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derachievers. Tat bull cannot reach his genetic potential for testes development. Tus 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 balanced diet for growth,” he says. On a practical basis, however, bull producers would need


to do some sort of creep feeding, especially for young bulls out of first-calf heifers. “Tis would help all bulls that have the potential to become breeding bulls, but especially any young bull that might not have a high-milking dam.” All the work with bulls was done with early weaning, so


testes and causes release of testosterone. What happens in a bull’s life, starting at about six to eight weeks of age, is an in- crease in this hormone, and it stays relatively high until about 20 to 25 weeks and then it decreases. Te nature of that in- crease, how high it gets, and how oſten 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 (the


group receiving 130 percent of their requirements, versus the group receiving 70 percent) we could substantially increase the amount of LH. Tere were profound differences in those bulls in that very early phase of growth. At that age, testoster- one levels are really 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 thereaſter.” What was interesting was that if the bulls were fed well


for the first 25 weeks and then backed off on the feed, this changed. “We put the profile of testicular development—mea- sured either as scrotal circumference or tes- ticular 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 percent of requirements (no extra). Te other interesting thing was that if we held them back in growth through the first 25 weeks (with only 70 percent of their requirements for energy and protein) and then supplemented them, giving them 130 percent of their nutrients, we had different re- sults. Tis would be like having a heifer for a mother (not as much milk) and then getting a lot of protein and energy aſter weaning,” he says. “We found that even with the additional


supplement we could not rescue those bulls. Te future course of testes development was already set; they were on a trajectory to be un-


20 | TEXAS LONGHORN JOURNAL | HERD SIRE EDITION


the exact amount of protein and energy in their diets could be calculated for the various groups in the studies. “Te bulls with heifer dams were weaned at about six weeks of age, for instance, and we worried about taking them off their moth- ers that early and putting them on dry feed. But we didn’t lose any calves during that early weaning and transition phase, over a four-year period with more than 150 calves,” he says. Te 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. “Tat was the easiest way to have complete control over what the various groups of bulls were eating. A practical method would be to just leave the calves on their mothers and supplement with both en- ergy and protein—not just straight energy—and boost their growth prior to about 25 weeks of age (weaning) and then back off,” he says. “You want them gaining close to three pounds per day


during that time frame, and then back off on the supplemen- tal feed at weaning, and the bulls are fine. Tat’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. Tis would allow breeders to achieve full genetic potential for testes development in the young bulls. “Tere have been many studies


looking at scrotal circumference and how it


is highly heritable, with results also showing that by selecting for bulls with large testes we also get earlier puberty in the daughters of those bulls. In addition, there is quite a bit we can do just with management. Te key is to supplement young bulls prior to weaning and then just back off on


the feed and grow them,” he says. “Tere has been a vast amount of work look-


ing at post-weaning nutrition, but that’s the time that we should not be ruining bulls by over-feed- ing. We tried that for several decades and it didn’t


work,” he says. Te time to augment their feed is when they are still on their mothers.


Brett Krause photo


Bear Davidson photo


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