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Educator discusses managing bull fertility and reproductive efficiency

by Wyoming Livestock Roundup

University of Wyoming Extension Beef Specialist Shelby Rosasco spoke to attendees on Feb. 8 during the Fremont County Farm and Ranch Days in Riverton about managing bull fertility and reproductive efficiency. 

Beef cow/calf production 

Rosasco shared with attendees the goal of beef cow/calf production is to wean a healthy, heavy calf with the greatest genetic potential to perform per cow, every year. 

In terms of reproductive efficiency, the goal is to optimize pregnancy rates early in the breeding season and select and/or develop highly-fertile replacement heifers at the lowest cost possible.

She explained reproductive efficiency is a result of management, selection pressure, nutrition and health and reproductive technologies. 

Producers’ expectations of replacement heifers in relation to management and selection pressure include attaining puberty prior to the first breeding season, calving by 24 months of age and maintaining a 365-day calving interval. 

Additionally, the calf must be genetically capable to perform; milk production must be appropriate for the environment, but provide sufficient resources for the calf to reach genetic potential; heifers must maintain body condition score for the production environment and/or management conditions and temperament. 

Rosasco encouraged producers to select heifers that will become pregnant early in the breeding season. 

“One option would be to only retain heifers bred in the first 30 days and market late-bred heifers,” she said. “Producers can also shorten the breeding season length to 30 days and market open heifers as feeders.” 

Additionally, producers can also utilize reproductive technologies such as estrous synchronization. She noted, in the long run, utilizing selection pressure for heifers that become pregnant early in the breeding season can allow for an increase in calf weaning weights and cow longevity. 

reproductive efficiency 

Rosasco shared with producers it’s important to identify good cows and to cull open cows. 

“Assessing reproductive efficiency allows for identification of any problems, as well as provides an opportunity to plan for the upcoming year and build on current success,” she said. 

A few things to help producers keep track of reproductive efficiency is to maintain records on the number of females exposed, dates when bulls were put in and removed, weaning and pregnancy rates, weaning weight and calving distribution. 

Reproductive technologies

She noted the power of reproductive technologies and management is well established, supported by research, field tested and works.

Several reproductive technologies producers can utilize include breeding season management, breeding soundness exams (BSE), pregnancy diagnosis, crossbreeding, reproductive tract scoring, ultrasounding, artificial insemination (AI), estrous synchronization, embryo transfer, in vitro fertilization and genomics. 

She has found very few operations across the U.S. are taking full advantage of reproductive technologies.

“Only 7.9 percent of U.S. beef operations use estrous synchronization, and only 7.6 percent use artificial insemination,” she said. 

Several advantages of estrous synchronization programs include allowing the use of superior bulls via AI or natural service; producing calves that were, on average, 13 days older and 21 pounds heavier than nonsychronized females (Schafer et al., 1990) and making AI programs practical by reducing time and labor for heat detection. 

Additionally, estrous synchronization programs allow producers to focus on concentrating the breeding season. Such programs can concentrate the calving period, allowing more uniform management of cows, produce a more uniform calf crop and concentrate labor. 

In order to supplement a good estrous synchronization program, producers will need a well-planned program, fertile females with a good body condition score (BCS) that are healthy and disease free, a good working facility, accurate records, with identification and people trained in detecting. 

She noted not all operations will use AI with estrous synchronization programs. 

If planning to use estrous synchronization with natural service, she encouraged producers to use a smaller pasture or lot to reduce bull energy expenditure, have a one to 15 or one to 25 bull to cow ratio, ensure bulls have a pecking order established prior to turnout and allow time for the bull to rest after intense activity. 

Bull fertility 

Rosasco explained 87.5 percent of the genetic makeup of a calf crop is determined by the last three bulls used. Of the 87.5 percent genetic makeup of the calf, the sire accounts for 50 percent, the grand sire for 25 percent and great-grand sire for 12.5 percent. 

Bull fertility includes the physical capability to mate, capacity to produce spermatozoa/sperm and the percentage of functionally normal spermatozoa. 

The goal of a BSE is to identify subfertile bulls. She shared bulls should have a BSE done every year, roughly 30 to 60 days prior to the breeding season. 

“It’s a cheap insurance policy,” she noted. 

She shared some research has shown high-energy diets fed post weaning influences scrotal growth and semen quality. 

During the off season, bulls should have an adequate nutrition management plan, as the plane of nutrition impacts mature bull fertility. She said bulls should be maintained at a BCS of a five or a six and have a good mineral program. 

Rosasco noted zinc plays a role in male fertility and is critical for sperm-cell plasma membrane integrity, tail morphology and mobility, in addition to selenium, which is critical for normal spermatogenesis. 

Itʼs important to have fertile bulls and offer adequate nutrition to maintain fertility. However, there are many factors that can impact a bull’s fertility.

Low temperatures and windy conditions can easily increase feed requirements 25 to 30 percent above normal maintenance requirements, and a lack of wind protection or bedding will increase the chance of frost damage to the scrotum and testicles. 

Additionally, cold weather and wind chill can result in bull infertility by causing tissue damage, including blisters and scabs from frostbite. 

Key points 

In closing, Rosasco shared efficiency and productivity within the herd is tied to reproductive performance and longevity. She encouraged producers to optimize the numbers of cows and heifers pregnant early in the breeding season and consider utilization of technology to increase efficiency. 

“Nutritional management of bulls is critical,” she said. “Consider the genetic contribution of bulls to a cow herd – his fertility is important.” 

Additionally, consider performing a BSE 30 to 60 days before the breeding season to identify subfertile bulls. 

Brittany Gunn is the editor of the Wyoming Livestock Roundup. Send comments on this article to 

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