The premiums paid for high-grading cattle have almost led to a situation of single-trait selection for intra-muscular fat (IMF), or marbling, in the U.S. beef industry. While history teaches us the folly of single-trait selection, the current rush for marbling poses significant potential for lowered fertility in U.S. beef cattle.
If we accept the premise that androgens (testosterone) have a demeaning effect on IMF, then the ultrasounding of bulls to determine IMF from a sire group could certainly lead to lower fertility and later puberty.
Consider this scenario: You have 100 bulls to ultrasound that are of varied frame sizes of 5 through 7. Age at scanning is 11½ to 12½ months.
The 5-frame bulls have probably reached puberty and are at high testosterone levels. The 6- and 7-frame bulls are perhaps at a lower level of testosterone and thereby show more IMF (if of equal genetic predisposition).
The early puberty bulls would show more activity because they are burning more energy. Testosterone, acting as a repartitioning agent, allocates more nutrients to muscle and less to IMF.
As a result, the later-puberty bulls — with lower testosterone levels — would show an advantage on ultrasound readings for IMF.
Even if all the bulls were of the same frame size and equal puberty, we still have a strong chance of bias. If some of the bulls are of a high libido nature, they may burn twice the energy as average or lower libido bulls. This would turn a buyer away from such bulls if he were looking for balanced traits.
The industry needs some basic research to determine:
When is an appropriate time to read IMF in relation to testosterone levels?
What happens to IMF at high testosterone levels?
How do low-fertility bulls read on IMF?
Would the use of heifers in determining a sire's progeny reading for IMF be less antagonistic to fertility?
What it boils down to is that while we are measuring variables in IMF, we may also be ignoring great variables in testosterone levels. Thus, we're putting ourselves at great risk of unconsciously avoiding high-fertility bulls.
I have found little research that tests the correlation between IMF and testosterone. It should be an industry research priority as there's no production trait with greater economic consequences for beef producers than fertility.
Jim Bradford owns and operates the Brad Z Ranch, a purebred Angus and purebred Gelbvieh operation in Guthrie Center, IA. The current chairman of the National Cattlemen's Beef Association Research Committee, Bradford has been a producer leader of national beef research efforts for the past decade.