Many of us have heard the old joke about the young bull and the old bull, and I will just leave it at that. However, the spring breeding season is upon us and 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 the dominant individual is 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.

A study conducted at the University of Nevada’s Gund Ranch looked at behavioral observations and recorded physical characteristics of bulls during the breeding season to understand the influence of social behavior, dominance, age, seniority, body condition, and semen quality on the fertility of bulls. Results from one breed season showed that out of 15 bulls, two bulls sired the majority of the calves, and 2 other bulls sired no calves.

Some of the preliminary findings of the UNR study showed that bulls (of relatively the same age) that did less fighting, spent more time grazing and spent more time within close proximity of the cows sired more calves. This is a good reason to observe your cattle as much as possible during the breeding season, in order to determine which bulls are doing their job and which ones are not.

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 to 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.


• Using young bulls in multi-sire pastures and cow-to-bull ratios, by Glenn Selk, Oklahoma State University Emeritus Animal Scientist.

• Bull Paternity predictors, by Meghan Gray, University of Nevada.