Abstract:
RELATIONS BETWEEN BODYWEIGHT AT MATING, AGE AND THE SUBSEQUENT REPRODUCTIVE PERFORMANCE OF MERINO AND CORRIEDALE EWES J. W. MCLAUGHLIN* Summary The reproductive performances of Merino and Corriedale ewes in relation to ewe age, bodyweight at mating and time of mating (Merinos only) were examined by multiple regression analyses. Each kg increase in the mean bodyweight of the flock was associated with 1.9 per cent and 0.9 per cent increase in the percentage of ewes with multiple births for Merinos and Corriedales respectively. At the same bodyweight, Merino ewes mated in April-May had 17 per cent more multiple births than ewes mated in November-December. Each year increase in the age of Corriedale ewes was associated with 2.5 per cent increase in the percentage of ewes with multiple births. Age did not influence the performance of Merino ewes. For both breeds, dry ewe percentages increased as the mean bodyweight of the flock decreased. I. INTRODUCTION In a review on flushing, Coop (1966) concluded that both bodyweight at mating and change in bodyweight before mating were associated with ewe reproductive performance. Most workers agree that, independently of change in bodyweight before mating, heavier ewes produce mure lambs (Coop 1962; Tribe and Seebeck 1962; Killeen 1967) or have higher ovulation rates (Edey 1968, Killeen 1967) than lighter ewes. Coop ( 1962) suggested th.at ewe age had no independent effect on reproductive performance, but de Haas and Dunlop ( 1969) considered that both ewe age and bodyweight at mating influenced ewe fertility and fecundity. An examination of Merino and Corriedale ewe reproductive performance in relation to age and bodyweight at mating is presented in this paper. * Department of Agriculture, Pastoral Research Station, Hamilton, Victoria. Present address : School of Agriculture, University of Melbourne, Parkville, Victoria. 366 II. MATERIALS AND METHODS (a) Ewe management and observations Merino and Curriedale ewe flocks are run on the Pastoral Research Station at Hamilton in south-western Victoria. In each of `four years (1963 to 1966), between 350 and 550 ewes of each breed were joined to 2-3 per cent of rams of their own breed. Each flock contained ewes varying in age from three to six years but, on some occasions, two year old ewes were joined (ewe age is considered as age at subsequent lambing). Approximately one third of the Merino ewes were joined in November-December each, year; all other ewes were joined in April-May each year. The geneial management of the ewes of both breeds and of all age groups was similar in each year. In each year, the ewes were weighed direct from pasture one to three weeks after the introduction of the rams and, at lambing, the ewes were inspected daily and new born lambs identified to their dams. At the end of lambing, all ewes were examined and dry (non pregnant) ewes recorded. Within breeds, th$e data from all years were pooled and, within ages and according to time of mating, the ewes were ranked in ascending order of bodyweight at mating. Ewes from each year's records and ewes of similar bodyweight were allotted at random in this ranking. Within breeds and ages, the ranked ewes were divided into groups each containing 45-50 ewes (Table 1) . The mean bodyweight of each group was calculated, and the percentages of ewes with multiple lambs and the percentages of dry ewes were determined from the lambing records (both calculated as percentage of number of ewes in each group). Ewes which were dry in two or more successive years usually gained weight rapidly and most were placed in the higher bodyweight groups. The records of such ewes (less than 1 per cent of total observations) were excluded from the analyses as it was considered that their reproductive performance was unrelated to their bodyweight at mating. TABLE 1 (b) Analyses of records 367 The data were examined by multiple regression analyses using linear and quadratic terms for bodyweight and age. Time of mating was considered as a pseudo-variate in the Merino analysis (value = 1 for November-December and 0 for April-May mating). The equations relating the percentage of ewes with multiple lambs (M) or the percentage of dry ewes (D) to bodyweight at mating (W), age (A) and time of mating (T-Merino only) were of the form: They were fitted using a stepwise regression programme and only those regression constants significant at the 5 per cent level were retained in th.e final equation. III. RESULTS The mean bodyweights of the gro:ups ranged from 28.5 to 50.9 