Reproduction

By Stephen Roberts.

Bovine infertility and re productive diseases exact an enormous yearly toll of over a quarter of a billion dollars annually on the cattle industry in the United States. Thus it is very important that cattle owners and breeders understand the normal bovine reproduction process in both a managerial and nutritional sense, as well as the adverse conditions and diseases which have a major impact on this process which is so vital to the success and profitable rearing of cattle.

Reproduction is a "luxury" function which is not necessary to the life of the bovine animal. So any severe condition affecting the cow or bull will have a marked effect on its reproductive functions.

The Estrous cycle commences in heifers at a variable period of from 6 or 7 to 18-25 months, depending upon the level of nutrition, rate of growth, and breed of cattle. Most dairy breeds are fed adequately, resulting in heifers weighing 500 to 750 pounds at 13 to 18 months of age, which is the usual time of breeding.

In older cows, the "heat" cycles usually commence and are observable about 30 to 60 days after calving; a wide range would be 20 to 90 days after calving. In cows suckling calves, the onset of these estrous cycles is usually delayed one or two months.

The "heat" or estrous period is characterized by the female standing to be mounted by other females or a male. It occurs about every 21 days with a range of 18 to 24 days. This "heat" period lasts about 10 to 21 hours but may range from 6 to 27 hours in certain breeds and individuals.

The estrous cycle and "heat" period is brought about by a complex chain of events caused by a variety of hormones and results in ovulation. This is the release of the egg from a follicle or egg sac on the cow's ovary about 10 to 15 hours (range 2 to 24 hours) after the end of the standing "heat" or estrus.

Insemination

Unless a fertile bull is placed with the cows to breed them during this "heat" period, the cows must be artificially inseminated. It is essential that semen used in breeding be of good quality, regardless of whether natural or artificial insemination is chosen.

If artificial insemination is used, semen should be obtained from a reputable bull stud employing skilled technicians. To be most effective, this insemination must be performed during the estrous or "heat" period, or within a few hours after the end of estrus, so that the living fertile male sperm cells are in the female's uterus and oviducts before the "egg" is ovulated from the ovary.

The "egg" or ovum can survive only a few hours without being fertilized, while the sperm cells usually survive for about 24 to 36 hours in the female reproductive tract. Therefore, two or three times a day the owner or herdsman must watch the cycling cows while they are in the barnyard or pasture to detect the occurrence of estrus so that they may be artificially inseminated at the proper time.

Careful, frequent heat detection and recording is extremely important for a successful reproductive program in herds using artificial insemination.

Causes of Failure

Failure to detect "heat" periods or estrum in nonpregnant cycling heifers or cows is most commonly due to a failure of the herdsmen or owners to observe these females frequently enough or for a long enough period of time (20 to 30 minutes) when the animals are able to interact and mount each other.

The second most common cause for failure of cows or heifers to show estrus is a lack of feed, energy, or total digestible nutrients, and occasionally protein and minerals to provide the nutritional needs of the female for growth, milk production, and a "luxury" level for reproduction. Usually these undernourished females are thin and in poor condition.

A third group of lesser causes for failure of estrus or heat includes a variety of abnormalities such as cystic ovaries (especially in dairy cows), severe chronic infections of the uterus, metritis or pyometra, pregnancy with or without death of the embryo or fetus, and severe chronic systemic or general diseases causing a marked loss of body condition.

Consultation with a knowledgeable veterinarian, or a university dairy specialist or nutritionist can be very helpful. They can give recommendations and advice on the diagnosis and prevention of problems associated with failure of estrus in dairy or beef herds which results in infertility and a loss of income.

A number of aids have been developed to assist farmers in detecting or regulating "heats," including hormones, Kamar pads or chalk on tail-heads, operations to produce "detector" bulls, and rectal examination of the genital tract to predict estrus and detect abnormalities. However, none of these can replace careful, frequent observation.

The gestation period in cattle, the period from conception to birth, varies from 273 to 296 days; the larger breeds have longer gestation periods.

Twinning and other diseases causing abortion may shorten the length of this period. Twinning occurs in 0.5 to 8 percent of bovine births. Beef breeds and dairy heifers produce fewer twins than the larger dairy breeds, and less than the mature or older cows of these breeds.

Occasionally, pregnant cows show signs of heat or estrus, especially during the first 3 to 4 months of gestation.

Early death of the embryo is quite common in cattle, especially during the first two months of gestation when the tissues and organs of the embryo are undergoing very rapid but defective development or when disease or infection in the uterus kills the embryo.

Venereal Disease

These early embryonic deaths and apparent infertility may signal the venereal diseases of vibriosis and trichomoniasis. This is especially true in cattle, usually of the beef breeds, that are bred naturally to older infected bulls.

The organisms introduced into the female genital tract by the bull at service may invade the uterus and cause infection of the uterus, death of the early embryo, and occasionally abortion around mid-gestation.

After a period of costly infertility lasting 2 to 6 months, most cows develop sufficient immunity to these two diseases to carry their calves normally. Effective vaccine given several months prior to breeding can control loss due to vibriosis, but no effective vaccine exists for trichomoniasis.

A number of other "wound-infection " organisms commonly found in the vulva of cows and the sheath of bulls may occasionally become virulent in certain herds and cause periods of infertility.

There are about 40 or more bacteria, viruses, molds, protozoa, toxic chemicals, hormones, and other agents known to cause abortion in cattle. Of these, the diseases of brucellosis, leptospirosis, infectious bovine rhinotracheitis (IBR), and bovine virus diarrhea (BVD), are most common and usually result in abortion of the fetus in the latter half of gestation. Vaccines are available commercially for these diseases and should be administered on the advice of and by the local veterinarian.

Bovine brucellosis due to Br. abortus can cause severe herd losses in nonvaccinated cattle and undulant fever in humans. An active campaign with blood and milk testing to detect infected cattle and herds is presently being carried out in the United States. Quarantine and depopulation are necessary to eliminate brucellosis in cattle.

If the incidence of abortion in a herd exceeds 5 to 7 percent, a highly competent diagnostic laboratory should be employed to determine the causative factors.