Bull fertility needs producers’ close attention

A bull sires 30 to 60 calves per year so a breeding soundness examination by a veterinarian should be done annually

SAN ANTONIO, Texas — A single bull affects the genetics of a herd much more than a single cow, so selection of the most fertile sires is critical.

“Reproduction is five times more important than growth traits and 10 times more important than product quality when we look at the finances on a cow-calf operation,” said beef reproduction specialist George Perry of South Dakota State University.

“Without reproduction, we don’t produce that calf and we can’t be sustainable,” he said during an education session at the National Cattlemen’s Beef Association convention in San Antonio.

A bull sires 30 to 60 calves per year and may work for about four years so a breeding soundness examination by a veterinarian should be done annually. The exam indicates the bull is a potentially satisfactory breeder.

The exam represents the semen and physical condition of the bull that day. If a bull was hurt earlier it takes 60 days to make good sperm again. If the bull is retested he could pass.

Testing the bull in December is not a good indicator of what is expected for the breeding season four to six months later, said Perry.

The current standards for breeding soundness are a physical exam, estimation of sperm production and scrotal circumference as well as an assessment of semen quality.

The bull needs to be able to see and smell.

“Visual estrus detection even for a bull is very important,” he said.

Smell helps him know which cows are in heat. The physical structure is important to know if the bull can get up and breed a cow. The testes and glans should be palpated and checked for inflammation or anything else unusual.

“If there is pus in the ejaculate, fertility rates go down,” he said.

Bulls should also be checked for trichomoniasis and vibrio and culled if infected.

Full extension of the penis is needed to see if there are warts, corkscrew abnormalities or other issues that might affect breeding ability.

Scrotal circumference is a measure of sperm production. There is a direct correlation between testicular mass and how much sperm a bull can produce each day. It does not indicate how many cows can be bred.

“If we stop selecting for scrotal circumference you can watch things can go the other way very quickly,” Perry said.

There are breed differences and size is highly heritable. Bulls with larger scrotums produce sons with larger scrotums and daughters reach puberty sooner.

Sperm quality should be assessed. Ejaculate volume and sperm concentration are needed.

“We need at least 30 percent of semen being progressively motile,” he said.

Motility means movement and progressive indicates the sperm head is going forward so it can reach the egg. Double or broken heads or tails may be seen under the microscope and these will not result in a pregnancy.

Poor nutrition during drought or wide temperature variation can affect semen quality. For example, during very cold weather if bulls are not bedded properly they can get frostbite on their scrotums and semen quality goes down. However they can recover.

It is possible for 10 percent of bulls to fail an exam even before they get to the semen evaluation. Another seven percent can fail the sperm test and 7.1 percent may have inadequate scrotal circumference.

Most producers figure out the physical abnormalities before any tests and remove them as breeders.

Serving capacity is difficult to measure.

Libido is the willingness to breed cows but it is not part of the exam. It is highly heritable and there are bulls that do not want to breed cows.

“What is his interest? Does he care where the feed bunk is or where the cows are,” Perry said.

About five percent of mature bulls with a satisfactory exam are physically unable to mate or do not want to breed cows.

“Breeding is a learned behaviour,” said Perry.

“Young bulls tend to fall in love and chase one cow around.”

Multiple sire pastures reduce serving capacity because many bulls might breed the same cow.

The bulls should be watched because there is social dominance within the herd.

“In most breeding pastures, there is one dominant bull who breeds the majority of the cows,” he said.

“If your dominant bull is subfertile, what will happen to your conception rates? It will drive it down.”

Breeding soundness is a good tool to detect sterile or infertile bulls but there is no guarantee those that passed are fertile. Below average and low fertility bulls are often classified as satisfactory.

Motility and morphology still explain abnormalities but researching fertility and conception rates is difficult.

A successful bull is good news but detecting the poor ones is more difficult.

Checking the sperm for longevity, normal morphology and progressive motility can be done but these do not account for everything that is seen.

New research shows there is variation in sperm proteins so scientists hope to learn if it is a marker for fertility or longevity. Epididymal versus ejaculated sperm are different but researchers are not sure how that affects fertility.

There are more than 400 proteins attached to sperm.

During ejaculation there are more proteins attached to the sperm but researchers are not sure what role they play in terms of sperm fertility or longevity.

When the sperm is deposited it runs into a mucus barrier that pushes it back. There are enzymes on the sperm that allow it to penetrate and reach the egg.

It was always thought sperm carries the DNA and delivers it to the egg but it is now known more is happening. Sperm plus the proteins also carry microRNAs, once considered junk.

“We have since learned they play a very important role in every tissue in the body,” Perry said.

“In other species we know embryo development will stop if they are not present,” he said.

This body of information is growing but will not replace the breeding soundness evaluation because this is the first step to eliminate infertile bulls.

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