by Stuart J. Burns, D.V.M.
implanting or injecting, always produce lower pregnancy rates ne of the main problems with A.I. is the than breeding on observed standing heat. Good records and tendency for the breeding season to last too identification are very important to prevent mistakes. Those cows long. Some purebred breeders will continue to not observed in heat within 10 days of the first injection, (their A.I. until a pregnancy results while others will CL’s were too young to respond) should be injected a second A.I. for one or two cycles and put in a “clean up” time and bred when in heat. Be sure not to inject a cow that has bull. The approach selected is determined by the importance just been bred for this will cause her to abort the first service. of a short calving period. The short calving season can be Heat detection remains the most important part of any A.I. encouraged by the use of synchronized breeding aids, which program. Failure to detect estrus, though it happened, remains manipulate the estrus cycle to “group” the ovulations closer the leading cause of failure to conceive when using A.I. together. One method involves the placement of a removable Remember that the interval between ovulations is 18-22 days implant that contains a synthetic progesterone. This hormone in cattle and the estrus period lasts for eight to 18 hours. Cows is slowly released and prevents the cow from cycling. When the ovulate about 12 hours after being in standing estrus. The best implant is removed in about nine-11 days, the cows begin to conception rates are obtained when the cow is bred within the cycle again but all about the same time, because the “brakes” 18-24 hours before ovulation, when the cow’s ovary sheds an were released at the same time. This source of progesterone egg. mimics a heat cycle and may provide a degree of stimulation The herd must be observed for animals in heat at a to the reproductive tract in non-cycling cows. This system does minimum of twice a day. This is best done in the early require that each cow be worked three times; once to implant, morning shortly after dawn and again in once to remove the implant and once late afternoon or early evening as this is to breed. the time cows are most active in showing A second popular approach is to signs of heat. It helps to confine the herd utilize a prostaglandin injection to in a small grazing area for ease of bring the cow into heat. These agents cause the destruction of the corpus One of the main problems with A.I. observation, and the person detecting estrus must be interested in the cattle and luteum (CL), a structure that is formed is the tendency for the breeding willing to observe their behavior. in the ovary after ovulation that season to last too long. The only sure sign a cow is in heat is produces the cow’s own natural her willingness to stand to be mounted progesterone. The cows must be cycling by other cows. There are several things for this drug to work. If there is no CL that might be observed that would present, there is no “target” for the drug suggest a cow is in the early stages of her to work on. The normal interval from estrus period. Notice of these signs and very careful attention one ovulation to the next is about 21 days in cattle. The to these cows on your next check will increase your chances of prostaglandin will only affect a mature CL. At any given time choosing the best time to breed them. about one third of the cows will have had a recent ovulation Cows that have been ridden as they approach standing heat one to seven days prior. These cows will not respond to a may have ruffled hair over their tail head. These cows may have prostaglandin injection. About one third will have ovulated mud over their ribs and flanks from the cows’ feet that are seven to 14 days before injection and the drug will destroy the riding them. Many of these animals will have already gone out CL and the cow will come into heat. The last third of the herd of heat. Clear mucus hanging from the cow’s vulva or tail may will have ovulated 14-21 days ago and are in heat or soon to be be observed just before, during, or after heat. A blood tinged in heat on their own schedule. The end result is that 2/3 of the discharge usually indicates that estrus is past, about two to injected cows will be in heat in two to five days after injection. three days before this change is seen. The vulva may appear These cows should be bred when found in heat and clearly swollen when in heat, in contrast to the dry wrinkled identified as having been bred. Timed inseminations in which appearance when not in heat. the whole herd is bred at a set time so many hours after
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