W2A-028 Shearing lambs-0.txt
Effects of Autumn Shearing of Spring-Born Ewe Lambs on Liveweight and Reproductive Performance

Introduction

Effective exploitation of the reproductive potential of ewe lambs can increase the overall efficiency of a sheep enterprise, particularly in self-contained flocks. However, the onset of puberty in spring-born ewe lambs is typically 6 to 8 weeks later than the onset of the breeding season in adult ewes (Dyrmundsson, 1973). As a result the lambing season in flocks consisting of both adults and ewe lambs may be very long unless the breeding of the adults is delayed. This is unlikely to be an option in most mid-season production systems. It has also been shown that conception rate and embryo survival are impaired in ewe lambs relative to adult ewes (Quirke and Hanrahan, 1977; Quirke, 1979; McMillan and McDonald, 1985). This increases the duration of the lambing season of ewe lambs with consequent inefficiencies in the use of feed and would also tend to increase the risk of perinatal infection under indoor lambing conditions. These difficulties would be minimised if the mating period of ewe lambs coincided with that for adult ewes. However, this would require advancement of the breeding season in ewe lambs.

The breeding season of ewe lambs can be advanced and controlled by the use of hormone treatment based on progestagen plus pregnant mare serum gonadotrophin (PMSG), by the use of exogenous melatonin (Ronayne et al., 1991) or by exploiting the ram effect (Hanrahan and O'Riordan, 1989). There is also some evidence that the onset of puberty can be advanced by shearing ewes lambs prior to joining (Gordon 1967; Kemp, Leve and Berger, 1991) although Dyrmundsson and Lees (1972) reported that autumn shearing was actually associated with a slight delay in the date of first oestrus. O'Doherty and Crosby (1991) and Pickard and Johnson (1992) have reported that autumn shearing improved overall reproductive performance, including conception rate, of ewe lambs.

The present experiment was designed to evaluate the effects of autumn shearing, under natural mating conditions, on mating date, ovulation rate, conception rate and embryo survival in March-born ewe lambs joined with rams on a date typical of that of adult ewes in mid-season production systems. The aim was to identify the components of the reproductive process responsible for any observed change in overall reproductive performance. The experiment was repeated in each of 3 consecutive years.

Materials and Methods

In September 1989 40 Suffolk x (Galway x Cambridge) ewe lambs, which had been born in a single flock in early March, were randomly assigned (13 September) to either shorn or unshorn treatments. The study was repeated in 1990, when 73 Cheviot-cross ewe lambs (sired by either Balclare or Suffolk rams and born in a single flock in late March) were assigned (21 September), within sire breed, to one of the two treatments. All the non-shorn ewe lambs had the crutch region clipped at the time of shearing to eliminate any possible effect of crutch wool on mating efficiency. Ewe lambs had been kept in isolation from males from weaning in July. Ferile, harnessed Texel rams were joined with the ewe lambs in 1989 and both shorn and unshorn lambs were managed together as one flock on a 3 ha unit throughout the study. In the second and third years, management was similar but Suffolk rams were used.

In all years the rams were joined on 16 October and removed in early December (5 to 7). Matings were recorded daily and within 10 days of the first recorded mating each ewe lamb was examined by laparascopy to determine ovulation rate. Ovulation rate was not determined after repeat matings. Ewe lambs which had not been mated by the end of the joining period were examined, by laparoscopy, immediately after ram removal to determine reproductive status.

Following ram removal the ewes were housed in a slatted shed and offered double-chop silage ad libitum plus a concentrate supplement (299g/day of a proprietary pelleted feed containing 160g crude protein per kg dry matter). In late January, pregnancy status and number of foetuses were determined by ultrasonic scanning after which ewes with twins were grouped separately from those with singles. Starting at about 8 weeks pre-lambing, the amount of supplement was increased gradually to a maximum of 400g and 600g/day for ewe lambs carrying singles and twins, respectively.

All animals were weighed before shearing in September (pre-shearing weight), and were weighed again prior to joining in October (pre-mating), at housing in December (post-mating) and in late January or early February (mid-pregnancy). In the case of shorn animals, the fleece weight was added to all subsequent liveweights to facilitate direct group comparisons. Lambing took place indoors under close supervision and lambs were weighed within 24 h of birth. Conception date and rate were verified by reconciliation of dates for mating with observed lambing date.

