W. F. Smith

Pre-mating herbage intake and the reproductive performance of north Country Cheviot ewes in different levels of body condition

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.

Effect of body condition and pasture type on herbage intake, performance during lactation and subsequent ovulation rate in Scottish Blackface ewes

Scottish Blackface ewes in uniform, good, body condition at mating were differentially fed during pregnancy, such that two groups lost 0·15 to 0·20 (groups A and B) and one group gained 0·05 (group C) of maternal live weight by lambing time. The treatments had little effect on the number of lambs born per ewe lambing (1·39, 1·46 and 148 respectively). Prior to parturition, group A was transferred to a poor hill pasture, and groups B and C to an improved upland reseeded pasture. Herbage intake by six single- and six twin-suckling ewes from each group was estimated in the 3rd, 6th and 8th week after parturition, and mean daily milk production was estimated on 1 day following each intake measurement period. Herbage intake by six dry ewes in each of groups A and B was also estimated. There were significant differences amongst groups A, B and C, respectively, in mean organic matter digestibility of the herbage consumed (0·701, 0·771 and 0·773), mean daily digestible organic-matter intake (1·06, 1·46 and 1·46kg), mean live-weight change over the period ( –2·9, +5·4 and +l·3kg) and mean daily milk yield (0·64, 1·93 and 1·95 kg). By the following mating time, after grazing the same pasture from weaning, there was still a difference in body condition score according to pasture type during lactation (2·08, 2·36 and 2·29, respectively). Ovulation rate measured after mating was consistent with the direct effect of body condition (1·34, 1·56 and 1·52, respectively).

Effects of age and its relationship with body size on reproductive performance in Scottish Blackface ewes

On a hill farm, the reproductive performance of Scottish Blackface ewes increases annually with increasing age up to 6 years and is positively related to pre-mating live weight. A group of Scottish Blackface ewe lambs was, from 5 months of age, preferentially managed over a 2-year period to accelerate the achievement of mature size, in order to study the relative importance of age and size to reproductive performance. Ovulation rate and early embryo mortality of these ewes at 31 months of age were compared with those of adult ewes of similar body size and condition at 79 months of age which had been reared conventionally in the same hill flock. Two comparisons were made, one between different age groups in the same year and the other retrospectively between different ages of the same age group. In the first comparison, the adult ewes had a slightly greater reproductive response which suggested a small effect of age per se but there was also the possibility of a genetic difference. In the second comparison, there was no difference in ovulation rate or potential lambing rate per pregnant ewe, but a trend towards greater wastage of twin-shed ova in the heavier and fatter younger ewes again suggested a small effect of age. At both ages, however, these ewes performed close to their adult genetic potential which suggested that the treatment of the young ewes was sufficient to overcome most of the limitations associated with age or size. Reproductive performance normally associated with young animals is therefore mainly an expression of their size relative to their maturity and the removal of the size limitation by higher levels of nutrition and more rapid growth can largely eliminate their poorer performance.

The effect of different durations and times of high- level feeding prior to mating on the reproductive performance of Scottish Blackface ewes

One hundred and sixty-four Scottish Blackface ewes from two different farm sources were brought into a uniform moderately-poor level of body condition by early November. Five groups of approximately 33 ewes were then fed different amounts of pelleted dried grass and hay on pasture to achieve either high (H) or live-weight maintenance (M) levels of intake for different durations and at different times in the 36 days prior to mating at a synchronized oestrus. One group of ewes was fed the M level throughout, two groups were fed the H level for 18 days either preceded or followed by 18 days of the M level and two groups were fed the H level for 27 days either preceded or followed by 9 days of the M level. After mating, all ewes were fed the M level until slaughtered on return to service or at 28 (± 8) days after mating for counts of corpora lutea and viable embryos relating to first mating. The gain in body condition and live weight after 18 days of H feeding was about half that achieved after 27 days. Time of H feeding had no differential effect on gain in condition or live weight. H feeding increased ovulation rate only if fed immediately prior to mating. When a period of M feeding preceded mating, most of the advantage was lost, particularly with 18 days of M feeding after 18 days of H feeding. Embryo mortality measured as ova loss was not significantly affected by either the duration or time of H feeding, although there was a trend for loss to decrease with increasing duration. Potential lambing rate to first mating was therefore significantly increased by H feeding for 27 days even when followed by 9 days of M feeding prior to mating but was not significantly increased by H feeding for only 18 days even when fed immediately prior to mating. Differences in response were obtained from the ewes from the two sources. H feeding only produced a significant improvement in reproductive performance when potential was high and this was partly related to greater size. In populations with low potential, the provision of H feeding in the pre-mating period is therefore of doubtful economic advantage.

A note on herbage intake by Greyface ewes on perennial ryegrass white clover swards in the autumn

The effects on herbage intake of changes of herbage mass associated with different stocking rates were studied in Greyface (Border Leicester × Scottish Blackface) ewes in the autumn. Intake was measured in 5-day periods in late September (PI), late October (P2) and late November (P3) on two established, perennial ryegrass/white clover swards (SI and S2). Both swards had previously been variably stocked to produce areas with a range in herbage masses, some of which were grazed continuously (G) at stocking rates declining from 18 to 6 ewes per ha. Intake was also measured in P2 and P3 on other areas of the swards which were ungrazed for 6 weeks in September and October (C) and then stocked at 18 ewes per ha. Intakes of dry matter, organic matter and digestible organic matter were 16 g, 10 g and 7·5 g/kg live weight, respectively. Organic matter digestibility was 075 in PI and P2 but declined in P3. Differences in herbage intake and digestibility arose largely because of differences between the swards in herbage mass, sward height and sward density. Ewes in condition scores > 3·00 in early September had lower intakes at all times than did thinner ewes. Changes in sward management associated with different strategies of use had little long-term effect on herbage intake. Keywords: digestibility, herbage, sheep, stocking rate.