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.
With Natural service, physical characteristics, such as scrotal circumference, mating ability, and semen quality play a role in a bull’s fertility. The best way to determine these factors is through a Breeding Soundness examination (BSE). The American Society for Theriogenology developed minimum guidelines for a bull to pass a BSE. To successfully complete a breeding soundness evaluation, a bull must have at least 30% sperm motility, 70% normal sperm morphology, and a minimum scrotal circumference based on age. Bulls meeting the preceding minimum requirements are classified as satisfactory potential breeders. If a bull does not pass one of these tests, he is classified as a “classification deferred” animal (meaning it is recommended that the bull be tested again) or he is classified as an unsatisfactory potential breeder. Bulls should be tested approximately one month to six weeks prior to the breeding season. This allows for time to retest bulls if unsatisfactory results are obtained or time to find a replacement herd bull.
The purpose of the physical examination portion of a BSE is to determine a bull's mating ability. Mating ability can be described as the physical capabilities needed to successfully breed a cow. A bull must be able to see, smell, eat, and move normally to successfully breed cows. The physical examination closely scrutinizes a bull’s eyes, teeth, feet, legs, and nutritional level (evaluated by body condition score). Any disease or injury that affects joints, muscles, nerves, bones, or tendons may cause a bull to be structurally unsound. In addition to structural unsoundness, diseases or injuries to the penis or prepuce can result in an inability to breed via natural service. These abnormalities will only be detected by careful examination or observing an attempted mating of a cow. A bull that has high quality semen but is unable to physically breed cows is unsatisfactory for natural service.
Whether natural service or AI is used, two of the most important indicators of bull fertility currently available are sperm motility and morphology. With AI identifying females in estrus and proper placement of semen are critical factors for obtaining desirable pregnancy rates in the cowherd; however, compromised semen quality through semen handling will negate the attention to detail of the two factors discussed previously. With natural service, structurally sound bulls with a large scrotal circumference and high semen quality should be selected as herd sires. Moreover, it is important to remember that semen quality of an individual bull changes over time and, for a bull to be fertile, desire to find cows in estrus (see Managing For Reproductive Success: Detecting Estrus Part I) and mating ability should be evaluated periodically.
Source: Jim Krantz
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