Data for all traits, except proportion mated and conception rate, were analysed by least squares procedures (Harvey, 1990) using a model which fitted shearing treatment and year-breed combination and their interaction. All animals joined with the ram were used in the analyses of liveweight. The analyses of date of mating (first service) excluded any individuals which were unmated or had failed to ovulate. Lambing date and birth weight were analysed using a model which included effects for shearing treatment, year-breed and litter size. Data relating to proportion mated, conception rate and the distribution of date of first mating were analysed by maximum likelihood procedures for categorical data using the CATMOD procedure (SAS,1988).

Results

Ewe liveweight

The effects of shearing and year-breed combination on liveweight are shown in Table 1. Shearing had no significant effect on liveweight at any stage to mid-pregnancy. However, when liveweights recorded after shearing were analysed, using pre-shearing liveweight as a covariate, the shorn ewes were significantly lighter at the pre-mating stage (about 4 weeks after shearing) but were significantly heavier at mid-pregnancy. The effect of shearing on mid-pregnancy weight was largely attributable to an effect in year 3 and this was reflected in a significant interaction component. Shearing had no significant effect on liveweight gain over the period from shearing to post-mating. Differences between years were significant and, within the second year, Suffolk-x ewe lambs were heavier than the Belclare-x ewe lambs. There was no evidence of any interaction between shearing treatment and either year or breed for any of the liveweight traits or for liveweight adjusted for pre-shearing weight except at mid-pregnancy, as noted above.

Reproductive performance

Information on reproductive performance is summarised in Tables 2 and 3. Only eight of the 166 lambs joined failed to mate and 78% of those mated produced lambs. Six of the unmated lambs had corpora lutea when examined at the end of the joining period. Of the 158 lambs mated, two did not have a corpus luteum at the post-mating laparoscopy. Thus 156 lambs had a date of first service which was accompanied by ovulation. Thirty-three of these animals did not lamb. Shearing treatment had no effect on proportion mated or on overall conception rate and these traits were unaffected by year. There was no evidence for any interaction of shearing treatment with either year or breed. Conception rate to first service was not affected by year or shearing treatment. However, the year-x-shearing interaction was significant (P < 0.05). In the first year the conception rate was higher for the shorn lambs (72% v. 43%) but the opposite pattern occurred in the second year (53% v. 68%). There was little difference due to shearing in the third year (50% v. 52%. Conception rate to first service was affected by ewe breed (second year) with Belclare crosses having a lower conception rate than Suffolk crosses (P 0.05). This was also reflected in overall conception rates.

There was no evidence for any effect of shearing treatment on the proportion of ewes mated within 14 days of joining (P < 0.5) but this trait varied significantly among years (P < 0.01), being lowest in the second year. Likewise, when date of first service was analysed directly, shearing had no effect and there was no evidence for any breed (second year) or shearing-x-breed interaction effects. Ovulation rate at first service was not significantly affected by autumn shearing and there was no year-x-shearing interaction. However, there was evidence for a breed-x-shearing interaction effect in the second year (P = 0.05) when shorn Belclare crosses had a higher ovulation rate than their unshorn counterparts while the reverse pattern occurred in the Suffolk crosses.

The reproductive performance of the ewes which lambed is summarised in Table 4. Ovulation rate, number of services to conception, conception date, lambing date and litter size were all unaffected by shearing. The number of services to conception was not affected by year or breed. Year had a significant (P < 0.05) effect on conception date due essentially to the later date in the second year. While this may be attributable to some extent to the later date for the Belclare crosses, the breed difference was only 4 days and was not significant. It was due to both a slightly later date of first mating and a higher number of repeats. Earliest conception date was seen in the third year although the number of services per conception was also highest in that year. Ovulation rate varied among years and while the F-test for years was not significant (P = 0.17) the mean test for 1991 was significantly greater than that for 1990 (P < 0.05). The pattern of effects on lambing date was similar to that for conception date.

Litter size was not significantly affected by shearing treatment. Year had no effect on litter size and the interactions breed-x-shearing and year-x-shearing were not significant. Shearing did not influence gestation length.

The effects of September shearing on lamb birth weight are summarised in Table 5. While shorn ewes gave birth to heavier lambs the differences were not significant for either birth-type category or overall. There was no evidence for a year-x-shearing interaction but in the second year Belclare crosses gave birth to heavier (P < 0.05) lambs than Suffolk crosses. Mean birth weight in the third year was higher (P < 0.01) than for the 2 previous years.

