MSU Extension Beef

What is your goal for length of your calving season? There are many advantages of a short calving season, including a more uniform group of calves, concentration of waiting and farm labor, older and heavier calves at a given sale date and cows and heifers that have more recovery time before breeding back. Achieving a short calving season using natural service takes sufficient bull power.

Bull power is not how much weight bulls have to throw around, or even how aggressive they are. It is not even the cost of a bull. Bull power is how much ability they have to inseminate cows and heifers. There are various factors that combine to produce bull power and that is exactly what a veterinarian assesses during a breeding soundness exam (BSE).

A video of a BSE clinic was recorded by Michigan State University Extension educators Durst and Thurlow to help farmers and others understand the process and importance. That importance is clear given the results of an MSU study of more than 2800 BSEs over a 10-year period by Roberts et al. published 2018 in Clinical Theriogenology titled, “Evaluation of bull breeding soundness examination.” In that study, only 82% of bulls passed, while 15% were deferred and 3% failed.

In spring 2021, the Michigan State University Extension beef team held annual BSE clinics for beef cow-calf producers. Whether on farms or at central locations where farmers brought their bulls, a veterinarian examined, measured, palpated, and looked through a microscope to determine if bulls passed minimum standards to be effective this breeding season.

It begins with a physical evaluation of the bull. Are they walking on good feet and legs? Feet and leg problems can hinder a bull from pursuing cows across a pasture and mounting them. We saw a bull that had corkscrew toes on a rear foot and another with a cracked toe. We watched bulls enter and leave the chute to see how they walked and whether any mobility issues were detected. We made observations of eyes, to determine if they were clear. A bull that does not see well will be less likely to mount cows. In addition, we assessed the body condition to see if a bull had sufficient reserves of energy for the work ahead.

Scrotal palpation and measurement check the sperm factory, the testicles. Minimum standards for scrotal circumference by age of the bull tell us not only whether he has the testosterone for sperm production, but also is positively correlated with the fertility of his offspring. Among the bulls tested during one local clinic was a bull with a degenerated testicle, severely limiting the bull’s ability to produce adequate semen during the breeding season. During the internal rectal exam, the bull is checked by palpation for abnormalities of the seminal vesicles and inguinal rings as well as for evidence of a hernia.

The bull is then prompted to ejaculate using the gradual build-up of stimulation from an electroejaculator and the semen is collected. During that process the ability of the bull to fully extend his penis is observed, looking for any problems with extending or of the penis. One year, we had a bull with a wart on his penis. A wart can make breeding painful for the bull and reduce his capacity for breeding as well as having potential to bleed into the cows. As a result, the owner decided not to keep that bull.

The semen sample is kept warm and is promptly taken to the on-site laboratory. That may be as simple as a table set on hay bales off to the side, or a lab in a trailer. Placing a drop on a slide and viewing it under a microscope will first indicate if a good sample has been obtained. Sperm motility, with a minimum of 30%, is checked to make sure that the sperm are alive. This is a liberal level for passing because handling can affect motility. Then the slide is stained so that a count of 100 sperm can be made, counting normal sperm and those with defects. A minimum of 70% normal sperm is required for passing.

Sperm abnormalities may be related to genetic, heritable defects, genetic factors that predispose the bull to defects that occur after an insult, and the effects of conditions such as cold stress, nutritional deficiency, fever, injury or disease. Sperm formation, or spermatogenesis takes 61 days in cattle and, therefore, a bull that is deferred based on sperm morphology should be examined again after 61 days. However, by that point, the breeding season may have passed him by. Testing early is important to allow enough time to pass if a recheck is needed.

As sperm is being checked, the presence of white blood cells is also recorded. At a certain level, these may indicate the presence of an infection or inflammation in the reproductive tract that will reduce fertility.

A BSE is important on all bulls that you plan to use for a breeding season or sell for breeding purposes, but as important as it is, it cannot tell you everything about a bull’s likelihood to breed cows. For instance, it does not measure the libido of a bull or its mating ability. Observe bulls, particularly if there are more than one in a group of cows or heifers. Older bulls may inhibit the libido of younger bulls making them ineffective. One farmer described a bull that passed his BSE, but apparently had little interest in breeding the herd. In addition, a BSE does not determine if a bull has a venereal disease that can be passed on.

Together, a breeding soundness exam and your observations will rule out the most common causes of cows being open as a result of not being serviced effectively. That is important so that you can achieve the desired calving window that optimizes returns for your calf business.