INDICATIONS

The purpose of examining cows for pregnancy is not to detect those that are pregnant, but to detect those that are not pregnant so that they can be inseminated again or culled from the herd. For profitable production, dairy cows should calve for the first time at approximately 24 months of age and should deliver subsequent calves at intervals of approximately 13 to 13.5 months. Thus, dairy cows should conceive within approximately 4 months or less of calving. Cows that are found to be not pregnant can be treated for any abnormalities of the reproductive tract discovered during the examination, observed closely for signs of spontaneous estrus, or treated with prostaglandin F_2α (PGF_2α) to predictably shorten the time until return to estrus.

An ideal test would accurately detect pregnancy before the first expected estrus after insemination (about 21 days) so that the cow could be reinseminated without further loss of time. Unfortunately, no tests are currently available that are practical to use and allow detection of pregnancy in cows before the first expected estrus. Most methods for pregnancy diagnosis are capable of detecting pregnancy between 25 and 40 days after conception, and most authors have traditionally recommended that nonpregnant cows be identified before the second expected estrus after insemination.² In a recent prospective study, however, cows that were diagnosed as pregnant by palpation between 30 and 36 days after breeding were found to have a 2-week longer calving interval than that observed in cows examined for pregnancy later.³

The frequency of embryonic death in cattle is high during the first months of pregnancy, perhaps because of loss of abnormal embryos or failure of maternal recognition of pregnancy.⁴ Thus, cows that are diagnosed as pregnant soon after insemination are more likely to suffer embryonic death and return to estrus, to be found not pregnant at a subsequent examination, or to fail to deliver a calf at the expected time than are cows in which pregnancy is diagnosed later. Owners frequently misinterpret this course of events and conclude that the test for pregnancy was inaccurate (i.e., the cow was not pregnant at the time of examination) or that the test affected the embryo and directly or indirectly resulted in termination of pregnancy. After approximately 60 days, fetal death rates are low, and in most, but not all, cows that are found to be pregnant after this time, the pregnancy proceeds to term with birth of a calf.

Beef cows usually are examined for pregnancy when their calves are weaned at 6 to 7 months of age, although in intensively managed herds, individual cows can be examined earlier. In most management systems, nonpregnant cows are culled from the herd to save the cost of maintaining nonproductive animals, but in herds that have spring and autumn calving seasons, cows sometimes are moved to the other group and given a second opportunity, although this decision may be difficult to justify economically. Although the cost of maintaining a nonproductive beef cow is variously estimated to be between $250 and $400 per year, a survey of herd owners in 18 states revealed that only 17.7% examine cows and that 15.9% examine heifers for pregnancy.⁵

MANAGEMENT METHODS FOR PREGNANCY DIAGNOSIS

As described later on, palpation of reproductive structures per rectum (probably to be replaced by ultrasound examination in the foreseeable future) has been the customary method of pregnancy diagnosis in cattle. Some cattle owners, however, may rely on the history, or on clinical signs that can be observed, for a presumptive diagnosis of pregnancy.

Exposure to a Bull or Artificial Insemination

A history of cohabitation with a bull, the observation of mating, or artificial insemination may be considered by some owners to be sufficient evidence that a cow has become pregnant. Although fertilization rates in cows typically are high, however, only approximately 50% of inseminations result in a detectable pregnancy. Conversely, unobserved, unplanned, or unrecorded matings are not uncommon, and cows frequently are presented with a complaint of infertility, only to be found pregnant when examined. Thus, history is not a reliable indicator of pregnancy status and sometimes may be deceptive.

Cessation of the Estrous Cycle

Bovine embryos signal their presence around 15 to 17 days after ovulation: The corpus luteum is maintained, and the maternal estrous cycle is suspended. Thus, failure of a cow to return to estrus at approximately 18 to 24 days after mating suggests that conception has occurred. In fact, the most common cause of failure of cows to have normal estrous cycles is pregnancy.⁶

In beef herds in which natural service is used, perceptive managers may observe that a greater-than-expected number of cows return to estrus after mating. This situation suggests an infertile bull, the presence of a venereal infection, or some other cause of infertility, and an opportunity exists to take corrective action before the breeding season ends. Conversely, undernutrition is a common cause of anestrus in lactating beef cows, and the observation that few cows return to estrus after the first few weeks of the breeding season may mislead less astute managers to believe that cows have become pregnant when they are in fact not cycling.

Failure of a dairy cow to be detected in estrus at approximately 3 weeks after insemination is viewed as a favorable event by managers, and anestrous cows are assumed to be pregnant. Unfortunately, although the efficiency of estrus detection varies among observers and among dairy farms, it generally is lower than desirable, so this approach is not sufficiently accurate to be of use in diagnosis of pregnancy. Most dairy reproductive herd health programs are based on the practice of examining cows that have not been observed in estrus after insemination as indicated by some other method (most commonly rectal palpation) that more accurately differentiates between pregnant and nonpregnant cows.

A few pregnant cows show mild to conspicuous signs of estrus and may be mistakenly thought to be nonpregnant. Artificial insemination of pregnant cows may result in abortion if the insemination instrument is passed completely through the cervical canal and the fetal membranes are disrupted.⁷

Metestrus Bleeding

A sanguineous vaginal discharge is common in cows 24 to 48 hours after estrus and is the result of hemorrhage from the caruncular capillaries due to the rapid decline in estrogen concentration that follows ovulation. Some owners subscribe to the notion that metestrus bleeding is a sign of conception failure; however, metestrus bleeding is observed in nearly all cows and heifers and is not an indication of whether conception occurred. If metestrus bleeding is observed in a cow that was not seen in estrus a few days previously, it is implied that estrus was unobserved and that the animal is not pregnant.

PALPATION PER RECTUM

Palpation of the reproductive tract through the rectal wall (rectal palpation) has been used for diagnosis of pregnancy since the early 1900s and has been the customary method used in cattle for more than 50 years.⁸^–¹¹ Depending on the skill of the examiner and the age and size of the dam, rectal palpation is useful to diagnose pregnancy as early as day 30 and thereafter until term. Although a number of changes occur in the size, texture, location, and content of the uterus during pregnancy, four positive signs of pregnancy that are detectable by rectal palpation are recognized:

• Palpation of the fetal membrane slip

• Palpation of the amniotic vesicle

• Palpation of placentomes

• Palpation of the fetus

The examiner must detect at least one of these four signs before declaring the cow pregnant.