Discussion

Autumn shearing was found to have a significant short-term effect on liveweight gain but this effect was relatively small and had disappeared by early December (post-mating). In contrast, Salman and Owen (1981) found that September-shorn lambs, offered a pelleted diet while confined outdoors for a 46-day period, had a higher growth rate than unshorn contemporaries but also consumed significantly more food. In the present study herbage supply was considered to be average, with sward heights of 6 to 9 cm (Grennan, 1989), and this is supported by the fact that liveweight increased substantially between September and the end of joining. The lower liveweight gain in the shorn group for the immediate post-shearing period is probably indicative of an inability of this group to increase herbage intake sufficiently to counter-balance the likely increased heat loss associated with shearing.

Shearing had no effect on the date of first service which is consistent with the results of Drymundsson and Lees (1972), who used Clun Forest ewe lambs. In fact, in both studies there was a slight delay in date of first service in shorn animals. Similarly, McMillan and Knight (1982) reported that shearing significantly reduced the proportion of 2-tooth ewes mated during the first 14 days of the joining period, even where joining date was in advance of normal onset of the breeding season. Thus ewe lambs or 2-tooth ewes mated during the first 14 days of the joining period, even where joining date was in advance of normal onset of the breeding season. Thus ewe lambs or 2-tooth ewes do not appear to respond to shearing in the same manner as adult ewes (Lees, 1967). This may be attributable to stress associated with increased heat loss during the early post-shearing period.

The proportions of ewes mated and lambing in the present study were unaffected by autumn shearing. Likewise shearing had no effect on overall conception rate. However, while there was no effect of shearing on conception rate to first service there was a significant interaction for this trait between year and shearing treatment due to opposite effects of shearing between the first 2 years. These results differ in certain respects from those of previous authors. Thus, Gordon (1967) found evidence that shearing increased the proportion mated with only a minor effect on conception rate. However the proportion lambing relative to those joined was only 10 percentage points higher for shorn ewe lambs. An analysis of the results given by Gordon (1967) shows that this difference was not significant (x = 1.62) while the effect on proportion mated approached significance (x = 3.69). Pickard and Johnson (1992) found that the proportion of ewe lambs which lambed was significantly increased by shearing 1 month prior to joining (89% v. 73%). O'Doherty and Crosby (1991), in a study involving synchronisation of mating using PMSG and progestogen sponges, also found that shearing significantly increased the proportion of ewe lambs which lambed to a synchronised oestrus. The proportion which lambed to all services was also increased. Some or all of these increases may be attributable to more effective mating of the shorn ewe lambs as it has been shown that 25 to 30% of ewe lambs which had been marked by rams were not in fact inseminated (Allison et al., 1975). In the present study the unshorn ewe lambs had wool clipped from around the crutch region prior to joining and this may have facilitated effective mating. In a study in New Zealand, using 2-tooth ewes, McMillan and Knight (1982) reported that shearing increased both the proportion which lambed to first service and the overall proportion which lambed following a 6-week joining period.

While litter size was not significantly affected by shearing, in agreement with the observations of O'Doherty and Crosby (1991) and Pickard and Johnson (1992), the shorn ewes had a litter size which was 0.13 higher than for their unshorn counterparts (Table 4). Contrasting results have been reported for the effects of shearing on the prolificacy of 2-tooth ewes by McMillan and Knight (1982) who reported that shearing significantly reduced ovulation rate and litter size. In order to further examine the components of the difference in litter size observed in the present study, data for the set of ewes which lambed to first service (n = 89), and therefore with information on the associated ovulation rate, were examined separately. The results are summarised in Table 6 and show that differences in litter size essentially reflected differences in ovulation rate and there was no evidence for any effect of shearing on embryo survival. This conclusion is based on the fact that the observed difference in litter size is equal to that predicted from ovulation rate, based on a common embryo survival pattern (Hanrahan, 1982), and is consistent with the absence of an effect of shearing on conception rate (Table 2). The observed mean litter size for all ewes in this set was significantly lower (0.10 ± 0.049, P 0.05) than that predicted from mean ovulation rate. This reflects the fact that the proportion of corpora lutea represented by lambs in the twin ovulating ewes in this set was 0.7, which is lower than the values for embryo survival typically observed in twin-ovulating adult ewes (Hanrahan, 1986) but higher than the survival rates observed by ewe lambs by Quirke (1979) and Quirke and Hanrahan (1977).