kg for Merinos and from 34.3 to 56.9 kg for Corriedales (Table 1). Each kg increase in bodyweight at mating was associated with an increase of 1.9 per cent in the percentage of Merino ewes bearing multiple lambs (Table 2). At equal bodyweights, Merino ewes mated in April-May had 17.1 per cent more multiple births than ewes mated in November-December. For Corriedale ewes, each kg increase in bodyweight at mating and each! year increase in ewe age were associated with 0.9 per cent and 2.5 per cent increase respectively in the percentages of ewes with multiple lambs. Because the regression equations (Table 2) predict negative values at low bodyweights, they only describe the data for bodyweights above 3 1.7 and 40.7 kg for Merino ewes lambing in the spring and autumn respectively, and for bodyweights above 36.2 kg for Corriedale ewes. For both breeds, the percentage of dry ewes increased as mean bodyweight of the flock decreased (Table 2). There is a significant quadratic component in the regression for Corriedale ewes. IV. DISCUSSION The proportion of ewes bearing multiple lambs increases as the mean bodyweight of a flock increases. For Merino ewes, the observed relationship is similar to that found by Lax and Brown ( 1968) who used bodyweight at 15-16 months of age in their analyses. Edey ( 1968) found that the ovulation rate of mature Merino ewes was rela-. tively constant below 35 kg and then increased rapidly before reaching a plateau at 55 kg. Between these levels, each kg increase in bodyweight was associated with an average increase of 2 per cent in ovulation rate. The present results are compatible with Edey's findings. Age of Merino ewe had no effect on reproductive performance. This suggests that relations between ewe age and fertility and fecundity (Turner and Dolling 1965) may be explained by ch,anges in ewe liveweight as sheep increase in age. Although this supports Coop's ( 1962) results, it conflicts with the results of the Corriedale analysis and with the conclusions of de Haas and Dunlop ( 1969) relative to multiple births. 368 (a!) Multiple births (b) Dry ewes TABLE 2 No definite explanation can be given for the differences between the Merino and Corriedale results. However, it is possible that, in flocks containing ewes of mixed ages, factors such as nutritional history, mating behaviour and genetic differences may produce or obscure possible age effects. Consideration of oth,er factors such as year effects (ignored here by pooling of the data) and change in bodyweight before mating (not available here) may allow a more accurate estimation of the influence of age and bodyweight at mating. Although the percentage of variance accounted for in the dry ewe regression equations is relatively low, particularly for Merinos, the results indicate that the proportion of dry ewes increases as flock mean bodyweight decreases. For Corriedale ewes, the dry ewe percentage increases rapidly below 40 kg, a weight similar to the 'critical' weight suggested by Coop ( 1962). Although the proportion of Merino ewes bearing multiple lambs was much higher for April-May than for November-December mating, the response to increasing flock bodyweights was independent of time of mating. Hence, provided ewes are in oestrus, management to achieve high.er bodyweight should produce similar improvements in reproductive performance irrespective of time of year. The results of the analyses show that the number of lambs born depends on the bodyweight of the ewes at mating, the time of mating and may depend on the age structure of the flo,ck. In southern Australia, highest fertility and fecundity should occur if Merino and Corriedale ewes are mated in the autumn at the highest practicable bodyweight. 369 V. ACKNOWLEDGMENTS Thanks are due to Messrs. W. Poynton .and R. Thompson for assistance in collecting and preparing the data for analysis, and to Mr. R. Jardine for analysis of the data. The study forms part of a project supported by the Australian Wool Research Trust Fund. VI. REFERENCES C OOP, I. E. ( 1962). N.Z. Jl. agric. Res. 5: 249. COOP, I. E. ( 1966). WZd Rev. Anim. Prod. 2: 4: 69. de HAAS, H. J., and D'UNLOP, A. A. (1969). Aust. J. agric. Res. 20: 549. E DEY, T. N. ( 1968). Proc. Aust. Soc. Anim. Prod. 7: 188. KILLEEN, I. D. (1967). Aust. J. exp. Agric. Anim. Hush. 7: 126. L AX, J., and B ROWN, G. H. (1968). Aust. J. agric. Res. 19: 433. T RIBE, D. E., and SEEBECK, R. M. ( 1962). J. agric. SC ., Camb. 59: 105. T URNER , Helen Newton, and D OLLING, C. H. S. ( 1965). Aust. J. agric. Res. 16: 699. 370