Summary 0[ Theoretical studies of large herbivore foraging assume that total daily grazing time is a key constraint on daily intake and diet choice[ We experimentally tested this assumption and investigated the e}ects of food availability on the ability of grazing sheep to compensate for restriction of available daily grazing time[ 1[ Foraging behaviour\ intake and diet digestibility by sheep\ were measured on grass pastures in a replicated 1 × 1 factorial experiment\ in which overnight access to pasture was varied "restricted overnight and continuous access# on two sward heights "4=4 and 2=9 cm#\ representing high and low food availability[ 2[ Regardless of food availability\ the overnight!restricted sheep fed for almost all of the available grazing time by grazing for fewer\ longer foraging bouts\ but still had much shorter total daily grazing time than the continuous access sheep[ 3[ In response to overnight penning\ the sheep had a signi_cantly higher instantaneous rate of intake achieved mainly via larger bites[ The continuous access sheep were hence not maximizing their short!term rate of intake\ whilst grazing according to the daily schedule considered normal for sheep[ 4[ The behavioural responses to overnight food restriction were able to counteract the reduction in daily grazing time only where food availability was high[ In contrast on short swards overnight grazing restriction led to a reduction in total daily intake[ We suggest that the interactions between the factors considered as constraints on foraging behaviour of herbivores are\ as yet\ only poorly quanti_ed[ Key!words] foraging constraint\ intake rate\ sheep\ sward height\ time limitation[ Journal of Animal Ecology "0888# 57\ 76Ð82
Over 3 years, a flock of Scottish Blackface ewes was managed so that one-third of the ewes received a high level of nutrition during the last 100 days of pregnancy (P), one-third received a high level of nutrition during the first 100 days of lactation (L) and one-third received a low level of nutrition as derived from a hill grazing over the same two periods (C). From these ewes, 496 ewe lambs were retained at weaning (19 to 22 weeks of age), balanced for treatment, year and birth type. At 6 months of age half of the lambs, similarly balanced, were transferred to an upland grazing system (U) for three breeding years. The other half was retained on the same hill grazing as their mothers for the same period (H). At the end of the study, all ewes were fed to reduce the range in body condition at a synchronized mating and ovulation rates were determined at slaughter.Differences in live weight of ewes due to treatment between groups P, L and C had disappeared by 18 months. There were no pre-mating differences between treatments in live weight or body condition score except that ewes in group P had higher live weights than those in group C at condition scores greater than 2·75. There was little effect of grazing system on the differences between treatments in live weight or body condition score at 30 and 42 months but H ewes were heavier and fatter at 18 months of age. Ewe mortality was consistently less in group L than in groups P and C and on system H than on system U, although the differences were not statistically significant. Group L had fewer barren ewes than groups P and C, and barrenness was significantly greater in the U than the H system. Group C had proportionally more single than multiple births than either groups P or L.At the end of the study there were no differences between the treatments in the proportion of ewes successfully mated at the final mating nor in their ovulation rates in relation to pre-mating body condition.It is concluded that nutrition during either the lactation period or late pregnancy period can influence subsequent lifetime reproductive performance and that this was expressed through an effect on embryo or foetal loss since there were neither residual differences apparent in live weight or condition score nor in ovulation rate potential.
Summary 1.The relationship between available biomass and short-term rate of intake (functional response) of herbivores is expected to provide a link between their food supply, and their distribution. 2. The functional response of captive wild rabbits (Oryctolagus cuniculus L.) on artificially produced broad-leaved (Lolium perenne L.) and narrow-leaved (Festuca ovina L.) grass swards was quantified. 3. The general prediction that habitat selection varied with biomass, and reflected the potential rate of intake defined by the functional response, was also tested. The main alternative predictions, that both intake rate and habitat selection increased asymptotically with biomass, or were biased towards intermediate habitat standing crop biomasses, were distinguished. 4. There was no relationship between biomass and short-term rate of intake on the narrow-leaved Festuca swards, but on the broad-leaved Lolium sward the short-term rate of intake increased asymptotically with biomass. 5. In a field experiment on Lolium swards, a population of free-living wild rabbits selected the shortest swards with the lowest biomasses, and which provided the lowest potential rates of intake. 6. Results demonstrate that free-living wild rabbits do not select habitats that provide the maximum potential rate of intake, nor did they select foraging habitat with intermediate standing crops. It is suggested that their selection of foraging areas in these grasslands which typify rabbit foraging habitat, is dominated by antipredator considerations rather than purely by rate of intake.
