Relationships in the sheep of ovulation rate with liveweight, breed, season and plane of nutrition

Abstract

This paper presents a study in the ewe of the relationship between liveweight and ovulation rate and factors affecting this relationship. A series of 11 experiments conducted in Victoria are reported in which 3017 Saxon or Peppin Merino, Border Leicester x Merino or Perendale ewes were used. There was an increase in mean ovulation rate per ewe of 0 to 0.44 for every 10 kg increase in liveweight of ewes within a flock. However, in most flocks the increase was in the order of 0.25 to 0.30 for each 10 kg increase. The response was least when mean liveweights of ewes were lowest and when ewes were studied either about the summer solstice or about the beginning of the breeding season. Breed of ewe effects on the liveweight-ovulation rate relationship were greatest when the Saxon Merino and Border Leicester x Merino ewes were compared at a January-February joining compared with an April-May joining. The Merinos had entered their sexual season in October-November whereas the crossbreds failed to exhibit regular oestrous cycles until January. In the one experiment in which face cover was studied, open faced Merino ewes had lower ovulation rates than woolly faced ewes at a January-February mating but had Mgher ovulation rates at the April-May mating. Liveweight alone was found to be as accurate a predictor of ovulation rate as both liveweight and body condition. Plane of nutrition at mating was not shown to affect ovulation rates significantly in the experiments reported in this paper. However, further studies on the interrelationships between nutritional inputs, skeletal size, season and genotype are strongly recommended.