decided through a choice function taking into account the relative availabilities and potential intake rates of the two upper horizons. The model simulates the animal–sward interactions from elementary parameters (bite mass, intake rate, etc.) to integrated outputs (sward height, daily intake, etc.). Hence, the interplay between characteristics of the vegetation and the internal state of the animal is dynamic taken from the level of a few bites to several successive days. Satisfactory validations were obtained on experimental data sets obtained in both rotational grazing with dry ewes and continuous grazing with lactating ewes. The sensitivity analysis highlights the balance between factors that control bite mass and intake rate and factors that control grazing time. Combining both series of factors in the same model represents significant progress in predicting intake at grazing in a mechanistic way. The model makes it possible to explore the balance between intake regulation by nutritional variables (animal needs and sward quality) and by the availability and structure of the sward. Finally, it is a promising tool to explore the sensitivity of the grazing process to characteristics of both sward (height, bulk density, nutritive value) and animals (weight, nutritional requirements, behavioural traits), and to management practices (stocking rate, rotational versus continuous grazing).

A mechanistic model of intake and grazing behaviour in sheep integrating sward architecture and animal decisions

The grazing process determines not only the nutrient intake of ruminants at pasture but also the intensity of their impact on vegetation. Grazing dynamics are the result of complex interactions between animal and sward characteristics. While sward is being depleted, a decline in intake rate is partly offset by longer grazing time. This behaviour may be controlled by nutritional feedback from digestion and nutrient absorption as the quality of the ingested herbage decreases. To simulate the dynamics of feeding behaviour and intake during sward exploitation, we developed a mechanistic model of intake rate that combines sward architecture and foraging decisions, and we linked this model with another focusing on control of intake. The sward was divided into horizons characterised by bulk density and nutritive value (NDF content and digestibility), enabling prediction of bite mass and potential intake rate for each grazed horizon. Animal decisions were simulated at two levels: (i) animal activity (eating, ruminating or resting) was self-regulated every minute by comparing a motivation-to-eat function with a satiation function based on a digestion and metabolic sub-model; (ii) while eating, the horizon to be grazed was decided through a choice function taking into account the relative availabilities and potential intake rates of the two upper horizons. The model simulates the animal–sward interactions from elementary parameters (bite mass, intake rate, etc.) to integrated outputs (sward height, daily intake, etc.). Hence, the interplay between characteristics of the vegetation and the internal state of the animal is dynamic taken from the level of a few bites to several successive days. Satisfactory validations were obtained on experimental data sets obtained in both rotational grazing with dry ewes and continuous grazing with lactating ewes. The sensitivity analysis highlights the balance between factors that control bite mass and intake rate and factors that control grazing time. Combining both series of factors in the same model represents significant progress in predicting intake at grazing in a mechanistic way. The model makes it possible to explore the balance between intake regulation by nutritional variables (animal needs and sward quality) and by the availability and structure of the sward. Finally, it is a promising tool to explore the sensitivity of the grazing process to characteristics of both sward (height, bulk density, nutritive value) and animals (weight, nutritional requirements, behavioural traits), and to management practices (stocking rate, rotational versus continuous grazing).

The effect of sward structure as influenced by ryegrass genotype on bite dimensions and short‐term intake rate by dairy cows

Abstract The effects of genotypic variation in ryegrasses on sward structure, bite dimensions and intake rate by dairy cows were investigated. Two experiments were conducted. In Experiment 1, swards were in a vegetative state whereas, in Experiment 2, they were partly reproductive and were taller with higher herbage mass but lower leaf proportion than in Experiment 1. Applicability of relationships between sward structure and bite characteristics, previously established from artificial or hand‐constructed swards, to field conditions were tested. Additional short‐term intake rates and/or sward structural characteristics were considered as indicators of potential intake for use in protocols for the evaluation of grass varieties.Four cultivars were studied: AberElan, Twins (diploid and tetraploid perennial ryegrasses respectively), Polly, a hybrid ryegrass (perennial × Italian ryegrass) and Multimo (Italian ryegrass), each established in 200‐m2 plots in four replicated blocks. Herbage intake rate was determined by short‐term liveweight change (taking account of insensible weight loss) using 16 dairy cows allocated to four balanced groups with each plot grazed by one group for a 1‐h assessment period. One block was grazed per day, over a 4‐d experimental period, with each group grazing each variety in a complete crossover design. Sward characteristics and bite rate were also measured in both experiments. Bite dimensions were subsequently estimated, with bite depth being determined as a function of extended tiller height (ETH) in both experiments.Within both experiments, bite mass and intake rate did not differ significantly between swards of different cultivars despite swards containing Multimo generally having a higher ETH and water‐soluble carbohydrate concentration and lower green leaf mass, sward bulk density and neutral‐detergent fibre concentration than the other swards. However, bite depth was significantly higher (P < 0·01) in swards containing Multimo swards than in the others and, in Experiment 1, bite depth, as a proportion of ETH, was higher in swards containing Multimo and lower in those containing Twins than in the other two cultivars, whereas there was no difference in Experiment 2. Taking both experiments together, the mean bite depth was 0·5 of ETH with sward bulk density accounting for almost half the variance in the relationship between bite depth and ETH. The bulk density of the bite (bite mass per unit bite volume), measured in Experiment 2, followed a similar pattern to sward bulk density, increasing in the order Multimo, Polly, AberElan and Twins.It is concluded that the relationships between sward characteristics and bite dimensions, derived from artificial swards, are applicable to field swards, although the range in natural ryegrass sward characteristics is usually not as wide as in experiments using artificial swards. Lack of precision in the measurement of short‐term intake and in sward‐based measurements is likely to preclude their use in the evaluation of grass varieties.

