Overweight bulls are not as fertile, and high-energy diets can create health and soundness problems.

Nutrition is important when developing young bulls; most seedstock producers market bulls as yearlings and want them well-grown and ready to breed at that young age.

Feeding strategies for young, growing bulls are complicated by the fact that pushing them too fast may negatively impact future fertility and longevity. We know that overfat bulls are not as fertile as they would be otherwise, and that high-energy diets can create health and soundness problems.

Pedro Fontes, PhD (Department of Animal and Dairy Science, University of Georgia) has conducted numerous research projects on reproduction and fertility in cattle. His efforts focus on understanding the factors that influence fertility and pregnancy, as well as developing novel strategies that can be adopted by cattle producers to maximize fertility in their herds. Many of his projects have been directed toward fertility in bulls.

“We get complaints from producers about young bulls that are too fat so I started looking into research that’s been done in this area. Research in Canada showed that when young bulls are fat this can have adverse effects on semen parameters we evaluate during a breeding soundness exam, such as sperm motility and morphology,” he says.

John Kastelic, Professor of Cattle Reproductive Health at the University of Calgary, experimentally induced these problems by feeding young bulls a high energy diet, and also by using scrotal insulation.

“Fat gets deposited in the neck of the scrotum–where blood vessels that help regulate temperature in the testes are located,” says Fontes.

In an overfat bull the scrotum looks square because the neck of the scrotum is filled with fat. Those bulls might not be able to raise and lower the scrotum as easily, and may have too much insulation around those blood vessels.

“The testes need to be cooler than the rest of the body, for optimum sperm production and health. If there is too much fat deposited there, sperm production could be suboptimal,” he says.

“We’ve taken this research a step farther. In humans, overweight is a common problem. When a couple has trouble conceiving a child and the man is overweight, doctors often recommend he change his diet and exercise more,” says Fontes.

IVF in cattle

In fertility clinics, IVF (in vitro fertilization) is often used when couples can’t get pregnant. In cattle we use IVF in females to increase the number of offspring in cows with superior genetics, but in humans IVF is mainly used to overcome fertility barriers. “In human IVF clinics, epidemiologic studies have noticed that if a male is overweight, embryo production is suboptimal. We decided to test this in bulls,” says Fontes.

Sometimes a bull might pass a breeding soundness exam but when he breeds cows, his ability to generate an embryo is impaired.

“We used mature bulls and fed them to gain about 4 pounds a day. Our control group was fed a maintenance diet, since they were already mature and didn’t need to grow. If we feed a mature bull to gain weight he simply adds fat. We saw a definite decrease in embryo production when semen from the over-conditioned bulls was used,” he says.

“We generally think a sperm cell simply needs to swim to the oocyte, fertilize it and its job is done, but we are learning that the bull plays a bigger role in early embryonic development after fertilization takes place. In our study, the oocytes that were fertilized, if they were fertilized by an over-conditioned bull, they were less likely to continue developing. This indicates that decreased embryo production is not only a consequence of low fertilization rate. The diet of the bull negatively impacts the ability of the embryo to grow and causes early embryonic mortality.”

This was done in an IVF setting. “Our initial study was in mature bulls, so we did a follow-up study on young bulls and saw similar results. The beef industry is interested in using yearling bulls when trying to increase the rate of genetic improvement. It is important that we avoid pushing them too hard and for too long on a high energy diet if we want them to settle as many females as possible.”

The study utilized new technologies to evaluate semen quality (computer-assisted sperm analysis, flow cytometry, etc.) and observed only subtle differences in semen characteristics between fat bulls and bulls with adequate body condition. “When we evaluated semen motility and morphology, we didn’t see any major changes. Yet when we used those semen samples, embryo production was quite a bit lower,” he says.

“We don’t know exactly what caused this, but its likely epigenetic changes in the sperm cell that influences its ability to induce early embryonic development. Seedstock breeders run into this when using bulls that do poorly with IVF and nobody knows why. Researchers are starting to realize the male is more important to early embryonic development than we previously thought,” says Fontes. It’s not just the female’s fault.

This is good reason to not buy an overfat bull just before breeding season. Bring him home a few months ahead of time, to gradually adapt to his new environment.

Buy bulls early

“Many bulls are developed on a predominantly concentrate diet, eating from a bunk and not moving around much,” he says. “When they get turned out to breed cows, they may have to travel many miles. We tell producers to buy their bulls as early as possible before turn-out with cows, and adapt them to the environment they will be working in.

Another study Fontes worked on looked at bulls consigned to bull test stations.

“We worked with veterinarians in several different bull tests to get BSE (breeding soundness exam) data and evaluated bulls based on subcutaneous back fat thickness when scanned for carcass ultrasound. We ranked the bulls on back fat thickness and noticed that the bulls with the most back fat were more likely to fail their BSE. They had more sperm morphology defects—which is what we also observed in experiments when we purposely got them fat.”

Some breeds put on more external fat than others. Breeds that don’t accumulate as much back fat include Charolais, Galloway, Piedmontese, Blonde d’Aquitaine and beef-dairy crosses. “Bull development programs in this study had several breeds, but most of them were Angus or Sim-Angus. To decrease the effect of breed variation, we only used Angus and Sim-Angus,” says Fontes.

Sometimes in bull development programs if diets get too “hot” some bulls have subclinical acidosis. Acidosis has negative impact on semen quality and can cause lameness issues (including founder) which decreases bull longevity.

Studies in Australia induced acidosis in bulls and did periodic semen collection.

“They found that semen quality goes down significantly. We start seeing negative effects when a bull starts looking like a fed steer,” he says. Longevity isn’t important for a steer, whereas bulls must stay sound for several years.

“We are interested in identifying the bulls that can gain weight quickly and efficiently; we want to know how their genetics for growth stack up and how well their offspring might do on a feedlot diet, yet we don’t want to feed bulls that kind of diet very long.” After the data is obtained and bulls are getting ready for sale, they can be slowed down in what they are fed, gradually adapted to a more forage-based diet.

“Techniques to evaluate semen don’t paint a full picture. New technologies are enabling us to learn more, but at this point are limited to research settings and semen companies. Most veterinarians don’t have access to these in the field so we are still limited in our ability to detect subtle differences in fertility between bulls. I think those technologies will become less expensive and as an industry we will get better at recognizing the bulls with superior fertility,” he says.