Feeding a bull a proper diet early in its life is critical to how it performs as a sire, says Dr. John Kastelic, head of the University of Calgary’s department of production animal health.
Studies have shown that bulls that get their full energy and protein requirements, or more, have larger testes and therefore will have more sperm available.
He cited studies in which bulls were weaned at eight weeks and then fed 70, 100 and 130 percent of their energy and protein needs, plus all the necessary minerals and vitamins, from the ages of 10 to 70 weeks.
“The bulls on the high diet consistently had larger testes,” Kastelic told the Saskatchewan Beef Industry Conference.
“The bulls on the high diet reached puberty about a month or more earlier than the bulls on the low diet.”
The testes of the bulls fed the high diet weighed 20 percent more than those on the low diet and also had 40 percent more sperm.
The study was repeated with the same diets but only from ages 10 to 30 weeks. After that, the bulls all received the medium diet, or actual requirements.
“Even though the differential feeding stopped at 30 weeks, we had persistent differences in testes size,” Kastelic said. “What they eat early on, prior to 30 weeks, is a huge, huge influence on testicular development and sperm production.”
Again, puberty was earlier for those that had been on the high diet, testes weighed 20 percent more and sperm production was 20 to 30 percent greater.
Sperm quality was similar in all the bulls.
Kastelic said producers want bulls to grow fast and gain about 2.5 pounds a day before weaning.
“Based on that, there is justification for creep feeding bulls both energy and protein,” he said.
He also said it is important not to underfeed bulls before weaning and then overfeed them to compensate.
“We have committed those bulls to delayed puberty, smaller testes, lower sperm,” he said.
“A bull that’s underfed prior to weaning, we can’t catch him up.”
High-energy diets fed to bulls after weaning are not recommended. They can lead to liver abscesses and other medical conditions, fat deposits in the scrotum and decreased sperm production and quality, he said.
Kastelic also reminded producers that testicular temperature should be 4 to 6 C below body temperature to produce normal sperm.
Common causes of testicular warming include scrotal inflammation from poor bedding or frostbite, fever from pneumonia or foot rot, or the bull spending a lot of time lying down because of lameness.
Late summer breeding in hot weather can also cause problems. Kastelic said temperatures higher than 25 C can lead to testicular warming.