With spring bull sales in full swing, cow calf operators are assessing their bull batteries and making needed purchases. Producers often ask about the use of young bulls in the same breeding pasture with older, larger bulls. In most instances, this is a practice that should be discouraged if at all possible. Young bulls will normally lose the battle of deciding who is the dominant individual in the breeding pasture. Ranchers report that in some cases young bulls that have been severely “whipped” are less aggressive breeders after that incident. Australian data on multi-sire pastures have shown that some young bulls gain a dominant role as they mature and breed a large percentage of the cows. Other bulls will not gain that dominant status, and only breed a very small percentage of the cows in a multi-sire pasture for the remainder of his stay at the ranch. The best solution is to always place young bulls with young bulls and mature bulls with mature bulls in the breeding pasture.
In some situations, the rancher may choose to use the mature bulls in the first two-thirds of the breeding season, and then rotate in the young bulls. This allows the young bulls to gain one to two months of additional age and sexual maturity. In addition the young bulls should have considerably fewer cows in heat at the end of the breeding season as the mature bulls will have bred the bulk of the cows or heifers. The young bulls will be in the breeding season only a few weeks and should not be as “run down” or in poor body condition at the conclusion of the breeding season.
Also a commonly asked question is: "How many cows should be mated to young bulls?" The old rule of thumb is to place the young bull with about as many cows as his age in months. Therefore the true “yearling” would only be exposed to 12 or 13 females. If he is a year and a half old (18 months), then he should be able to breed 15 – 18 cows. By the time the bull is two years of age, he should be able to breed 24 or 25 cows. Realize that tremendous variability exists between bulls. Some are capable of breeding many more cows than what is suggested here. and sadly enough, a few bulls will fail when mated to a very few cows. Hopefully, a breeding soundness exam and close observation during the first part of the breeding season will identify those potential failures.