Studies of the artificial insemination of sheep: The effects on fertility of diluting ram semen, stage of oestrus of the ewe at insemination, and injection of synthetic oxytocin

Abstract

Ewes were artifically inseminated with semen diluted in skim milk. In the first experiment, with an insemination dose of 50 x 106 spermatozoa, 10-fold dilution of semen did not reduce the fertility of the diluted sample if it was reconcentrated by centrifugation to the same volume as the undiluted controls. However, three later experiments conducted under extensive pastoral conditions showed that dilution in milk lowered the fertility of semen, and reconcentration of the spermatozoa had no effect. Only 8.8% fertility followed the insemination of 50 x 106 spermatozoa in semen diluted 10-fold and stored at 5°C for approximately 36 hr. The administration of 50 or 200 mU of synthetic oxytocin to ewes at an early stage after insemination had no effect on fertility. The intensity of marking with the crayons carried by vasectomized teaser rams, thought to be a measure of the intensity of display of oestrus, was positively correlated with the fertility of ewes at insemination. The greater the consistency and opacity of the vaginal mucus at the time of insemination, the lower was the fertility of the ewe. The fertility of ewes was highest during approximately the first 12 hr of oestrus, and dropped progressively through the two subsequent 12 hr periods.