HERBIVORE FUNCTIONAL RESPONSE IN HETEROGENEOUS ENVIRONMENTS: A CONTEST AMONG MODELS

Variation in the spatial arrangement of plant tissue modifies the functional response of herbivores. In heterogeneous environments, this variation can occur at multiple spatial scales. We used likelihood-based approaches to examine the strength of evidence in data for models of herbivore functional response to spatial variation in plants. These models represented different hypotheses about plant characteristics controlling intake rate, including biomass, bite mass, plant density, and the composite effects of plant density and bite mass. Models were fit to observations of the food intake rate of elk (Cervus canadensis), white-tailed deer (Odocoileus virginianus), prairie dogs (Cynomys ludovicianus), domestic rabbits (Oryctolagus cuniculus), and lemmings (Dicrostonyx groenlandicus) feeding in plant patches where plant density and plant mass ranged over at least two orders of magnitude. The model representing composite effects of bite mass and plant density had more support in the data than any competing mod...

Mechanisms of functional response and resource exploitation in browsing roe deer

Summary The functional responses of roe deer were examined using 11 plant species. A technique to discriminate between encounter‐ and handling‐limited processes was used, and it can be concluded that the functional response applicable to patch browsing by roe deer is governed not by the rate of encounter but by the rate of oral processing. The large differences between plant species were due to variations in both parameters of the functional response: h , the time lost in biting, and R max , the maximum processing rate. Removing the thorns from three of the species affected these parameters differently, according to the size and density of thorns. Animals took larger bites from larger patches (branches), and bite mass declined as patch exploitation progressed, implying that animals were selecting the larger items to eat first. It was demonstrated experimentally that depletion of the larger bites does occur first, and it was concluded that prey selection is an important component of herbivore foraging behaviour. The gain curves for deer feeding on the different plant species are calculated as being virtually linear. Patch depression did not, in general, occur because increasing bite rate compensated for declining bite mass. Our mechanistic approach is contrasted with other approaches to describing the gain curve in the literature. A priori and empirical grounds are presented for rejecting the hypothesis that resource exploitation by browsing mammals is governed by optimal patch use. Diet optimization, involving a trade‐off between diet quality and quantity, offers a better explanation of herbivore foraging behaviour.

Distribuição horizontal e taxas de crescimento, senescência e desfolhação de azevém perene e festuca, puros e em associação

O experimento foi realizado no INRA (Institut National de la Recherche Agronomique) em Theix, França. Duas gramíneas (azevém perene e festuca)foram semeadas em caixas (0,13 m²) usando-se três distribuições horizontais do pasto (pura, linhas alternadas e faixas alternadas) e três intervalos de desfolhações (3,5, 7 e 14 dias), com quatro repetições por tratamento. Quatro meses após a semeadura, a cada data de desfolhação, as caixas foram oferecidas, individualmente, a quatro ovelhas secas e retiradas após a realização de 340 bocados.m-2. Para cada gramínea estudaram-se a densidade populacional e a massa de perfilhos, a altura e a produção de matéria seca, a profundidade e a massa do bocado e as taxas de crescimento, senescência e desfolhação. A maior altura do pasto, gerada por desfolhações menos freqüentes, promoveu a realização de bocados mais profundos e de maior massa. Por outro lado, em desfolhações mais freqüentes a altura do pasto foi mantida mais baixa, e a profundidade e massa do bocado foram menores. A distribuição horizontal das plantas promoveu respostas diferenciadas em relação as taxas de crescimento, senescência e desfolhação. Quando em linhas alternadas a festuca teve a taxa de crescimento limitada, provavelmente desfavorecida pela competição.

