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Ensure Breeding Success

04/08/2014 @ 11:19am

Selecting herd sires with good fertility and breeding ability plays a key role in the profitability of a beef herd.

“The number of cows bred during the breeding season plays the largest role in percent of calf crop weaned and is the single-most important factor influencing profitability of a beef herd,” says George Perry, South Dakota State University Extension beef reproductive management specialist.

When using sires by natural service, he suggests having your veterinarian perform a breeding soundness evaluation on bulls in advance of turn-out time.

“A breeding soundness evaluation includes a physical examination, the measurement of scrotal circumference, and the evaluation of semen quality,” says Perry. 

Young bulls 15 months or younger should have a minimum scrotal circumference of 30 centimeters. Bulls 2 years and older should have a scrotal measurement of at least 34 centimeters.

The sperm should have at least 30% motility. The semen should also have at least 70% normal sperm morphology. “This means 70% of the sperm evaluated should be normal in appearance,” he notes.

Research suggests bulls with an even higher percentage of normal sperm will produce higher pregnancy rates. One study showed that bulls with 80% normal sperm had pregnancy rates 5% to 6% higher than randomly selected bulls passing a breeding soundness evaluation.

Keep in mind that the results of a breeding soundness evaluation provide a marker for a set point in time. Results can change over time because a bull’s sperm production is a continuous process affected by external conditions. For instance, injury, disease, fever, and extreme environmental conditions can decrease sperm production.

“Testing a bull annually – usually a month prior to the start of the breeding season – is a good idea,” says Perry.

Ability to Produce

Young bulls vary in their ability to produce semen of good quality. Nutrition and breed differences affect the age when bulls reach puberty.

“A practical indication of imminent puberty is when scrotal circumference is between 27 and 29 centimeters,” says Perry. “Simply because a bull has reached puberty and can produce semen does not necessarily mean he is highly fertile.”

Sperm quality and quantity continue to increase for several months after a young bull starts producing semen. Research suggests about 35%, 60%, and 95% of 12-, 14-, and 16-month-old bulls, respectively, are reproductively mature and produce good-quality semen.

At the start of the breeding season, bulls should be in good condition and, ideally, have a condition score of six. Such body shape gives them a cushion for losing weight. Yet, it ensures they are not overly fat, which could cause fatigue and result in a poor conception rate during a demanding breeding season.

Aside from measurable differences in semen quality, unknown factors also play a role in a bull’s fertility. Semen from some bulls seems to have reduced viability when used by artificial insemination (AI) in a fixed-time breeding protocol. “With timed AI, there could be as much as 15% to 20% difference in pregnancy rates between bulls,” says Perry.

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Farm Science Review, Day Two