The nutritional management of bulls in a cow/calf operation is critical, as bulls contribute half the genetics of a cow herd. Their fertility is just as important as cows.
Cattle producers should consider performing a breeding soundness exam (BSE) on each of their bulls 30 to 60 days before the start of the breeding season. Bulls also need to be watched closely and given protection in cold weather to avoid frostbite.
In a presentation at the University of Nebraska-Lincoln (UNL) Extension 2022 Cow-Calf College, Kacie McCarthy, UNL Extension cow-calf specialist, detailed some of the reasons why bull management is important. Bulls need proper nutrition from the start to ensure they are able to breed.
All three time periods of pre-weaning, post-weaning and breeding season are important times to ensure the proper nutrients are being provided to bulls, she said.
Pre-weaning nutrition is important, as rapid Sertoli cell development occurs when the bull is 4-20 weeks of age. These cells will allow for higher sperm levels in bulls, she said. Poor pre-weaning nutrition can lead to smaller testicle size, lower sperm production and a higher age of puberty.
McCarthy said proper forage quality during this time is key, and the use of supplements can ensure cow and bull nutrient needs are met. She also recommended that bulls from mature cows be selected over bulls born to heifers.
In post-weaning environments, a high energy diet is needed. This will influence scrotal growth and semen quality, McCarthy said.
Breeding season nutrition
Nutrition during breeding season is important, as studies show bulls can lose anywhere from 100 to 400 pounds during the season. This weight must be regained before the next breeding season.
Bulls should remain in the range of 5.5 to 6.5 in body condition score (BCS). This is a moderate condition, and cows should also be in this range, she said.
“The age and weight of bulls matter when thinking about breeding season nutrition,” McCarthy said.
A mature 2,000 lb. bull has different nutritional requirements compared to a 1,200 lb. yearling bull, she said.
According to research, a young bull gaining 2 lbs./day will need 26 lbs. of dry matter intake, 63 percent total digestible nutrients (TDN) and 8.4 percent crude protein (CP). The older bull, meanwhile, is not gaining any more weight and will need 31 lbs. of dry matter intake, 48 percent TDN and 6.8 percent CP.
McCarthy said if bulls are on a concentrate diet in the pre-breeding season, they should gradually be stepped off of this diet. This should be done 30 to 60 days prior to turnout.
During the breeding season, producers should continue to assess the BCS of bulls. With bulls losing weight during the season, it is important to keep a close eye on them, she said.
“There can be a ton of variability with bulls, so watch them closely,” she said.
Post-breeding season nutrition also needs to be considered, McCarthy said. Cattle producers should consider separating bulls by age groups and feeding them differently, as young bulls are still gaining weight.
Another nutritional aspect to consider in the post-breeding season is a mineral program for bulls.
Several minerals are critical for bulls, she said. Selenium is important for normal spermatogenesis (production of sperm), while zinc plays a critical role in sperm cell plasma membrane integrity and tail morphology.
McCarthy said the bull-to-cow ratio depends on the capability of each individual bull. A study compared the ratio of 1:15 up to 1:50, and the report showed 1:25 was the best dollar value for the bull-to-cow ratio.
The impacts of age, social dominance and libido all play roles in the effectiveness of bulls, she said.
Complete a BSE
McCarthy said cattle producers should do a BSE annually. BSE is a uniform method of assessing a bull’s likelihood of establishing pregnancy in an appropriate number of open, healthy and cycling cows or heifers in a defined breeding season.
The exam checks to make sure they can do their jobs and to make sure nothing is wrong with them physically or with sperm production. Some producers do not utilize this exam because they believe it costs too much money, but research shows about a $6-7 per-cow return on the BSE investment, she said.
“We recommend doing a BSE four to six weeks prior to breeding season,” McCarthy said. “This way you can still have some time to purchase bulls before the season.”
Most BSEs are made up of four parts: physical exam, scrotal circumference, sperm motility and sperm morphology. All body parts are examined, including the eyes, ears, teeth, mouth, feet, legs and reproductive parts. In addition, internal accessory glands are checked.
McCarthy said semen is collected, and morphology and motility are checked. To pass a BSE, bulls should have 70 percent normal sperm morphology and at least 30 percent sperm motility.
Watch weather and bulls
Bulls can become infertile during periods after cold weather due to frostbite on the scrotum, McCarthy said. Cold weather and wind chill can result in lower sperm counts for bulls. Frostbite can cause blisters and scabs on the scrotum and can cause discoloration, she said.
Spermatogenesis in bulls is a 61-day process after a frostbite injury, which, depending on when the injury occurred, could affect the breeding season. McCarthy recommends for bulls with a frostbite injury to be reevaluated with a BSE about 45 to 60 days after the injury.
“Bulls need some time to recover from frostbite,” she said. “We encourage producers with bulls a few years old to get BSE so we can catch these issues.”
McCarthy said producers need to use some cold weather prevention practices to ensure their bulls are protected. These would include giving bulls plenty of space, shelter and windbreaks to get out of the cold and providing bedding to ensure bulls and their reproductive parts are not laying on cold, frozen ground. — Russ Quinn, DTN staff reporter