Effect of type of supplement offered out of parlour on grazing behaviour and performance by lactating dairy cows grazing continuously stocked grass swards

AbstractTwo 4-week experiments were conducted to examine the effect of type of supplement, offered through automatic feeders at pasture, on supplement intake and grazing behaviour by multiparous spring-calving Holstein-Friesian cows grazing continuously stocked grass pastures. Four supplement formulations were used: a dairy concentrate containing 180 g crude protein per kg fresh weight (treatment DC), a high digestible undegraded protein supplement (treatment HP), a high starch, low protein supplement (treatment HS) and a high fibre supplement (treatment HF). In experiment 1, groups of eight cows were provided with access to one of diets DC, HP or HS, with a maximum of 4 kg being available during each of two periods, between 16:00 and 03:30 h and between 03:30 and 14:30 h. During experiment 2, groups of eight cows were provided access to diet DC, HP, HS or HF, up to a maximum of 8 kg between 16:00 and 14:30 h the following day. Detailed measurements of grazing, ruminating and supplement eating behaviour were made using jaw movement recorders and the transponder-controlled out-of-parlour feeder software. Treatment had no effect on intake rate per min or daily intake of supplement in either experiment, but did affect the temporal pattern of concentrate meals. Treatment did not affect grazing bite mass, bite rate or intake rate in either experiment. During experiment 1, compared with treatment DC, treatment HS reduced total eating time, total grazing jaw movements and daily herbage intake and increased ruminative mastications per bolus and ruminative mastications per kg grass OM intake. In experiment 2, compared with treatment DC, only treatment HS reduced total eating time, total grazing jaw movements and daily herbage intake. Treatments HS and HF both increased ruminative mastications per bolus. The results indicate that when supplements are available at pasture there is a conflict between the time required to consume supplements and herbage and that supplement type can affect both the temporal pattern of supplement intake and subsequent grazing activity.

DINÂMICA DE UMA ASSOCIAÇÃO DE AZEVÉM PERENE E FESTUCA SUBMETIDA A DESFOLHAÇÃO ANIMAL

O experimento foi realizado no INRA (Institut National de la Recherche Agronomique) em Theix, França. Dua gramíneas (azevém perene e festuca) foram semeadas em caixas (0,13 m2) usando-se três distribuições horizontais pastagem (pura, linhas alternadas e faixas alternadas) e três freqüências de desfolhação (3, 5, 7 e 14 dias), com quatro repetições por tratamento. Quatro meses após a semeadura, a cada de desfolhação, as caixas foram oferecidas, individualmente, a quatro ovelhas secas e retiradas após a realização de 340 bocados.m-2. Para cada gramínea estudou-se a densidade e a massa de perfilhos, a altura e a produção de matéria seca, a profundidade e a massa do bocado e os fluxos de crescimento, senescência e desfolhação. A maior altura da pastagem, gerada por desfolhação menos freqüentes, promoveu a realização de bocados mais profundos e de maior massa. Por outro lado, em desfolhações mais freqüentes a altura da pastagem foi mantida mais baixa, e a profundidade e massa do bocado foram menores. A distribuição horizontal das plantas promoveu respostas diferenciadas em relação aos fluxos de crescimento, senescência e desfolhação. Quando em linhas alternadas a festuca teve o fluxo de crescimento limitado, provavelmente desfavorecida pela competição.

Pattern of herbage intake rate and bite dimensions of rotationally grazed dairy cows as sward height declines

To allow improved prediction of daily herbage intake of dairy cows in rotational grazing systems, intake behaviour was assessed throughout the day in 24‐h paddocks. Herbage intake in 16 lactating Holstein–Friesian cows was assessed using the short‐term (1‐h) weight gain method at four predetermined natural meal times throughout the day (early morning, T1; late morning, T2; mid‐afternoon, T3; and early evening, T4). The study comprised two 4‐day experiments, each with a cross‐over design of four blocks. In both experiments, cows grazed a 24‐h paddock daily, and the effect of the immediately previous grazing experience on intake behaviour was investigated throughout the day, taking account of daily fluctuations in the short‐term physiological condition of the cows. Experiment 1 was carried out to investigate overall grazing behaviour during meals as a sward is progressively depleted during the day, with intake being assessed within the paddock and, hence, on a depleted sward. Experiment 2 similarly investigated the effect of sward depletion and physiological condition throughout the day on intake, but cows were removed to fresh, undefoliated swards during intake measurement periods; thus, intake rate was not influenced by differences in sward condition. Intake behaviour from both experiments was compared to establish the effect on herbage intake of changes in sward state and non‐sward factors. In Experiment 1, sward surface height, available herbage mass, proportion of leaf and green leaf mass declined as the day progressed. Bite mass declined with sward depletion, and mean intake rate was 1·64 kg dry matter (DM) h–1, which was significantly lower at T3 (P < 0·01) than during other meals. In Experiment 2, plot sward conditions did not change throughout the day, and intake behaviour also remained constant, with a mean intake rate of 2·11 kg DM h–1. Mean bite depth as a proportion of pregrazing extended tiller height was constant throughout the day (mean 0·32). The results show that, although cows grazed throughout the day on progressively depleted swards, indicative of rotationally grazed paddocks (Experiment 1), bite mass declined linearly and intake behaviour was variable. However, where intake was assessed on high‐quality, undefoliated swards (Experiment 2), intake behaviour was similar regardless of the time of day and the immediately previous experience. There was some indication of an interaction between the effects of the sward and the physiological condition of the animal on herbage intake.

