Across much of cow country, it’s a done deal. Many spring-calving cow herds underwent pregnancy diagnosis, at weaning time or soon afterward, and decisions were made. Cows that failed to become pregnant during the most recent breeding season were identified. Many of those open cows have been sold already, and the remainder are likely destined for market in the not-too-distant future.

Eliminating nonproductive females from the breeding herd is the practical thing to do, and weigh-up prices have been pretty good. That helps, of course, but it’s still concerning when the cull pen contains too many relatively young and structurally sound females of a kind well-suited to the particular environment.

It makes a conscientious manager wonder what the reason or reasons might be. Could it be related to health, nutrition, or stress due to weather or handling? Did pregnancies fail because of genetic abnormalities? Or, was it the bull?

When pondering possibilities, it’s best to remember that it might not be the cow’s fault. Sometimes, it seems, pregnancy loss is paternally driven.

The bull’s influence

When the male’s contribution to bovine reproduction was discussed during the Applied Reproductive Strategies in Beef Cattle (ARSBC) conference hosted last September in North Platte, Neb., speakers explained that the bull’s influence is more far-reaching than most producers realize. Involving more than fertilization of the female’s ovum (egg), factors associated with the bull’s sperm also influence how an embryo develops and whether a pregnancy is maintained.

According to University of Tennessee animal scientist Saulo Zoca, the veterinarian’s complete breeding soundness examination (sometimes referred to as a BSE) is “the standard and only method” for predicting bull fertility. An important part of that is evaluation of semen quality, including assessment of sperm motility and morphology.

However, Zoca cited evidence showing even among bulls that pass a breeding soundness exam, there are different levels of fertility based on differences in the live sperm that each bull produces.

A given bull’s ejaculate [or semen dose used for artificial insemination (AI)] contains three populations of sperm cells:

• sperm that cannot initiate fertilization;
• sperm capable of initiating fertilization but unable to produce a viable embryo; and
• fully competent sperm capable of fertilization and generation of a viable embryo.

A bull’s fertility depends on the proportion of each type of sperm present in the ejaculate. Zoca said researchers are now trying to look beyond motility and morphology assessment to find ways of determining which bulls are capable of delivering higher proportions of fully competent sperm.

“We are looking for what it takes to make the most-fertile bulls. We know it is multifactorial, but we don’t yet know how many characteristics are involved. A bull’s sperm must have enough of all desirable characteristics, and we don’t yet know how much ‘enough’ is,” said Zoca, noting that the answers will likely be found at the molecular level.

According to Zoca, the “_omics” are being applied to discover relationships between sperm biology and bull fertility. This includes investigation of differences in the proteins (proteomics) and metabolites (metabolomics) in sperm cells and accessory gland secretions. Through genomics, researchers hope to identify genetic markers linked to superior fertility.

“We need to find biomarkers to add to the morphology and motility assessment used now,” said Zoca, “and we need to adapt what we find as a diagnostic tool for evaluating bulls in the field.”

Feeding concerns

During the North Platte conference, University of Georgia animal scientist Pedro Fontes reminded producers of the potential fertility problems associated with bulls overfed during development. He cited observational studies and controlled experiments evaluating the effects of excessive growth and fat deposition on yearling bull fertility outcomes.

Results suggest excessive subcutaneous backfat (greater than 0.5 inch) is associated with increased sperm cell abnormalities and a reduced chance of bulls passing their first breeding soundness exam. However, research also suggests consequences of bull overnutrition also negatively affect postfertilization embryo development.

“That was mind-blowing to me. Not only is overnutrition affecting the sperm, but we’re seeing carryover effects in resulting embryos,” said Fontes, noting an increased incidence of delayed cell development in embryos resulting from eggs fertilized with sperm from overly conditioned bulls.