Study finds seasonal differences affect Brahman bull sexual maturity

December 03, 2002

OVERTON - Fall-born Brahman bulls require more than a month longer to reach sexual maturity than Brahman bulls born in the spring, according to recent research by an internationally recognized authority on the reproductive physiology of Brahman cattle.

A study conducted by Dr. Ron Randel, Texas Agricultural Experiment Station researcher, found that sexual maturity was delayed by as much as 39 days in fall-born bulls compared to spring-born bulls.

"It's important information for commercial cattlemen if they buy young bulls and want to use them at an early age," said Randel, who is based at the Texas A&M University Agricultural Research and Extension Center at Overton.

"Time is money. If a cattle producer buys a young bull born in the fall, it may not be sexually mature enough to breed cows during the coming winter months," Randel said.

He also noted that the delay in sexual maturity would have little to no effect on long-term reproductive efficiency.

Experts on the Brahman breed of cattle have long known that heifers are affected during winter months, with a shorter duration of estrus and decreased incidence of estrus. Although some have suspected that there are seasonal effects on Brahman bull reproductive physiology, Randel's study is the first to quantify the effect.

Most cattle breeds show physiological responses to seasonal change. Brahman cattle seem to be more sensitive to these differences. Exactly what factor triggers these physiological changes is unknown, but many animal scientists believe it has something to do with photoperiod; that is, the animals' biological clocks are responding to the changing proportion of hours of night and day.

For the purposes of the study "sexual maturity" was defined as when the bull's sperm count increased to 500 million or more, the benchmark amount sufficient for freezing for artificial insemination.

The study compared eight Brahman bulls born between August and October 1998 and 10 Brahman bulls born between March and May 1999.

Both groups of bulls were maintained in the same pastures and fed a three-to-one ratio of corn/soybean meal. Their rations were supplemented with 200 milligrams per day per head of the ionophore lasalocid, a commonly used feed additive that changes the characteristics of a ruminant's intestinal flora and fauna to improve feed efficiency.

Both groups of bulls were given free access to water, Coastal Bermuda grass hay and a salt/mineral supplement.

Measurements of body weight, body condition score, scrotal circumference and sperm concentration were taken every two weeks. Only age at sexual maturity was significantly different between the bull groups.

The fall-born Brahman bulls did tend to be heavier (as much as 104 pounds) at maturity, but Randel noted that the increased weight was due to the additional time it took them to reach that stage of development.

The Overton Center is one of 13 regional research and extension centers in the state. As with the other regional centers, the Overton Center facilities serve many purposes. In its research role, the center is headquarters for eight Texas Agricultural Experiment Station scientists and their support staff.

Located two miles from downtown Overton in Rusk County, the facilities include labs for the study of forages, fruits and roses, legume-breeding lab, reproductive physiology (beef cattle), small grains and soils. Thanks to a grant from the Bruce McMillan, Jr. Foundation, the center has more than 1,200 acres on which to perform horticultural, forage and livestock research.

Randel is a nationally honored researcher. His awards include the Regents Fellow Service Award in 1999 and the American Society of Animal Science Award in animal physiology and endocrinology in 1996. In 1998, Randel was named a Faculty Fellow of the Texas Agricultural Experiment Station.
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Texas A&M AgriLife Communications

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