Effects of mating behaviour and ram libido on the fertility of young ewes

Two groups of 92 and 93, 2 1/2-year-old maiden Merino ewes were each mated to a team of two, 13-year-old Corriedale rams. Both rams of each ram team were either of high or low libido and mating dexterity as determined by a series of pen libido tests. Continuous observations of mating behaviour were carried out on each group for 9.5 days and at 4-5 weeks post coitum (p.c.) the pregnancy rate was determined by laparotomy and hysterotomy. Although the ram pairs differed in libido in the pen tests, under the conditions of paddock mating there was found to be no significant difference between the two mating groups in the total number of mounts and services, the proportion of ewes detected in oestrus, the proportion of ewes pregnant, or the mean duration of oestrus. The duration of oestrus was shown to be dependent on the time of day at which the ewe was first detected in oestrus. Some ewes, although probably in oestrus, were not detected by the ram during periods of little mating activity and therefore their subsequent duration of oestrus was significantly shorter. The pregnancy rate of ewes with a duration of oestrus of less than one hour was significantly lower than that of ewes with a longer duration of oestrus.