The average bull in a commercial beef herd stays around for four years and mates at least 100 cows. That makes him key to genetic direction of the herd and his reproductive potential the true source of his value. But, all of that potential can literally go cold in a perfect storm of freezing temperatures, lack of acclimation and management missteps.
"Anytime there is a period of extreme temperature change, bulls can have a period of subfertility, especially if they lack good nutrition or protection from the environment," says Lew Strickland, Extension veterinarian with the University of Tennessee.
While producers in Southern states rarely see temperatures cold enough to produce anything as extreme as frostbite on a bull's scrotum, Strickland says they still need to be aware that excessive cold may have affected the herd bull's ability to reproduce. He believes it's important to go into the winter with a strong body condition score (BCS) of around 6 to 7 and to schedule that bull for a breeding soundness exam (BSE) at least 60 days prior to the start of the season.
"It takes that bull at least 60 days to make a brand new set of spermatozoa," Strickland explains. "If he has an issue, this will give him 60 days to recover, after which we'll recheck him. This also gives a producer time to find another bull if he has to without impacting his breeding season."
The veterinarian says his biggest wintertime concern for herd bulls, aside from body condition, is the fast-changing weather pattern so common in the South. This can lead to illness, and a bull with a fever or pneumonia may not recover sufficiently in time to breed by January, a month many in Strickland's state start exposing cows to the herd bull.
"When we are going from 60°F one day to 30°F the next, a bull can develop pneumonia," he says. "If he runs a fever that elevates body temperature, that can easily interfere with spermatogenesis [sperm production and development]."
Once the animal's temperature gets to 102°F or above, stress becomes an issue, and there may be an impact on fertility. However, given time, bulls are often able to produce and mature a new set of spermatozoa.
Strickland says in his area, most producers provide some type of shelter for the herd bull, and there is almost always, at the least, a protective windbreak. He adds an ag-enhancement program in Tennessee has helped provide funds for many of the state's cattle producers to build shelters for animals, as well as their hay.
Acclimation Is Key. While producers in the South have to be aware of potential fertility loss during the winter, in the North, it's almost a given the herd bull is going to experience some extremely cold temperatures and, as a result, will need more attention to prevent problems.
Greg Lardy, head of North Dakota State University department of animal sciences, says the biggest issue is for those bulls that can't get out of the wind and are frostbit, and those that become malnourished.
"Both cause serious problems with fertility. If a bull is extremely thin and malnourished, it affects sperm production and sperm maturity," Lardy says. "Frostbite can do the same thing. Of the two, it's more common to see damage from frostbite."
This type of damage will often produce scabs on the lower part of the scrotum. Absence of scabbing doesn't necessarily mean there was no frostbite, however. Lardy says they recommend a BSE 45 to 60 days after injury or before breeding season. The BSE will look at both sperm motility, morphology and population. He says it's important to wait the 45 to 60 days after an injury to check because it takes that long for damage to be evident.
"Damage becomes clear as the sperm cell matures, and that takes a couple of months to see," he explains. "At the same time, don't wait to take action with regards to treatment. If there's evidence of frostbite, bed the bull up properly, and increase nutrition to try to help him overcome the harsh conditions and begin to recover."
Asked if there is a rule of thumb as to what equals "prolonged exposure" for a bull to the point it creates harm, Lardy says there is no certain number. Acclimation and a winter where the change is more gradual means cold conditions are usually going to be well tolerated. Extremes are the issue.
"Say some sort of storm system comes in and temperatures drop quickly. The bull, all the cattle, suffer more because they are not acclimated. Their hair coats haven't had time to grow in, and they aren't ready for the cold."
He points out an animal can have hypothermia when temperatures are above freezing. This can lead to loss of cattle.
"In 2013, we had the storm they called 'Atlas' in early October. It really impacted cattle producers," Lardy recalls. "We had a rain/snow mix, and temperatures were right around 32°F with wind. The cattle were simply not acclimated, and many died. They got wet, they couldn't insulate themselves. As soon as they get wet, you have problems."
Even without big swings in the weather, Lardy says acclimation can be an issue for bulls brought to the northern climate from other parts of the country. He advises producers buying bulls from other regions to allow two weeks to a month acclimation time.
"Don't go to Tennessee, for example, buy a bull for your ranch in North Dakota, and bring him back with the expectation he is ready to start breeding your cow herd. That bull needs time to acclimate to everything—the weather, the feed, his overall environment."
Body Condition And Fertility. Lardy likes to see bulls at a 5 or 6 body condition going into the winter, same as for mature cows. He stresses overfeeding can be an issue and is another way to reduce fertility. It can also increase feet and leg issues for bulls going into breeding season.
"Too much fat means a bull can't regulate its temperature properly, and that can reduce fertility as well," he says.
Strickland, who likes a little higher body condition on bulls in his area, notes that for producers who want a bull to start breeding in January or February, it's important to remember that a bull would rather breed than eat. That extra body condition under these conditions is a good insurance policy. Spring-bred herds, he adds, can aim more for a 6 BCS on the bull.
"This goes back to something I tell producers when it comes to their herd bull," Strickland says. "Remember that sex is a luxury. If the body condition gets below a certain level, that bull won't produce the spermatozoa he needs to keep up with the cows."
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