Over 4 years, a flock of mature North Country Cheviot ewes was grazed on different swards between weaning in August and mid October to achieve a range of condition scores at 5 weeks before mating (post-weaning period). During the subsequent 5 weeks before a synchronized mating (pre-mating period), all ewes were grazed on a perennial ryegrass/white clover sward, provided at high (H) or low (L) herbage masses (1600 to 2100 or 1200 to 1400 kg dry matter (DM) per ha, respectively). Ovulation rate was derived from the number of corpora lutea in 101 ewes slaughtered after first mating. Conception and lambing rates were derived from the number of lambs born to first mating of 406 ewes. In 2 out of the 4 years, 79 ewes were used to estimate herbage intake.There was no effect of post-weaning management on reproductive performance. Herbage intake and reproductive performance were examined in relation to the body condition of the ewes at 5 weeks before mating. On an H herbage mass pre-mating, herbage intake in the period immediately prior to mating was greatest in ewes in low body condition and declined significantly with increasing condition. On an L herbage mass pre-mating, herbage intake did not vary significantly with condition. Ovulation rate was low in ewes in low body condition (scores =≤·25) at 5 weeks before mating, increased significantly with increasing condition up to scores 2·50/2·75 but did not change significantly above such intermediate scores. Conception and lambing rates increased significantly with increasing body condition up to scores 2·50/2·75 and then decreased significantly as condition increased above these levels. These effects of body condition were greater on an L herbage mass pre-mating than they were on an H herbage mass. Ewes in the intermediate body condition scores of 2·50 and 2·75 at 5 weeks before mating had a low sensitivity to contemporary nutrition pre-mating. Such condition levels are therefore considered to be above the range of condition in the North Country Cheviot breed within which reproductive response is affected by contemporary nutrition pre-mating.The lower reproductive performance of ewes in high body condition (scores 3=3·00) at 5 weeks before mating was related to an increase in estimated ova and embryo loss and may be associated with a reduced herbage intake.
The effects of high-low (HL) and low-high (LH) patterns of nutrition during the 5 weeks prior to a synchronized mating were studied on the reproductive performance of 115 Welsh Mountain and 114 Brecknock Cheviot ewes in a range of body condition at the start of treatment. For 2 months prior to treatment imposition, ewes were grazed in a systematic way at different stocking rates on different sward heights to obtain a range of body condition scores. Target score groupings were 2·00 to 2·25, 2·50 to 2·75 and 3·00 to 3·25 and although scores of 1·50 to 3·25 were obtained, most lay in the 2·00 to 2·75 range. Two treatment groups, balanced for live weight and body condition score, received high and low levels of nutrition for 16 days from mid October, achieved, respectively, by a low stocking rate on a sward with a high surface height plus ad libitum concentrate and a high stocking rate on a sward with a low surface height plus 200 g hay per head per day. Treatments were then reversed for the 17 days prior to mating. Live-weight and body condition-score changes were recorded and reproductive performance at first mating was measured from counts of corpora lutea and viable embryos at slaughter either at return to service or at 4 weeks after first mating. There were no differences due to nutritional pattern in live weight or body condition score at mating. The LH treatment significantly increased the rates of ovulation, conception, multiple ova survival and potential lambing per ewe pregnant and per ewe mated in the Welsh Mountain breed but significantly increased only the rates of ovulation and potential lambing per ewe pregnant in the Brecknock Cheviot breed compared with the HL treatment. The relatively poorer response in the latter breed was partially due to a lower ovulation rate potential coupled with non-significantly lower rates of conception and survival of single-shed ova associated with the LH treatment. Much of this relatively poorer reproductive performance in the Brecknock Cheviot breed was in ewes with body condition scores 3= 2·75. Reproductive performance increased with increasing body condition at the start of treatment over the range of scores =≤ 2·25 to 2·50 in both breeds and, while continuing to improve in scores above 2·50 in the Welsh Mountain breed, in the Brecknock Cheviot breed it started to decline. Ewes with a body condition score of 2·50 in both breeds showed the greatest response in potential lambing rate to the LH treatment.
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