A bull’s fate as a potential herd sire is determined in large part by genetics, but its nutritional status during calfhood determines whether it will realize its full genetic potential.
Nutrition at this early age, defined as the first 30 weeks of life, is often taken for granted because beef calves are usually happily nursing and nibbling grass during this period. That being the case, most research on bull development has been geared toward the post-weaning period, says Dr. John Kastelic, professor of cattle reproductive health and theriogenology with the University of Calgary’s faculty of veterinary medicine.
He and bovine theriogenologist Dr. Albert Barth, now retired from the University of Saskatchewan, supervised a study by PhD candidate Leonardo Brito, who investigated how nutrition during a bull’s first year and a half affects metabolic hormones and sexual development.
Beef bull calves were weaned at six weeks of age and fed diets containing either 70, 100 or 130 per cent of the recommended energy and protein requirements. All rations supplied adequate minerals and vitamins.
“Those fed the 130 per cent diet during the first 25 weeks reached puberty one month earlier and had 20 to 30 per cent larger testes and more sperm than those fed the 70 per cent diet,” Kastelic says. “Interestingly, bull calves that received the 70 per cent diet for the first 25 weeks, couldn’t make up the deficit even if fed the 130 per cent diet for the second 25-week period.”
In short, the study detected profound differences in hormonal secretions associated with sexual development that explained the difference. The higher plane of nutrition increased or sustained production of important hormones and improved functioning of two endocrine glands in the brain, whereas, the deficient diet had the opposite effect in every respect.
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Overfeeding bull calves for the first eight to 30 weeks and then easing off to the 100 per cent diet from weeks 30 through 74 (the peripubertal period) didn’t result in overly fat bulls. These bull calves and those fed the 100 per cent diet for the entire period had similar average body weights by the end of the 74 weeks, even though the average body weight of the group started on the 130 per cent diet was significantly higher from 26 to 58 weeks of age than that of the group consistently fed the 100 per cent diet.
“Back in the day, producers aimed for post-weaning gains of more than three pounds a day with the notion that if a bull wasn’t fat or didn’t have a high growth rate, then he was thought to be a poor-doer,” Kastelic says. It is now known that overfeeding bulls after weaning on high-energy diets to achieve high rates of gain comes at the expense of reproductive health (reduced semen quality, fewer sperm), abnormal foot and bone growth (laminitis, lameness), and digestive disorders (rumenitis, liver abscesses).
Since boosting diet energy and protein above the recommended level for the first 30 weeks of life benefits sexual development without having these types of harmful side-effects, Kastelic recommends creep feeding bull calves with a ration that provides both energy and protein starting when the oldest bulls are about a month of age. Target an average daily gain on the high side of 2.5 pounds through to weaning, then a moderate growth rate of less than three pounds a day after weaning.
He reminds producers that it’s always important to ensure the cows have been properly vaccinated and well fed during pregnancy and that the calves receive adequate colostrum, ideally at least five per cent of the calf’s weight within five hours of birth.
Researchers are now digging even deeper, into how nutrition in utero affects calf development, Kastelic adds. For example, recent studies have shown negative effects on reproduction potential of heifer calves born to undernourished dams. After birth, the calf’s body tries to conserve energy, which appears to predispose it to metabolic imbalances. c