Feed for Fertility
Preweaning feed for young bulls affects future fertility.
Young bulls must be adequately fed for proper growth and development and future fertility, but we are still learning about the best ways to feed for optimum fertility.
John Kastelic, veterinarian and professor of cattle reproductive health at the University of Calgary, has done many studies on nutrition in bulls. He says breeders need to understand the importance of the preweaning phase of the young bull’s life.
“There was some work done in the 1950s in dairy bulls and then we started our studies 20 years ago. Most beef bulls are still on their mothers during the first months of life, and we assumed that she is going to feed him adequately,” Kastelic says.
“Some work in the 1970s at Colorado State University, looking at scrotal circumference in yearling bulls, came up with an adjustment formula; if the bull had a heifer for a mother, you added 1.5 centimeters to the yearling scrotal circumference (since heifers’ calves tend to be smaller as yearlings and catch up later) and if he had a really old dam you also added a small amount of correction for maternal nutrition,” he says.
The work in dairy bulls showed clearly that bulls that were really well-fed early in life reached puberty quicker, had larger testes and produced more sperm. This was also our finding with beef bulls. If they were fed very well prior to about 25 weeks of age — roughly 6 months — puberty would hasten a little, but more importantly their testes size and number of sperm produced would increase.
To prove this, they used a feeding trial with several groups of young bulls. One group received 100% of protein and energy requirements. Another group received just 70% of those requirements and a third group about 130% of requirements, but all bulls had adequate minerals and vitamins.
“We fed that ration from about six weeks of age to about 25 weeks. We found that bulls on the 130% ration — compared to bulls that were on 70% — reached puberty about a month earlier and their testes were about 20% to 30% bigger and produced 20% to 30% more sperm, with 20% to 30% more epididymal sperm reserve,” he says.
Looking at what might be changing due to extra feed, the researchers found that it involved luteinizing hormone (LH).
“This hormone is released from the pituitary, at the base of the brain, and goes to the testes and causes release of testosterone. What happens in a bull’s life, starting at about six to eight weeks of age, is an increase in this hormone, and it stays relatively high until about 20 to 25 weeks and then decreases. The nature of that increase, how high it gets, and how often it is released has a big impact on what happens later in the bull’s life,” says Kastelic.
“We found that when we fed bulls really well (the group receiving 130% of their requirements vs. the group receiving 70%), we could substantially increase the amount of LH. There were profound differences in those bulls. At that age, testosterone levels are quite low, but what happens at that time sets the bull up for the rest of his life. So by feeding really well during those weeks, we bumped that LH up, and saw all the other changes thereafter,” he explains.
What was interesting was that if the bulls were fed really well for the first 25 weeks and then were backed off on the feed, this changed.
“We put the profile of testicular development — measured either as scrotal circumference or testicular volume — on a completely different trajectory. By feeding young bulls very well until about 25 weeks of age and then backing off, their testes continued to grow rapidly, even though they were on a normal diet of 100% of requirements (no extra). The other thing that was interesting was that if we held them back in growth through the first 25 weeks (with only 70% of their requirements for energy and protein) and then supplemented them, giving them 130% of their nutrients, we had different results. This would be like having a heifer for a mother and then getting a lot of protein and energy after weaning,” he explains.
“We found that even with additional supplement, we could not rescue those bulls. The future course of testes development was already set; they were on a path to be under achievers and could never reach genetic potential for testes development,” he says.
Thus, the key to optimizing a bull’s future fertility is to provide extra nutrition early in life from about six weeks of age to about 25 weeks, and from that point on just feed a balanced diet for growth, he adds.
To accomplish this, breeders could creep-feed young bulls out of first-calf heifers. This would help all bulls, but especially any bull that doesn’t have a high-milking dam.
This is the opposite of what’s best for heifer development; the young heifer with a high-milking dam or creep feed may put too much fat in her young udder and never milk as well in adulthood.
This research with beef bulls was accomplished with early weaning, so the exact amount of protein and energy in their diets could be calculated for the groups. The calves were put on a silage-based diet, adding various amounts of grain and canola or soy meal to bump up the energy and protein.
“That was the easiest way to have complete control over what the groups were eating. Producers can leave calves on their mothers and supplement with energy and protein — not just straight energy — and boost their growth prior to about 25 weeks of age and then back off,” he says.
“That’s the key, but you don’t want to feed heifer calves that same way, so you’d have to separate them and their mothers into a different pasture,” says Kastelic. This would allow breeders to achieve full genetic potential for testes development in the young bulls.
Editor’s Note: Heather Smith Thomas is a cattlewoman and freelance writer from Salmon, Idaho.