Ingestive behaviour of heifers grazing monocultures of ryegrass or white clover

The ingestive behaviour of dairy heifers grazing monocultures of either perennial ryegrass or white clover was characterised. Two different types of jaw movements were characterised: bites (i.e. when herbage was gathered in to the mouth and then severed) and chews (i.e. when herbage severed by a bite was processed before swallowing). The heifers had a total jaw movement rate (i.e. bites+chews) between 70–80 min −1. Of these, 80–90% were bites, with bite mass varying between 0.15–0.30 g DM per bite. Heifers grazing grass spent longer eating (536 min per day versus 436 min per day) compared to those on clover. As average instantaneous dry matter intake rates min −1 were identical on grass and clover swards (both 12.9 g DM min −1), this gave rise to greater daily intakes of grass compared with clover (6.93 kg DM per day versus 5.61 kg DM per day). However, clover had a higher digestibility than grass (59.9 DOMD versus 77.2 DOMD, % DOM in DM) and so animals on the two different swards had similar intakes of digestible organic matter (4.17 kg per day versus 4.27 kg per day). This gave rise to similar daily liveweight gains (0.97 kg per day versus 0.99 kg per day) for heifers kept on grass and clover, respectively. The masticatory processing (chewing) of the herbage appeared to be delayed from grazing to ruminating, with the heifers kept on grass spending longer ruminating (526 min per day versus 267 min per day) and tending to have more ruminating chews per kg DM ingested (5906 per day versus 3117 per day) than those on clover. These findings suggest that the heifers were regulating their intake by adopting a foraging strategy aimed at minimising grazing time.

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 (R2=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.

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

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

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

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

Further development of the Spalinger-Hobbs mechanistic foraging model for free-ranging moose

Spalinger and Hobbs proposed a mechanistic model of forage intake based on the mutually exclusive actions of biting and chewing. A necessary consequence of this model is that an animal postpones the intake of more food by biting when it is processing food by chewing. In previous work, the Spalinger-Hobbs model successfully predicted short-term intake in controlled experiments. Application of the model to an entire foraging bout requires the following assumptions: (i) biting and chewing are independent events; (ii) there are no periodicities in the length of consecutive bite or chew sequences; (iii) the average bite size is constant; and (iv) the bite rate does not change with the number of bites in the sequence. To test these assumptions, we videotaped entire foraging bouts of two free-ranging moose (Alces alces) feeding on dense swards of Epilobium angustifolium in midsummer. From these videotapes, we measured the time spent biting and chewing, the rates of biting and chewing, the frequency distributions of consecutive bite and chew sequences, and lengths of E. angustifolium shoots above the point of cropping. Plant samples were collected in order to determine bite mass. A total of 1050 bites and chews were analyzed for moose 1 and 1925 bites and chews for moose 2. For both moose, three chewing events occurred, on average, for each bite event. Given this 1:3 bite:chew ratio, the frequency distributions of consecutive bite and chew sequences were as expected from a geometric distribution of independent events. There were no time-series correlations or dominating frequencies in the lengths of bite and chew sequences. These findings fulfill the first three assumptions required to extend the Spalinger-Hobbs model to entire foraging bouts. However, the fourth assumption was not fulfilled, in that time spent per bite increased asymptotically with bite-sequence length. We therefore incorporated the effect of bite-sequence length on bite rate into the Spalinger-Hobbs model. The new model predicts that to simultaneously maximize the marginal intake rate with respect to both bite rate and bite mass, a moose should take single bites most often and bite mass should be approximately 3.6 g. Eighty-two percent of bite sequences were composed of single bites for both moose, and we and others independently observed a bite size of 3.24-3.75 g for free-ranging moose. These observations lend credibility to our modified model.

