Reproductive success can depend on sexual motivation

It would be wonderful if we had a simple test for libido in bulls, but unfortunately we don’t. | File photo

Reproductive success is key to profitability for cow-calf operations. To achieve this success, two things have to happen.

First, our breeding cows and heifers must cycle early in the breeding period. Second, they must conceive when bred.

The first factor is largely influenced by the nutritional program and body condition of the cows and heifers. The second factor (conception) is where the bull plays a significant role.

We often emphasize the importance of having your veterinarian perform a breeding soundness exam on bulls before the breeding season. The veterinarian will assess the bull’s overall physical condition along with scrotal circumference and an evaluation of the semen.

Scrotal circumference of bulls is directly linked to the age of puberty and lifetime fertility of female offspring and has become an important criteria of all breeding soundness examinations.

Research by Dr. Cheryl Waldner at the Western College of Veterinary Medicine in Saskatoon showed that cows exposed to bulls with smaller scrotal circumference were less likely to be diagnosed pregnant and had a longer interval from calving to conception.

Data collected by Dr. Albert Barth, who pioneered much of the research on bull evaluation in Western Canada, has shown that our selection for higher scrotal circumference over the years has created a steady trend toward bulls having larger scrotal circumferences in all major beef breeds.

When your veterinarian looks through the microscope at each semen sample, he or she is evaluating the concentration of the semen, the motility of the sperm, the percentage of live sperm and the morphology of individual sperm cells.

Some of the sperm morphology problems identified by microscopic examination may be permanent and others may be short-lived, which may then require a re-test at a later date.

However, one thing that the veterinarian cannot do during the breeding soundness examination is assess libido or sexual motivation of the bull. This is largely a behavioral characteristic that includes behaviours such as detection of females in estrus, mate seeking, courtship and the act of mating itself. A bull could have a very good breeding soundness examination with an acceptable scrotal circumference and good semen quality but still have poor libido.

There have been attempts over the years to develop various tests to assess libido in bulls with serving capacity tests developed in the 1970s by the Australian veterinary researcher, Dr. Michael Blockey.

This test, which became known as the Blockey test, involved observing multiple bulls that were exposed to a small number of heifers in estrus and recording the number of matings each bull performed in a prescribed time period. The test was effective at predicting which bulls would serve more cows or have higher libido, but it was a time-consuming process that was not practical or efficient and as a result is rarely used in commercial operations.

With the advent of genetic parentage testing, we can now examine the calf-crop to see retrospectively which bulls were more active as breeders. A study of multi-sire breeding pastures by Dr. Van Eenennaam and colleagues from the University of California, Davis, showed that the average number of calves produced per bull was approximately 19. This was remarkably consistent over time, but there was a great deal of variability between bulls.

In 4.4 percent of the time, individual bulls produced no calves during a breeding season, even though they had all passed a breeding soundness examination. Some bulls consistently sired more than 50 calves in a breeding season.

In 40 percent of breeding opportunities, at least one bull sired only one calf and at least one bull sired more than 50 calves.

The researchers grouped bulls into low prolificacy, medium prolificacy and high prolificacy groups based on their ability to impregnate cows and showed that these bulls tended to be fairly consistent in their prolificacy over years and over breeding seasons.

Some of these differences that are detected in bull prolificacy are likely due to libido. However, what are the various factors that affect libido?

Many factors that affect libido are also behavioral. One review paper written by an Australian researcher (Petherick) summarizes many of the factors and to my surprise the bull-to-cow ratio was only demonstrated to have minimal effects at most.

There was evidence that having multiple males on a pasture results in a greater expression of libido but most of that research was conducted on dairy bulls and is dated.

Social relationship between males and their dominance hierarchy would likely be important factors and there is some limited evidence that having mixed ages of bulls in a multi-sire pasture may have the potential for dominant bulls to restrict breeding access to subordinate bulls.

There have been very few studies that have looked at temperament or herd dispersion factors and topography of breeding pastures, which all could have potential effects on libido or on the social dominance hierarchy of bulls. Other factors, such as heat and humidity, may depress libido to some degree in Bos taurus (European) breeds.

It would be wonderful if we had a simple test for libido in bulls, but unfortunately we don’t. Your veterinarian should evaluate bulls for physical soundness issues such as conformation, lameness, or back and leg injuries as part of the breeding soundness examination.

These musculoskeletal issues in breeding bulls can obviously have a major impact on their libido. After that, the rest is up to you as the stockperson to make the effort to observe bulls on the breeding pasture to ensure that they are indeed breeding cows that are displaying estrus. The use of parentage testing can be very revealing in terms of gaining the understanding of which bulls are indeed siring your calves but it still has the limitation of being a retrospective measurement.

John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.

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