Pregnancy Diagnosis of Bison by Rectal Palpation

Dr. John Berezowski

Reprinted from Smoke Signals, volume XI, number 5, October, 2000, pages 89-91


This article is a summary of Dr. Berezowski's IBC 2000 presentation in Edmonton. He discusses the rationale behind pregnancy checking bison. He also explains the best time of year to be checking your bison cows and the facilities required to perform the task safely.

Should Bison Producers Routinely Preg Check Their Bison?

Annual preg checking of beef cattle is a management practice that has become very common among North American cattle producers. The reason for this is that cattlemen can realize an economic gain by preg checking their cows in the fall of the year and culling any non-pregnant cows. Culling non-pregnant cows allows cattleman to replace the non-pregnant cows with pregnant replacements. The goal of this management procedure is to ensure that all of the cows that are carried over winter produce a calf during the next calving season. The money that is saved is in the cost of the feed that would have been fed to non-pregnant cows that do not provide any economic return for that year. The economic incentive occurs because the cost of feeding a cow over winter is large relative to the cost of a pregnant replacement or the return from a marketed weaned calf.

Annual preg checking of bison cows has not become a common practice because there is little economic incentive for bison producers to routinely preg check their cows. There are several reasons for this:

  1. The market value of pregnant bison cows or heifers is considerably greater than the market value of replacement beef cows or heifers.
  2. The cost of feeding a bison cow over winter is less than the cost of feeding a beef cow over winter.
    • Bison are much more cold tolerant than cattle and are less likely to require supplemental feed in cold weather to maintain their body temperature
    • Bison lower their metabolic rate in winter, and require less feed to maintain themselves than cattle.
    • Bison have a more efficient digestive system than cattle. This allows them to do better on poorer quality feed, and to require less feed than cattle.
  3. The reproductive life span of a bison cow is much longer than that of a beef cow.
  4. The market value of culled, non-pregnant bison cows is small relative to the market value of pregnant replacement bison cows or heifers.
  5. There is a greater chance of injuries occurring to bison during the handling and restraining required for preg checking, than there is for cattle.
  6. Producing home raised bison replacement heifers takes one year longer than it does to produce home raised replacement beef heifers.

The bison industry is in a phase of rapid expansion. At this time the main focus of the bison industry is on increasing the size of the breeding herd. Many producers, especially new producers, are more interested in keeping bison cows as breeding animals than culling them for poor reproductive performance. They are willing to bear the expense of feeding a non-pregnant bison cow over winter, since this expense is small compared to the cost of buying a replacement. If bison producers receive no economic benefit from culling non-pregnant cows, there is little reason for them to preg check their cows on every year.

One place that rectal palpation can be used is in herds that have reproductive problems. In many herds it is not possible to identify which cows do not calve or do not wean a calf. In, these herds it is possible for cows that have reproductive problems to remain in the herd for years without becoming pregnant. Preg checking and recording the identity of open cows: will allow the identification and removal of those cows that are unable to become pregnant.

Preg checking by rectal palpation

The anatomy of the bison cow's reproductive tract is very similar to that of a beef or dairy cow. Any differences that are present can be attributed to the differences that occur in the size of adult beef or dairy cows versus adult bison cows. Bison cows being smaller have a smaller vagina, cervix, pelvic opening and non-pregnant uterus. The distance, or the amount of reach, from the opening of the anus to the cervix is less than that of a beef or dairy cow.

Any veterinarian who is proficient at preg checking beef or dairy cows by rectal palpation will be able to preg check bison cows. Bison cows are not as sedate as beef cows and are capable of a surprising amount of motion even while re-strained in a squeeze. Bison often have drier feces and a drier rectum than cattle, making the use of lube essential. The depth of penetration required to grasp the cervix and uterus is not as great as that of cattle. Even in late pregnancies the palpator rarely has to insert his/her arm into the rectum any further than up to the elbow. This allows the palpator to stand to the side of the bison while performing the palpation. It reduces the risk of the palpator being kicked and allows him/her to quickly extract their arm from the rectum if the cow lies down.

When to test

Pregnancies can be detected as early as 35 or 40 days by rectal palpation. It is, however, difficult to consistently diagnose early pregnancies in bison because the constant motion of bison cows in the squeeze makes it difficult to detect the subtle changes in the uterus that are associated with early pregnancy.

There have been some reports that there may be fetal losses associated with preg checking of bison during the early stages of pregnancy. At the present time it is not known whether these losses are actually caused by rectal palpation or whether they would have occurred if the palpation had not been performed, as early pregnancy is the time when most fetal loss occurs. However, it is probably safer to wait until the cows are bred more than 60 days before performing rectal palpation.

The bison breeding season begins in early to mid July. Most of the breeding is done by mid to late August. By mid October most of the cows will have been bred for at least sixty days. After this time of the year pregnancies will be advanced to the point where pregnancy diagnosis can be performed quickly, accurately and with minimal chance of fetal loss.


Bison squeezes should be safe for both the bison and the palpator. The squeeze operator should be able to consistently catch, restrain and release bison cows with little or no injury to the cow. The side of the squeeze should have large panels or doors that can be opened to allow the palpator easy and safe access to the back end of the bison. The doors should open the entire height of the squeeze so that the palpator can stand outside of the squeeze and still reach bison cows at various heights. The squeeze should be capable of preventing bison cows from dog sitting. Bison cows will frequently dog sit when confined in a squeeze even for a short period of time. When they do this it is not possible to preg check them safely. Extra time and the use of a stock prod are often required to get the cow into a standing position. Increasing the time spent in the squeeze and using a stock prod will result in an increase in the amount of stress placed on the cow and an increase in her risk of injury. Most manual squeezes are unable to squeeze bison cows with enough pressure to ensure that the cow remains standing during preg checking. Hydraulic squeezes are two to three times more expensive than manual squeezes. A well designed hydraulic squeeze, however, will often pay for itself in a short period of time by reducing bison injuries and deaths as well as reducing veterinary charges for preg checking.

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