Not all bulls are created equal. Some are more fertile than others and some have more sex drive (the desire to breed cows). Fertility can be readily checked by a veterinarian during a breeding soundness examination, collecting and evaluating sperm (quantity and quality). But sex drive is just as important as fertility, and not as easily tested.
Good fertility in a bull does not necessarily mean he also has high sex drive.
For many years stockmen have realized there are differences in sex drive in bulls and that this can affect breeding and conception rates. Just because a bull is highly fertile doesn't mean he will get his cows pregnant; he first has to breed them.
Some universities and research centers have been doing libido testing in recent years, and there are now some fairly reliable ways to test and score a young bull's sex drive.
The earliest studies on bull libido were begun due to problems in maintaining the interest and sex drive in dairy bulls being collected for artificial insemination. Dairymen were using AI much more extensively and much earlier than beef producers, so much of the early work was done with dairy bulls. This work led to development of procedures to stimulate bulls and collect as much semen as possible, and later led to developing tests for young beef bulls to determine sex drive. Sex drive in bulls is a heritable trait, aggressive and efficient breeders tend to have offspring that are also good breeders.
Some early work with testing of beef bulls was done by Dr. Peter Chenoweth, an Australian, while he was professor of animal reproduction at Texas A & M. He developed a system of cattle behavior management and scoring to determine and predict sex drive in beef bulls. He observed and scored bulls for their interest, actions, and the time it took them to actually mount a cow in heat (or not mount her, or not actually breed her). His research showed that one out of five young bulls is a failure at breeding cows. He said high libido in bulls is advantageous to herd fertility and is beneficial to the fertility of the bull's female offspring.
Later studies at the Meat Animal Research Center (MARC) in Clay Center, Neb., classified tested bulls into three groups high, medium, and low libido. When presented with 50 in heat heifers over a three week period, high and medium bulls mated with 87.5 and 95 percent of the heifers, while the low group bred only 70 percent.
But libido test scores are not always accurate in determining what a bull's actual breeding abilities will be.
Several factors can temporarily reduce a bull's libido. For instance, young bulls that are too fat or very thin, or suffering from pain or disease, or from stress (even the emotional stress from improper handling at testing) may have poor scores.
Dr. Duane Mickelsen, Veterinary Clinical Medicine and Surgery at Washington State University, Pullman, specializes in bovine reproduction, and he says that some researchers questioned the usefulness and accuracy of libido tests, especially at first. But as more work as been done in this field, Dr. Mickelsen feels the tests are valid. Libido tests are not as consistent on yearling bulls, for as he points out, they are still learning how to breed cows. But the tests are fairly accurate for 2 year olds. Mickelsen says that 15 20 percent of young bulls tested for the first time are failures at breeding, but only about four percent of older bulls tested were questionable breeders.
Libido tests do have some shortcomings. There can be major differences between bulls, but detecting these differences when first evaluating a bull can be difficult. For instance, most bulls will be enthusiastic when first turned in with females, but there is no way of predicting whether a bull will keep up his efforts throughout the breeding season.
Also, the testing set up may not give accurate results. Testing bulls that are apprehensive or agitated can lead to low scores, as can testing of bulls immediately after subjection to other stressful procedures such as vaccination, breeding soundness exams that include use of electro ejaculator, etc. Testing during adverse weather (very hot, very cold, or stormy) may also make a bulls score low, as can any other factor that prevents him from focusing on the job at hand. A dominant bull (even if confined in a separate pen, but still nearby) may intimidate a more timid or subordinate bull and distract him from trying to breed the cow.
Dr. Chenoweth, after his early work with libido testing, discarded the traditional philosophy of using a bull for every 20 25 cows in the herd, and called this a "waste of bull power," since one good bull with sex drive and healthy sperm is very capable of breeding 60 80 cows within a 3 6 week period. He stated that a good bull can breed up to 25 or even 30 cows in a single day. But you can't expect this kind of performance from just any bull, and there are few bulls who can keep up this kind of pace for very long. You have to observe and know your bulls, and know whether you can depend on them. The ultimate test of a bull is how many cows he actually breeds and settles.