Which bulls perform in breeding season?

The old saying that your bulls are “half of your herd” is certainly true when talking about genetics.

However, to pass on those genetics, a bull first has to get cows pregnant. If the bull can’t perform this task, 
his genetic potential will count for nothing.

The latest edition of the Journal of Animal Science has an interesting article evaluating bull prolificacy on multi-sire pastures.

In this study, Dr. Alison Van Eenennaam from the University of California, Davis, performed parentage testing on 5,052 calves on three northern California ranches to evaluate which bulls were getting cows pregnant in these multi-sire breeding pastures.

The cow herds were predominantly Angus and had both fall and spring calving herds. As a result, the re-searchers were able to evaluate 15 calf crops over a three-year study.

There was about one bull to every 25 cows and breeding seasons varied from 60 to 120 days.

On two of the ranches, the breeding pastures were about 100 acres and typically contained two to five bulls in each breeding group.

On the bigger ranch with 700 total cows, the breeding pastures were larger and typically contained five to nine bulls.

Breeding soundness examinations were performed on all bulls before the breeding season and hair samples were collected from bulls and calves for DNA testing to determine the parentage of each calf.

The results of this study are remarkable.

In total, there were 275 bull breeding opportunities over the three years of the study. Because these were working commercial ranches, bulls were used multiple times over those three years, so some of these bulls were turned out in different breeding pastures for both fall calving and spring calving herds over the length of the study.

The average number of calves produced per bull breeding opportunity was about 19. This was remarkably consistent between ranches and over different breeding seasons and years.

However, there was much variability between bulls.

Individual bulls produced no calves during a breeding season 4.4 percent of the time. These bulls had total reproductive failure and did not impregnate a single cow.

Some bulls consistently sired more than 50 calves in a breeding season. In 40 percent of the breeding opportunities, at least one bull sired only one calf and at least one bull sired more than 50 calves.

Researchers grouped bulls into low, medium and high prolificacy 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.

The study demonstrated a remarkable degree of variation between bulls in how successful they were getting cows pregnant. Remember, all of these bulls had previously passed a breeding soundness exam, so they presumably had fertile semen.

Some of this variability can be accounted for by the libido or desire to breed of the individual bulls.

What else could account for these differences?

The age of the bulls had a small effect on the average number of calves produced by each bull. Young bulls in their first breeding season varied in age from 1.4 to 2.9 years and, on average, produced 14.9 calves per breeding opportunity.

However, even in young bulls there was remarkable variability, ranging from a bull producing only one calf in a breeding season to some producing as many as 40 calves in a season.

The only factor measured that was related to prolificacy was scrotal circumference. It certainly did not explain all of the variation, but it was statistically associated with prolificacy of the bulls. No other breeding group management factors could explain this variability.

Scrotal circumference has been related to sperm motility, semen quantity and heifer maturity. This study provides more evidence of the importance of scrotal circumference.

The study also demonstrated that the high prolificacy bulls also had calves earlier in the calving season than the lower prolificacy bulls. Therefore, these bulls might be more likely to be represented in the heifers that are selected for replacements providing some measure of reproductive selection.

Prolificacy was found to be moderately repeatable and the authors of the study wondered about whether it would be cost effective to use parentage testing as a method of bull selection/culling.

The prolificacy of the bulls was by far the most important factor in determining the total weight of calves weaned and demonstrated the im-portance of reproductive success to the economics of the herd.

Using parentage testing or other means to assess libido of bulls re-mains an important area of research for the cow-calf industry.

I hope we will see more studies that provide insights into this topic.

John Campbell is head of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.

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