Control of ovulation, storage of semen, and artificial insemination of fibre-producing goats in Australia: a review

Abstract

Artificial insemination (AI) allows individual bucks to be exploited widely and so is a potentially useful tool for the rapid genetic improvement of fibre goats. In Australia, where there is a desire by farmers to improve the productivity of their goats, AI may best be adopted under extensive grazing conditions using control of ovulation to allow efficient and accurate timing of the deposition of frozen-stored semen. Although ovulatory activity is influenced by the manipulation of environmental factors, the time of ovulation is synchronised most accurately by the combined use of intravaginal progestagens and pregnant mares' serum gonadotrophin. However, the costs of these exogenous hormones remain high, which justifies investigation of alternative methods to control ovulation. Bucks show strong seasonality in the quality and quantity of their sperm production, and so there is limited time in which semen may be collected for storage and AI, but this can be extended by optimising nutrition and management. There appears to be no improvement in the fertility of stored semen when seminal plasma, which contains egg yolk coagulating enzyme, is removed and an extender containing only a low concentration of egg yolk is used for dilution. Simple methods have been developed for 1-step dilution and freezing of buck semen. However, the post-thawing viability of spermatozoa frozen in pellets on dry ice is higher than for semen frozen in straws in liquid N2 vapour, although straws are preferred for commercial trade. For frozen-thawed semen, fertility after laparoscopic insemination is high, whereas the fertility after cervical insemination is considerably lower but improves by the deeper placement of semen into the reproductive tract. Does are best inseminated 5-10 h before the expected time of ovulation. A dose as low as 1 x 106 motile spermatozoa may be used for laparoscopic insemination of thawed semen that was previously diluted at rates (semen: diluent) of 1:2 to 1:23. However, for the cervical method, a low dilution rate of 1:2 allows a sufficiently small, highly concentrated dose of at least 120 x 106 motile frozen-thawed spermatozoa t o be deposited into the reproductive tract of the doe. Cervical insemination is cheaper and simpler than the laparoscopic method, and this warrants the development of an improved technique for the consistent, deep deposition of frozen-thawed semen through the cervix in a high proportion of does.