Ewe genotype effects on seasonality of oestrus and ovulation in south western Victoria.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 19 EWE GENOTYPE EFFECTS ON SEASONALITY OF OESTRUS AND OVULATION IN SOUTH-WESTERN VICTORIA L. J. CUMMINS, S. A. SPIKER and J. M. WILSON Dept of Food and Agriculture, Pastoral and Veterinary Institute, PB105, Hamilton, Vic. 3300. SUMMARY Two experiments, one examining oestrus and ovarian activity in 5 genotypes of young ewes exposed continuously to teaser rams and the other examining the onset of the breeding season in 4 separate strains of young Merino ewes, were carried out at Hamilton. In each experiment there were genetic differences in the duration of the non-breeding season. In experiment 1, Merinos and Comebacks had a shorter and less intense anoestrus season than Corriedales, Romney Corriedale cross and Border Corriedale cross ewes. In experiment 2, South Australian strain 3 had an earlier onset and Peppin strain 1 a later onset of the breeding season than Western District strain 1 or South Australian strain 1 ewes. Key words: breeding season, genetic differences, sheep. INTRODUCTION Genetic differences in the seasonality of reproduction in sheep have been recognised for a long time. Photoperiod is the primary cue but this can be modified by other factors such as nutrition, the ram effect and melatonin (Scaramuzzi 1988). Advisors need to be well aware of the basic pattern of reproduction in the genotypes and environment where they are working before they can predict the likely outcome of variation in mating management. This is particularly important in those artificial breeding programs which precede normal mating periods. MATERIALS AND METHODS Experiment I Oestrus and ovarian activity were monitored in 2 drafts of young ewes. The 5 genotypes involved were fine wool Merinos, Comebacks (Obst et al. 1991), Corriedale, Romney Corriedale cross and Border Leicester-CorriedaIecross (Spiker et al. 1987). In autumn each year, 15 or 16 maiden ewes of each genotype which had been bred on the Pastoral Research Institute (PRI) were selected from those available and run as a common flock. From April 1983, when the ewes were about 15 months of age, until June 1984 these ewes were run with teaser rams fitted with mating crayons. Fresh raddle marks were recorded at between weekly and fortnightly intervals. At monthly intervals ovarian activity was recorded by laparoscopy after overnight fasting. From April 1984 until May 1985 a second draft of 15 maiden ewes of these genotypes were subject to the same treatment. The ewes were grazed on perennial pastures at PRI (location 37'49' S.) and provided with supplementary feed as required to prevent excessive weight loss. The data reported here are the proportion of ewes showing oestrus each month, the proportion ovulating each month and the ovulation rate per ewe ovulating. Since endoscopy was at monthly intervals, both current corpora lutea and well defined corpora albicans were included as measures of ovarian activity. Experiment 2 Ewes from the following 4 Merino genotypes: Western District 1 (Victoria), Peppin 1, South Australian 1 and South Australian 3. were used in an experiment to determine the onset of the breeding season. These ewes were an extra sample of the genotypes used in a wether production trial (Wilson 1986). There were 15 or 16 ewes from each genotype and all were grazed as part of a common flock from birth. On 16 December 1982, when the ewes were about 16 months of age, teaser rams with mating harnesses were run with the flock until May 1983. One year later teaser rams were run with the mob from mid December until May. Under these conditions the occurrence of oestrus in early January would be a function of the ram effect (Pearce and Oldham 1984). The ewes were examined by laparoscopy in May 1983, December 1983 and January 1984 and May 1984. In both experiments there were a few occasions when not all ewes were present. Chi-squared procedures were used to examine the significance of oestrus activity and proportion of multiple ovulations. 