The effect of liveweight on ewe fertility

Abstract

The reproductive-performance of ewes is determined by ovulation and fertilisation rates and by the number of embryos which survive. However, many studies of the efficiency of reproduction or management concern traits which involve only gross measures of performance such as average lambing rate, per cent ewes lambing and per cent lambs born per ewe mated. As a result the physiological basis of difference observed is often not clear.

On the other hand, direct measurements of these traits by laparotomy or slaughter have limitations as the former may impair subsequent reproductive performance whereas the latter precludes any further records being available from the experimental animals. Often, too few sheep have been used and the chances of detecting real differences due to treatments imposed have been slight. There is little available information concerning ovulation rates under controlled experimental conditions in New Zealand although reports involving lambing percentage data are more numerous.

Recently Wallace (1961) and Coop (1962, 1966a and 1966a) have shown that liveweight per se affects reproductive performance in that twinning rate is increased and barrenness is decreased as liveweight increases. This is especially important in connection with husbandry and management aimed at increasing lambing percentage. Together with the possibility of liveweight being a useful criterion in selection for fertility this warrants a more exhaustive investigation of the relationships involved.

Liveweight may have its alleged effect at any stage from the shedding of the ovum to the birth of the live lamb. Coop (1962; 1966a and 1966b) has measured only the end result of this complex chain of events and Wallace (1961) measured ovulation rate in only one quarter of his experimental sheep. Consequently this trial was designed to compare the ovulation rates between two groups of sheep which differed greatly in mean liveweight. An attempt was made to eliminate the variables in order that any differences obtained would be predominantly due to the liveweight effect. Firstly, ewes were randomised into their respective groups on the basis of liveweight, age and previous reproductive performance; and secondly a large liveweight difference was obtained in the preliminary period of the trial, before the onset of the breeding season. During the period of reproductive activity all ewes were grazed together and thus the nutritional variable which is of paramount importance in this type of experiment was reduced to a minimum.

Recently the development of the laparotomy technique facilitated the gaining of more than one record from each sheep. This allowed for all ewes to be synchronised and laparotomised following marking by a teaser ram. By slaughtering the ewes during the oestrous cycle following laparotomy, half at 3 and half at approximately 28 days following mating with an entire ram, it was hoped to elucidate further the physiological relationships between liveweight, ovulation rate and embryo survival.

In addition, the experiment provided the opportunity to study types of ova produced and embryonic mortality.