Young bulls need to be fed adequately for proper growth and development, and future fertility, but we are still learning about the best way to feed them, according to John Kastelic, a professor of theriogenology and head of the department of production animal health, in the University of Calgary’s faculty of veterinary medicine.
“In beef bulls, historically, the breeders focused on nutrition after weaning. There has been a great deal of study on this, going back to the 1970s when Dr. Glenn Coulter (at the Lethbridge Research Centre) and others did a lot of work on post-weaning nutrition,” says Kastelic.
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“Back then, seedstock producers typically fed their young bulls what would be considered a feedlot ration, to see how fast they gained, to achieve gains up to four pounds per day. With the studies that were done, there is good evidence to show that if you feed bulls high-energy diets after weaning, you get very rapid weight gain but also create a lot of problems. This kind of feeding results in excessive fat in the scrotum (which interferes with proper thermoregulation), poor semen quality, and therefore reduction in fertility,” he explains. In some cases these bulls end up with permanent reduction in number of sperm produced.
High-energy diets also create more risk for laminitis and founder, damaged feet, acidosis, liver abscesses, rumenitis.
“Feeding this much energy is counterproductive,” says Kastelic. “Today many breeders recognize this and realize that a bull should be fed for a long life of breeding service rather than being fed like a steer destined for slaughter. A bull needs to be athletic, not fat.
John Kastelic.
photo: Supplied
“There is no justification in overfeeding bulls in the post-weaning period, yet some people still feed too much during feed trials to see which bulls have the best rate of gain and feed efficiency. If young bulls are fed post-weaning on a mostly forage diet, the bulls with genetic potential for rapid and efficient gain will still gain the fastest, although the difference between the top and bottom is smaller. You can still do a feed test — but on a forage-based ration with modest amount of grain — and still identify the top-performing bulls. Thus there is no excuse to push them with a feedlot ration, because you can do permanent damage, and it’s a waste of feed resources.”
The second important thing breeders need to understand is the importance of the pre-weaning phase in a young bull’s life.
“There was some work done in the 1950s in young dairy bulls looking at this, and then almost nothing until we started our studies with beef bulls nearly 20 years ago. For most beef bulls, that’s when they are still on their mothers. We just assumed that the cow is going to look after her calf and feed him adequately,” Kastelic says. His emphasis on feeding young bulls has mainly been in the post-weaning stage of their lives.
Researchers have recognized that fast-maturing bulls tend to have greater scrotal circumference as yearlings and tend to be more fertile than slower-maturing bulls with smaller scrotums as yearlings. Thus they realized that a bull with a heavy-milking dam tends to grow bigger and mature a little more quickly than a bull raised by a heifer or an old cow that may no longer milk as well as she did in her prime.
“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 about 1.5 centimetres to the yearling scrotal circumference (since heifers’ calves tend to be smaller as yearlings and catch up later in growth) and for a mature cow there was no adjustment. If the bull had a really old dam you also added a small amount of correction — for the lack of maternal nutrition,” he says.
“The earlier work in dairy bull showed very clearly that the 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 we fed them very well prior to about 25 weeks of age (roughly six months), we could hasten puberty a little, but more importantly we could increase testis size and increase the number of sperm produced.”
To prove this, Kastelic and his colleagues did a feeding trial with several groups of young bulls, putting them on diets with different levels of nutrition. One group received 100 per cent of the protein and energy requirements as a baseline. Another group of young bulls received just 70 per cent of those requirements and the third group was fed about 130 per cent of requirements. All had adequate minerals and vitamins.
The calves were weaned early off the cows and fed these different diets, to see how they performed.
“We fed that ration from about six weeks of age to about 25 weeks. We found that the bulls on the 130 per cent ration — compared to the bulls that were on 70 per cent of their requirements — reached puberty about a month earlier and had testes that were about 20 to 30 per cent bigger and produced 20 to 30 per cent more sperm than the bulls on the 70 per cent ration. The number of sperm produced per ounce of testes is fairly consistent, within a fairly narrow range. Therefore in general, when the testes are bigger, those bulls produce more sperm,” he explains.
“We found no difference in semen quality among the three groups, so there was no indication that feeding the bulls extra at an early age in any way harmed future sperm quality. If we feed bulls really well prior to 25 weeks of age they might reach puberty a month or so earlier than they would otherwise, with testes 20 to 30 per cent bigger, with 20 to 30 per cent more epididymal sperm reserve. Thus the quality seems to be fine, but the quantity increases,” he says.
Looking at what might be changing due to the extra feed, the researchers found that the difference involved the hormone called LH (luteinizing hormone).
“This hormone is released from the pituitary, a small gland at the base of the brain, and goes to the testes and causes release of testosterone. What happens in a young 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 of age and then it 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 the bulls really well (i.e. 130 per cent of requirements) we could substantially increase the amount of LH. There were profound differences in those bulls in that very early phase of growth. At that age, testosterone levels are really quite low, but what happens at that time actually 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.”
What was interesting was what happened when the bulls were fed really well for the first 25 weeks and then backed off on the feed.
“We put the profile of testicular development — measured either as scrotal circumference or testicular volume — on a completely different trajectory. Their testes continued to grow rapidly, even though they were on a normal diet of 100 per cent of requirements.
“The other thing that was really interesting was that if we held them back in growth through the first 25 weeks (at only 70 per cent of requirements for energy and protein) and then supplemented them, giving them 130 per cent 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.
“We found that even with the additional supplement we could not rescue those bulls. The future course of testes development was already set; they were on a trajectory to be underachievers. That bull cannot reach his genetic potential for testes development. He doesn’t really catch up. 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 then from that point on we can just feed a nice balanced diet for growth,” Kastelic says.
On a practical basis for beef cattle, however, these bulls are still on their mothers at this stage of their lives. Bull producers would need to do some sort of creep feeding to ensure they have adequate energy and protein, especially for young bulls out of first-calf heifers.
“This would help all bulls that have the potential to become breeding bulls, but especially would help any young bull that might not have a high-milking dam.”
This is the opposite of what we’ve learned about optimum heifer development; a young heifer with a high-milking dam or eating creep feed may put too much fat in her young udder and never milk as well in adulthood as a heifer that was not allowed to get so bloomy fat as a calf.
All of the work with beef bulls was done with early weaning, so the exact amount of protein and energy in their diets could be calculated for the various groups in the studies.
“The bulls with heifer dams were weaned at about six weeks of age, for instance, and we were a bit worried about taking them off their mothers that early and putting them on dry feed. But we didn’t lose any calves to health problems during that early weaning and transition phase, and this was over a four-year period with more than 150 calves,” says Kastelic.
The early-weaned 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 for us to have complete control over what the various groups of bulls were eating and we could modify it to suit our trial. A practical method when raising bull calves would be to just leave the calves on their mothers and supplement with both energy and protein — not just straight energy — and boost their growth prior to about 25 weeks of age and then back off,” he says.
You don’t want to feed them this much post-weaning, as they are becoming more mature, because that’s when they start putting extra fat in the scrotum.
“You probably want to have them gaining close to about three pounds per day during the time they are still nursing their mothers, and then just back off at weaning, and then the bulls are fine. That’s the key, but you don’t want to feed your heifer calves that same way — so you’d have to separate them and their mothers into a different pasture,” says Kastelic.
“There have been many studies looking at scrotal circumference and how it is highly heritable, and that by selecting for bulls with large testes we can make rapid genetic progress and also get earlier puberty in the daughters of those bulls. In addition, however, there is quite a bit we can do just with management. The key is to supplement the young bulls prior to weaning and then just back off and grow them,” he says.
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