WHAT CONSTRAINS DAILY INTAKE IN THOMSON’S GAZELLES?

We tested whether cropping or digestion by Thomson’s gazelles (Gazella thomsoni) constrains daily energy intake under sward conditions normally encountered during the growing season. Distinguishing between these alternatives is important in understanding grass–grazer interactions and modeling grazer energetics. Grazing trials on artificial swards showed that gazelles had a monotonically saturating functional response, but that relationships between grazing rate and forage density changed with grass height. Grazing rate was positively related to biomass on short swards, yet there was no significant relationship for tall swards. Bite mass and bite rate also differed in their relationship to biomass across sward heights, with the strongest relationships being found on short swards. Bite rate and bite mass were inversely related, as predicted by current theory for dense grass swards. Voluntary energy intake on a daily basis was a positive function of the digestible energy content of forage, but a negative function of sward biomass. Therefore, our results indicate that daily energy intake is constrained by digestive processes on swards with biomass >25 g/m2, whereas intake is constrained by cropping processes at lower sward biomass. Our data additionally suggest that variation in bite rate and bite mass with sward height could permit a small ruminant to select high-quality grass, thereby achieving high energy gain on immature swards.

The effect of physiological state (lactating or dry) and sward surface height on grazing behaviour and intake by dairy cows

The effect of physiological state lactating vs. non-lactating (dry) on grazing behaviour and herbage intake by Holstein-Friesian cows was examined on grass pastures maintained at 5, 7 or 9 cm sward surface height (SSH), typical of those provided under continuous variable stocking management. Intake rates were estimated over periods of 1 h by weighing the animals before and after grazing, retaining the faeces and urine excreted, and applying a correction for insensible weight loss. Grazing behaviour during these periods and over 24 h was recorded automatically using sensors to measure jaw movements. Bite mass (BM) did not differ significantly between lactating and dry cows but decreased ( P<0.001) from 0.42 to 0.30 g organic matter (OM) bite −1 as overall mean SSH decreased from 9 to 5 cm. An increase ( P=0.040) in grazing jaw movement (GJM) rate, from 75.3 to 80.3 GJM min −1, as SSH decreased, did not compensate for reductions in bite mass, and intake rate declined linearly ( P=0.006) from 24.6 to 18.9 g OM min −1. Lactating and dry cows compensated for the reduction in intake rate, by increasing total grazing time and total number of bites per day. As SSH decreased from 9 to 5 cm, lactating and dry cows increased total eating time (528 to 607 and 419 to 510 min), total GJM (40 400 to 49 300 and 31 300 to 40 600 GJM) and total bites (31 100 to 37 900 and 24 600 to 31 200 bites, respectively). As a result, there was no significant effect of SSH on daily intake of OM, although lactating cows had greater intakes than dry cows; 12.9 vs. 9.3 kg day −1, ( P<0.001). The increased time spent grazing as SSH decreased was associated with a reduction in the time spent ruminating ( P<0.001), despite similar levels of daily intake being achieved across SSH treatments. Although dry cows had much lower daily intakes, they spent only about 30 min less each day ruminating than the lactating cows ( P=0.060), allowing them 120 to 160 min more idling (i.e., non-grazing, non-ruminating) behaviour ( P=0.001).

Patch depression in grazers: the roles of biomass distribution and residual stems

1. In this paper the results are presented of two experiments designed to examine the relationship between sward structure variables and the nature of patch depression experienced by a large grazing herbivore, Bos taurus, grazing from small patches of forage. In experiment 1 the spatial distribution of leaves within patches was varied. In experiment 2 the role of reproductive stems of different lengths as potential intake rate inhibitors was examined. 2. Both experiments were conducted by offering cattle hand‐constructed swards of Orchardgrass (Dactylis glomerata) leaves attached to plywood boards. Each of three experimental animals was allowed to take 10, 20, 30, 45 or 60 bites from a patch and intake was estimated on the basis of pregrazing and postgrazing dry‐matter. All trials were videotaped and the trial length in seconds was determined from playback of the tapes. 3. In both experiments patch depression (reduction in instantaneous intake rate) was due to decreases in bite mass concomitant with patch depletion. Handling time per bite was unaffected by depletion level. In experiment 1, different spatial configurations resulted in markedly different gain functions ranging from linear to sigmoid. In experiment 2, the presence in the sward of long (15 cm) reproductive stems slowed cumulative gains by restricting bite mass and by increasing the per‐bite handling time relative to patches with shorter (5 cm) or no stems. 4. The study of patch depression mechanisms now needs to be generalized over a range of herbivore body sizes and feeding styles.