Cattle producers should consider creep feeding bull calves that could someday be sires, says a recent study from the University of Calgary.
“These findings have important implications for management of young bulls prior to puberty,” said PhD student Alysha Dance, who looked at the effects of nutrition on hormone production, age of puberty, semen quality, and testicular characteristics in 26 bull calves.
Her research draws on previous work by Dr. John Kastelic, who found bulls on a high-nutrition diet in early life had more reproductive hormone, which led to earlier puberty and increased testicular weight and sperm count.
“We had testes that were 20 to 30 per cent larger and 20 to 30 per cent more sperm in those testes,” said Kastelic, head of the university’s faculty of veterinary medicine.
“We can really profoundly get bulls with larger testes and more sperm by just feeding them really well.”
In Dance’s study, the calves were put on three levels of milk — four, six, and eight litres a day — until nine weeks of age. Then the calves were transitioned to a silage diet that corresponded to 70, 100, and 130 per cent of the recommended daily intake for bull calves.
“Throughout the entire duration of the feeding period, the high-nutrition group was larger in terms of their body weight than the other two groups,” said Dance.
Scrotal circumference and luteinizing hormone — which are both related to reproduction — were also higher in the calves fed a high-nutrition diet.
“In the high-nutrition diet, the early rise (in luteinizing hormone) was occurring earlier and at a higher rate than in bulls on a low-nutrition diet.”
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Testosterone production, which is driven by luteinizing hormone, was also higher in the high-nutrition diet.
“The high-nutrition bulls had more testosterone coming up earlier than those on the low-nutrition diet.”
Dance also evaluated the age of puberty using semen from the bull calves and found a difference of 45 days in puberty progression between the high-nutrition diet and the low. And when evaluating age of puberty based on scrotal circumference, she found a difference of 81 days.
“This is a quite significant difference between the high and the low diet in terms of progressing the bulls to puberty a little bit sooner.”
But there was no difference in calf fertility between the high- and low-nutrition diets, she said.
“Once they were past puberty, we used that semen for in vitro fertilization in the lab, and there was no difference between the amount of embryos produced and the high, low, and medium groups,” said Dance.
“We’re causing the bulls to come into puberty earlier, but it’s not at the compromise of fertility at all.”
There’s also little risk of overfeeding bulls prior to weaning.
“Virtually all of the work done on nutrition in bulls… was done after weaning. That’s really the wrong time to be overfeeding bulls,” said Kastelic. “Overfeeding bulls after weaning is very detrimental.
“Prior to 30 weeks, what you feed them is hugely important.”
And though some studies on creep feeding suggest that calves fed a lower-nutrition diet will catch up after weaning to their creep-fed counterparts, the work done by Dance and Kastelic shows otherwise.
“If you fed them poorly prior to 30 weeks and then at a much higher level after 30 weeks, you couldn’t make up for that earlier deficiency,” said Dance.
“Something critical is happening prior to 30 weeks in these bull calves.”
Dance will be completing further research on the mechanisms that are creating these effects in bull calves.