by: Heather Smith Thomas
It's always a good idea to have a breeding soundness evaluation and semen check for any bull you plan to use—not only for bulls you purchase, but also the bulls you kept over from last year. Even though they might have been fine last year, things can change. Injuries or infections can change that picture. Make sure each bull is good to go—before your breeding season.
Most producers make sure that every bull passes a breeding soundness examination before putting him out with cows. As stated by Dr. Dee Whittier (Professor, Large Animal Clinical Sciences, Virginia Tech), it is always good to start with the bull breeding soundness exam that is outlined by the Society of Theriogenology.
“This is based on good research and is a place to start, when assessing a bull. It basically looks at five things: physical soundness (feet and legs, eyes, etc.), reproductive tract soundness, scrotal circumference that meets the minimum requirement, minimum percentage of sperm cells that are normal, and acceptable motility. This is where we start, and we recommend that every bull, every year, have a breeding soundness exam.”
During that exam, a semen sample will be collected, and checked. The semen will be evaluated for motility, velocity, total number of sperm, and any major sperm defects that will significantly affect fertility.
“The Western College of Veterinary Medicine in Saskatchewan did a study a few years back, and put together all their data on bull breeding soundness exams. They found that basically the same percentage of bulls failed at each age, across the years. Some stockmen feel that they only need to test the young bulls or the older bulls; they think the middle-aged bulls won't have problems. But just as high a percentage of those bulls fail every year, as well,” says Whittier.
Several things can interfere with fertility, including high temperature and cold weather. Temperatures outside the optimum for sperm production may reduce sperm quality and quantity, especially if the bull can't keep his testicles cool enough in hot weather or warm enough in cold weather (by raising and lowering them away from the body).
Scrotal frostbite (which can often occur in northern climates with cold weather and wind, if bulls don't have adequate windbreaks) can result in scar tissue that makes it impossible for the bull to raise and lower his testicles, rendering him infertile. This may be temporary and not a serious problem (if the damage heals before the next breeding season) or permanent if the damage is extensive. “The bull may be out for the next breeding season, or maybe out for his lifetime, depending on the damage,” says Whittier. If your area had cold weather—especially if there was wind during the cold—bulls should be checked before the next breeding season to make sure they have recovered, if there's a chance they may have suffered scrotal frostbite.
Optimum sperm production and sperm health depends on the testicles being a few degrees cooler than body temperature. This is why the bull must be able to lower his testicles in hot weather, and draw them up closer to the body for warmth in winter. Hot weather can be detrimental to fertility, as can a high fever if the bull is sick or suffers from an infection like foot rot. The sperm that were forming at the time he had a fever will be abnormal, and he will have an infertile period about 60 days following the fever.
Summer heat may render a bull temporarily infertile. “I live in a part of the country where fescue is our major grass. The toxin in fescue inhibits cattle's ability to shunt blood to the outside of the body so they can get rid of heat,” says Whittier. It is harder for the animal to regulate its temperature, and there is more risk for heat stress.
“Cows suffering from heat stress may not become pregnant (or may lose the embryo) but bulls are also adversely affected. We constantly talk to producers about introducing clover into their fescue pastures, and having a fertilization program that doesn't stimulate the fescue to outcompete the other plants. There is lots of management involved in dealing with this problem. One whole side of our state has largely abandoned spring calving just because of the problem of getting cows bred during the heat of summer. Many bulls show decreases in fertility earlier than cows do, in the same setting. When they are incapable of cooling their testicles their semen quality is quickly diminished,” says Whittier.
Age can make a difference in sperm quantity and quality, according to Dr. Ron Skinner (veterinarian and seedstock producer near Hall, Montana). A bull's highest fertility is at two to four years of age, on average. “After four years there may be some decline in fertility, but this is not very noticeable until a bull gets to be five or six years of age. At seven years, on average, you start to see a more rapid decline,” says Skinner.
This will depend on the individual bull. “There are some bulls that will have good fertility at that age and others that will fall apart on you by then, according to trials that were done in Oklahoma by T.D. Rich,” says Skinner. “The five and six year old bulls may not have declined significantly from their peak fertility at age four, but it will depend on their genetics.” It's always wise to do a semen check on older bulls before the breeding season.
Some stockmen figure that if a bull has large testicles, he will be fertile, but this isn't always true. Be wary of a young bull that has huge testicles at a year of age, like 44 centimeters (average size would be 36 centimeters). These individuals are predisposed to subsequent testicular degeneration. That bull might be fertile at first, and then become infertile too soon as he gets older.
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