The relationship of age, body weight and factors associated with oestrus to non-return to oestrus of artificially inseminated Merino ewes

Abstract

In the course of three experiments involving the artificial insemination (Al) of flocks of 1230, 1240 and 1170 Merino ewes, observations were made at the time of A1 and were related to subsequent 28 to 45 day non-return rate (NRR). Ewes in oestrus were detected by vasectomized rams harnessed with marking crayons and the flocks were examined twice daily at 7.00 a.m. and 3.00 p.m. for 17 days. Significantly more ewes came into oestrus per unit time in the interval 7.00 a.m. to 3.00 p.m. than in the other 16 hours of the day. Age (range 2 to 7 years) and body weight (range 31 to 49 kg) of ewes did not affect NRR significantly. The intensity of the crayon marks left on the rumps of ewes by teaser rams was scored (1, faint; 2, distinct; 3, intense) and NRR was lowest where ewes had faint colour marks. The appearance and volume of vaginal mucus found at the time of A1 were scored on a scale (1 to 4) related to the sequence of changes occuring in mucus during the oestrous period. NRR was highest when a volume of 10 to 15 ml of mucus was present in the depths of the vagina. The structure of the vagina and external opening of the cervix was classified into one of six categories and NRR was significantly lower where the vagina was too narrow to permit the introduction of a speculum. In these cases the semen dose was deposited into the vagina without locating the cervical opening.