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Don’t just assume bulls are operating at the top of their game

Proper care and management of bulls is crucial for optimum fertility. Some bulls are naturally more fertile than others, due to factors such as genetics, and scrotal conformation, 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 their ability to breed cows. Young bulls need adequate nutrition to develop properly, but should never be fat.


Ron Skinner, a veterinarian and seedstock producer near Hall, Montana, recommends monitoring the condition of all your bulls. “Semen production of an underfed young bull is only 77 per cent of the production of an adequately fed bull,” he says. “If a young bull is allowed to get too thin, he’ll lag behind in his 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.”

“Poor energy levels delay puberty,” Skinner adds. “A young bull that is not developed properly nutritionally will be slower maturing. Purebred breeders who are developing bulls need to have them on a proper energy ration — not too much and not too little. 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 being fed to gain three to 3.5 or even four 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 poor fertility.

Skinner says he may have less actual testicle size. Testicle size will be larger with fat deposition in the scrotum, but lack of circulation due to the extra fat has a tendency to decrease actual testicular tissue (hence less size — less scrotal circumference — after the fat is lost). Therefore you end up with less semen production.

Most people know that fat decreases fertility, but bull breeders still want to have their bulls 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. In practicality, however, a daily gain of 2.6 pounds is optimum. “At the University of Missouri, Robert Larson did some 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 until they get some weight off,” 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 is a greater chance of sub-optimal fertility,” says Skinner. “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 per cent 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 your nutrition management. You need a balanced program with the appropriate levels of trace minerals to complement your feeds. The bulls should be on the same mineral program as your cows, to make up for any

A Properly Fed Bull

lack in your soils (and pasture plants) or feeds.


“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),” says Skinner. “Reproduction in a bull is no better than the weakest link in that chain.”

Bulls need to be vaccinated before breeding season. Often they are neglected. It comes time to turn bulls out and people realize they haven’t been vaccinated. It’s not wise to vaccinate bulls 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. If the bull has stresses, or reactions from vaccination, this may compromise his abilities during the start of breeding.

“IBR vaccine, for instance, 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 is going into a herd that is not properly vaccinated, he is apt to be challenged, from that cow herd.” You want bulls to have peak immunity, to protect them during that first 30 days, as well as no temporary impairments from their own vaccination that might hinder fertility at this crucial time, so vaccinate them several weeks ahead of turnout.


Age can make a difference in fertility. 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,” says Skinner. “At seven years, you may start to see a more rapid decline but this will depend on the individual bull, depending on his genetics. There are some bulls that will have good fertility at that age and others that fall apart on you by then, according to trials that were done in Oklahoma by T. D. Rich.

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,” says Skinner. “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.”

Weather is also a factor. Pay attention to bulls in cold weather, especially if there’s wind. Wind chill can lead to scrotal frostbite, especially in older bulls with large testicles, if they are unable to draw them up close enough to the body for protection against the cold. If weather is severe, bulls need good windbreaks, and bedding, to protect their testicles from frostbite.

Hot weather can be detrimental also, since heat is detrimental to sperm production. High fever will also make a bull infertile for a while. “His semen may be okay for a few days after the fever (because the sperm that were already mature will be fine), but he will be in trouble later,” says Skinner. “It takes 60 to 63 days for sperm cells to develop.” Closely monitor your bulls. If a bull gets sick and has a fever, have 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 you can get into if you don’t check bulls — even if you have four bulls out with 100 cows — 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 per cent of the calves,” says Skinner. “If he’s infertile, he may keep the other bulls from breeding, even though he’s not settling the cows himself.”

“Some bulls with sub-optimal fertility may not be detected with a typical semen test and breeding soundness exam, however,” he says. “A bull may have plenty of semen on that day, since he hasn’t been breeding cows.” 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, Skinner says. Producers have to watch the bulls after they are out with the cows. This is especially important in single sire breeding groups. Make sure that young bulls figure it out, he says. Some work done 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 issues. A bull may be fertile, but if he can’t get around or is in pain or uncomfortable from feet/leg issues or becomes unsound, he won’t breed many cows. Always check feet and legs before bull turnout.

Heather Smith Thomas ranches with her husband Lynn near Salmon, Idaho. Contact her at 208-756-2841

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