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, Perry says.
"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," Perry said. "Clearly there are differences among bulls in their ability to achieve pregnancy success."
Perry points to research conducted at the USDA-ARS research center in Miles City, Mont.

This research 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," he said. "The ability of sperm to become capable of fertilizing, binding and penetrating an oocyte all influence a bull’s fertility."
Perry says that research is 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," he said. "Nevertheless, the only current method for determination of fertility differences between bulls requires the insemination of several thousand animals under the same management practices."
Currently, Perry says the best method for cattle producers to acquire semen with good fertility is to buy it from a reputable source and make sure it has all been handled correctly.
Fertility level and natural service
With Natural service, physical characteristics, such as scrotal circumference, mating ability, and semen quality play a role in a bull’s fertility," says Perry.
He says 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 percent sperm motility, 70 percent 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," Perry said.
The overall 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," he said.
Sperm motility and morphology
Whether natural service or AI is used, Perry says 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," Perry said. "With natural service, structurally sound bulls with a large scrotal circumference and high semen quality should be selected as herd sires. 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.

For more information related to detecting standing estrus contact, Jim Krantz, SDSU Extension Cow/Calf Field Specialist at [email protected] or 605-995-7381 or Dr. George Perry, SDSU Extension Beef Reproductive Management Specialist at [email protected] or 605-688-5456. To listen to a recent iGrow Radio Network interview on this topic with Dr. George Perry, and to review all four articles in this four-part series released by SDSU Extension visit

Part 3 of a four-part series

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,’ says George Perry, SDSU Extension Beef Reproductive Management Specialist.
Perry explains that the ‘Equation of Reproduction’ includes the following four areas:
Percentage of animals detected in standing estrus and inseminated;
Inseminator efficiency;
Fertility level of the semen and;
Fertility level of the herd.

Each of the areas will be discussed in the four-part series on managing for reproductive success by SDSU Extension.