by: Heather Smith Thomas

Proper care and management of bulls is crucial for optimum fertility. Some bulls are naturally more fertile than others, due to genetics, scrotal conformation, etc. but poor management can reduce a bull's fertility or his chances of siring a high number of calves. Bulls too fat or too thin won't have optimum fertility and may have other issues that impede ability to breed cows. Young bulls need adequate nutrition to develop properly, but should never be fat.

Nutrition -- Ron Skinner, DVM, (veterinarian and seedstock producer, Drummond, Montana) recommends monitoring the condition of all bulls. “Semen production of an underfed young bull is only 77 percent of the production of an adequately fed bull. If a young bull is allowed to get too thin, he'll lag behind in semen production for the rest of his life, even if nutrition is brought back up to speed. This was shown in a study many years ago by T. D. Rich at Oklahoma State,” he says.

“Poor energy levels delay puberty. A young bull that's not developed properly nutritionally will be slower maturing. If bulls are going to work as yearlings, they need to be developed at a proper level. A 2.6 pound average daily gain is ideal. Yet most young bulls are fed to gain 3 to 3.5 or even 4 pounds per day. You want a bull to be bloomy, but you don't want him fat.” A young bull that is overly fat may have fertility problems.

He has less actual testicle size. “Testicle size will be larger with fat deposition in the scrotum, but lack of circulation due to the extra fat may decrease actual testicular tissue (hence less scrotal circumference after the fat is lost)--and less semen production.”

Most people know that fat decreases fertility, but bull breeders still want their bulls to look good in the spring; ranchers still tend to prefer bulls that are big for their age and have shown how much they can gain.

A gain of 2.6 pounds is optimum. “At the University of Missouri, Robert Larson did research on this, following bulls for an extended period of time, evaluating semen production and semen quality later in life,” says Skinner.

“Excessive energy levels, getting bulls too fat, will also decrease libido, especially in older bulls. If they are too fat in the spring they will be lazier because they can't move well,” he says. Bulls need some reserves, however, especially a young bull out for the first time breeding cows; he needs something to draw on when he doesn't have time to eat. You don't want him fat, but you want him in good condition.

“If we don't take care of bulls properly there's a greater chance of sub-optimal fertility. We end up with less cows settled in their first cycle and a strung out calf crop—and lower average weaning weight due to a higher percentage of late calves. You should have at least 85 percent of your calves from the cows' first two cycles, depending on the condition and fertility of the cows, the type of breeding pastures, etc.”

A good mineral program is an important part of nutrition management. You need the appropriate levels of trace minerals to complement your feeds. Bulls should be on the same mineral program as your cows, to make up for any lack in your soils or feeds.

Disease Protection – “Several things play a role in health and fertility of a bull—nutrition, genetics, disease, and stress (the latter can hinder the immune system and make an animal more vulnerable to disease, or cause recrudescence of latent disease). Reproduction in a bull is no better than the weakest link in that chain,” says Skinner.

Bulls should be vaccinated before breeding season but it's not wise to vaccinate the day you turn them out. Depending on age of the bull and what you vaccinate for, the time it takes for immunity to develop after vaccination can vary. A booster shot does not take as long as a first time shot.

“IBR vaccine should be given long enough ahead that the animal, if stressed, can get past any recrudescence of the IBR virus and any shedding of the virus. This may take two weeks. If you vaccinate 30 days ahead, the bull should no longer be shedding virus by the time you turn him out,” says Skinner. “Some people say it's better to vaccinate at least 60 days ahead, so that if the bull has a fever, he'll have a new batch of sperm cells by breeding time. But he'll be going through a stress period as soon as he goes out with cows, that first 30 days, and he needs some immunity. We find that IBR recrudesces in a cow during the stress of estrus, and if the bull goes into a herd that is not properly vaccinated, he is apt to be challenged, from that cow herd.” He needs peak immunity, to protect him during that first 30 days, as well as no temporary impairments from his own vaccination that might hinder his fertility, so vaccinate him several weeks ahead.

Other Factors That Affect Fertility – Age can make a difference; a bull's highest fertility is at two to four years of age, on average. “A decline in fertility may be noticeable when a bull gets to be five or six years of age. At seven years, you may start to see a more rapid decline but this will depend on the individual bull's genetics. Some bulls have good fertility at that age and others fall apart on you by then, according to trials that were done in Oklahoma by T.D. Rich,” says Skinner.

Overuse can also decrease fertility, though this decline is usually temporary. “A bull may have poor fertility because he is exhausting his semen supply, especially if he is not producing semen as well as he should,” says Skinner. Overuse can be a bigger problem in a sub-fertile bull than a highly fertile bull.

“If a bull is being used very heavily, usually a seven day rest will bring him back to full speed on semen count and semen production. Rotating bulls in and out of the cow herd during the peak of breeding season (in for a week, out for a week) is not a bad strategy,” says Skinner.

Hot weather can be detrimental, since heat is detrimental to sperm production. High fever will also make a bull infertile for a while. “His semen may be ok for a few days after the fever (because sperm that were already mature will be fine), but he'll be in trouble later. It takes 60 to 63 days for sperm cells to develop,” says Skinner. Closely monitor bulls. If one gets sick and has a fever, do a semen check before he breeds cows.

Semen evaluation should be done on all bulls before turnout. Everyone tries to figure out ways to cut expenses, but the expense of a breeding soundness exam usually pays off. One problem, if you don't check bulls--even if you have four bulls out with 100 cows—is that if the dominant bull is infertile or sub-fertile, this may negatively affect your calf crop even though the other bulls are fertile. “The dominant bull usually sires 60-plus percent of the calves. He may keep the other bulls from breeding, even though he's not settling the cows himself,” explains Skinner.

“Some bulls with sub-optimal fertility may not be detected with a typical semen test and breeding soundness exam, however. A bull may have plenty of semen on that day, since he hasn't been breeding cows,” says Skinner. As soon as he goes out to breed, he tends to run out of sperm because his production may be poor.

“Libido is also something that can't be evaluated during that exam. You have to watch the bulls after they're out with the cows. This is especially important in single sire breeding groups. Make sure young bulls figure it out,” he says. Research at Miles City, Montana showed that crossbred bulls reached sexual maturity more quickly than purebred bulls and have more libido.

Feet and soundness are also important. A bull may be fertile, but if he can't get around or is uncomfortable from feet/leg issues or becomes unsound, he won't breed many cows. Always check feet and legs before bull turnout.


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