181 Proc. Aust. Sot. Anim. Prod. Vol. I9 RESULTS Experiment 1 The results are shown in Fig. 1. As the animals entered the non-breeding season there was a close association between observed oestrous behaviour and ovarian activity. However as the ewes entered the breeding season in summer the initiation of ovarian activity preceeded oestrus behaviour. Reporting ovulation rate in terms of per ewe ovulating leads to several discrepant observations during the nonbreeding season (e.g. Border cross ewes September 1984 and Comebacks October 1983 and November 1984) due to small number of ewes ovulating at these times, but it seems a good description of ovulation rate during the breeding season. Based on the somewhat arbitary classification of less than 50% of ewes showing oestrus in any month, the non-breeding season lasted from August until February with little difference between the breeds. There was a suggestion that the Merinos and Comebacks had a slightly shorter anoestrus period. The degree of anoestrus was slightly less in the Merino and Comeback groups. During the breeding season the Merinos and Comebacks had a slightly lower proportion of ewes showing oestrus, particularly the maiden groups in 1984. Based on an arbitrary classification of less than 50% of the ewes showing ovarian activity, the nonbreeding season lasted from August in 1983 or from July in 1984 until January in each year. The Border cross ewes were slightly later than the other groups in commencing ovarian activity. In the winter and early spring of 1983 some Merinos and Comebacks continued to ovulate. During the peak of the breeding season the Border cross ewes had the highest proportion of ewes with multiple ovulations and the Merinos the lowest. Table 1 shows the mean for the 2 drafts of ewes observed in April or May of each year and classified as either 15-month old or 27-month old ewes. Table 1. Proportion of multiple ovulations during the peak of the breeding season in young Merino, Comeback, Corriedale, Romney Corriedale cross and Border Corriedale cross ewes. Within columns, means followed by the same letter are not significantly different (P > 0.05). Experiment 2 In both years the proportion of ewes showing oestrous behaviour within 3 weeks of joining was depressed in the Peppin group. Two months after the start of joining oestrus activity was high in all groups except the Peppin group in 1983 (Table 2). When ovarian activity was examined in May 1983, approximately 60% of the ewes were ovulating with no obvious genotypic effects. At the start of the next season in December 1983 more ewes in the South Australian 3 group were ovulating than in the Peppin group (Table 3). Later examinations in January and May 1984 showed most ewes were ovulating with little difference between the genotypes. Very few multiple ovulations were observed in these ewes. Table 2. Proportion of ewes detected in oestrus or ovulating by given dates Within columns, means followed by the same letter are not significantly different (P > 0.05). 182 Proc. Aust. Sot. Anim. Prod. Vol. 19 DISCUSSI0N Experimenf 1 showed a clear pattern of seasonality in ewes at pasture in south-western Victoria which were continuously exposed to rams. There was a period of anoestrus and anovulation over spring, although the intensity of this was less in Merinos and Comebacks than in the Corriedales and Crossbreds. There were moderate differences due to genotype in the duration of the non-breeding 183 Proc. Aust. Sot. Anim. Prod. Vol. 19 season. To some extent this may be a consequence of the experimental methods, however in a previous study using similar methods in northern N.S.W., a larger difference was observed between Merino and Border Merino cross ewes (Cummins 1983). Experiment 2 demonstrated differences between strains of Merino ewes in reproductive ability when rams are introduced in December. In this experiment the Peppins were lighter than the other strains (Wilson 1986), but it is not clear to what extent this contributed to their poorer performance. Watson (1962) also reported a later transition to the breeding season in Peppin ewes compared with other Merino strains. REFERENCES CUMMINS, L. J. (1983). Ph.D. Thesis University of New England. OBST, J. M., THOMPSON, K. F., CAYLEY, J. W. D., THOMPSON, R. L., SAUL, G. R., BROCKHUS, M. A., CUMMINS, L. J., WILSON, J. M. and BUTLER, K. L. (1991). Aust. J. Expt. Agric. 31: 15-23. PEARCE, D. T. and OLDHAM, C. M. (1984). In ` Reproduction in Sheep.' (Eds D. R. Lindsay and D. T. Pearce.) p. 26 (Aust. Acad. Sci., Canberra.) SCARAMUZZI, R. J. (1988). Proc. Aust. Sot. Anim. Prod. 17: 57-73. SPIKER, S. A., CUMMINS, L. J. and BROCKHUS, M. A. (1987). Proc. Aust. Assoc. Anim. Breed. Genet. 6: 297-9. WATSON, R. H. ( 1962). Aust. Vet. J. 38: 3 10-23. WILSON, J. M. (1986). Wool Tech. Sheep Breed. 34: 88-91. 184