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Does cow size really matter?

Gene Johnston 03/16/2011 @ 10:01am On the scene at the 2012 Cattle Convention, Nashville

Want to get a good argument going? Ask a group of cattle producers about the ideal size for a mature beef cow.

For several reasons, cattle have been getting bigger over the last generation. Cheap feed for most of recent history has dictated extended feeding periods. Packers can process big cattle more efficiently. Many producers assumed more pounds equal more income. As calves and feedlot cattle have grown bigger, mama cows have followed.

Now, with more expensive feed, beef producers are giving more thought to the question of cow size. Is a smaller cow more efficient in terms of feed utilization, provided she can still wean a big calf? With smaller cows, can you automatically expand herd size?

The answers aren't simple. Texas A&M specialists at the King Ranch Institute looked at numerous studies that might have some relevance to the question of best cow size.

In theory, they say, bigger animals have an advantage because they use energy more efficiently. For instance, says Jennifer Johnson, a graduate student at the King Ranch, 100 cows that weigh 1,000 pounds each are equal to 87 cows that weigh 1,200 pounds each in terms of maintenance feed requirements. That means bigger cows have 20% more weight but only take 13% more feed.

Kleiber's Law

The reason this is true is stated in a biological truth known as Kleiber's Law. It calculates the geometric relationship between body surface area and volume.

Even given the truth of this law, says Johnson, maintenance requirements alone are not a measure of a cowherd's efficiency. There are many other variables that are far more important, the most significant of which is reproductive efficiency.

“It's the single biggest proponent of overall herd efficiency,” Johnson says. “If your cows can't get bred and produce a calf every year, they are not going to be economically efficient regardless of size.”

Johnson and her colleague, J.D. Radakovich, a graduate student, point out that one of the most common measures of efficiency in a beef herd is the ratio of calf-weaning weight to cow weight.

“It's flawed in several respects,” Johnson says. It assumes two cows of equal weight consume the same amount of feed. But feed intake depends on body condition score, sex, stage of production, age, quality of forage, and environmental stress.

“This ratio ignores the most important thing, which is reproduction,” she notes. “Both cows are much more efficient than their counterparts, which didn't breed. A 50-pound difference in weaning weight is minimal compared to no calf at all.”

She adds that the cow with the bigger calf may produce more milk, meaning she might eat more even in her dry season, and she may be more at risk for not re-breeding. Research has hinted that cattle breeds that are more moderate in genetic potential for growth and milk production are more efficient overall because of higher conception rates.

The bottom line is cow size doesn't have much to do with profitability, provided cows are within certain guidelines (roughly 1,000 to 1,400 pounds).

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