CONTROLLED BREEDING IN CATTLE: A REVIEW

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Abstract
The use of progeny-tested sires through artificial insemination (AI) is an important tool for genetic improvement in cattle. Use of AI in beef cattle and dairy heifers is generally not practical because of difficulties associated with estrus detection. This situation will change only if breeding by AI can be done under controlled conditions, preferably an insemination at a fixed time relative to a treatment. Sufficient data on hormone secretion patterns during the cycle of the cow are available to provide a reasonably detailed understanding of hypothalamic–pituitary–ovarian interrelationships. Two methods, although both are at the development stage, are providing encouraging results. Much of the early work using natural and synthetic progestogens orally or by injection was disappointing because fertility was depressed after treatment. However, by reducing the period of treatment to 9 days and combining the synthetic progestogen implant with an injection of estrogen plus progestogen, it has been possible to get good synchrony and apparently normal fertility. Further studies need to be conducted to verify that fertility is normal and to determine if a single timed insemination relative to time of implant withdrawal would give normal fertility. A second method is based on use of potent luteolytic agents. Prostaglandin F2α (PGF2α) or synthetic analogues act by causing regression of the corpus luteum when they are injected between about days 5 and 18 of the cycle. Animals with a functional corpus luteum will come into estrus about 72 h after the injection and will ovulate about 23 h after that. Since injections of PGF2α are ineffective during the remainder of the cycle, a single injection can not provide complete synchrony. Two injections of PGF2α at 10- to 12-day intervals should in theory give complete synchrony since all cycling animals would have a functional corpus luteum at the second injection. This theory appears to be valid and fertility with a single (80 h) or double (70 and 88 h) insemination is comparable to normal.