Fertility is influenced by many factors, and one of the best methods to look at factors that influence fertility is with the “Equation of Reproduction”. The Equation of Reproduction includes the following 4 areas: 1) Percentage of animals detected in standing estrus and inseminated, 2) Inseminator efficiency, 3) Fertility level of the semen, and 4) Fertility level of the herd. Each of the preceding areas will be discussed in this series. Parts l and ll of this “Managing for Reproductive Success” four-part series dealt with estrus detection and inseminator efficiency as limiting factors in successful artificial insemination programs. “Fertility Level of the Semen” is another factor which also determines success or failure for breeding programs.
The bull influences overall herd fertility more than any other single animal, and loss of fertility by a bull or straw of semen can cause substantial loss to a potential calf crop. Furthermore, the bull supplies half of the genetics to all of the calves he sires, and bull selection can be the most powerful method of genetic improvement in the herd. Clearly there are differences among bulls in the ability to achieve pregnancy success. Research conducted at the USDA-ARS research center in Miles City, MT has shown tremendous variation in pregnancy rates between bulls when they were used either in a fixed-time AI breeding program or used following detecting cows in estrus. However, all of these bulls in this study looked normal when evaluated under a microscope for motility and morphology.
For several decades seminal traits have been studied to try to predict reproductive success. Research is being conducted to identify characteristics of semen that influence fertility rates. The ability of sperm to become capable of fertilizing, binding and penetrating an oocyte all influence a bull’s fertility. Research is also underway to develop tests that will more accurately determine the fertility of individual bulls. Our ability to predict the fertility of individual bulls either by a semen sample or a DNA sample may eventually be possible. Nevertheless, the only current method for determination of fertility differences between bulls requires the insemination of several thousand animals under the same management practices. The best method to for AI protocols the best method to acquire semen with good fertility is to buy it from a reputable source and make sure it has all been handled correctly.