Selection and Development of Rangeland Heifers
Heifer size not the best tool for selecting heifers for longevity in the Southwest.
RUIDOSO, New Mexico (Aug. 29, 2018) — Heifers should have their first calf by the age of 2. However, that is dependent on their reaching puberty early, and puberty is a function of age and body weight, noted Bruce Carpenter, Texas A&M University animal science associate professor and extension livestock specialist, speaking to cattlemen gathered Aug. 29-30 in Ruidoso, N.M., for the Applied Reproductive Strategies in Beef Cattle workshop.
“A lot of the pastures in extensive ranch systems are big, and labor is not available to watch each cow and record calving dates,” Bruce Carpenter explained. “Seeing those cattle is pretty challenging sometimes, so you don’t usually know calving dates.”
Heifers born in the first half of the calving season should be more physiologically mature and thus more likely to be pubertal. However, Carpenter pointed out, a lot of mitigating circumstances often make this a flawed and inefficient approach.
One of the most challenging aspects of selecting heifers is knowing what to base selection on. In an ideal world, it would solely be on birth date, selecting the heifers born earliest in the calving season as a sign of early fertility.
“A lot of the pastures in extensive ranch systems are big, and labor is not available to watch each cow and record calving dates,” Carpenter explained. “Seeing those cattle is pretty challenging sometimes, so you don’t usually know calving dates.”
This leads to selection decisions based on picking the bigger, heavier heifers in hopes they are more mature and will take less feed to get to breeding weight, said Carpenter. However, this can lead to bigger cows. For the Southwest, where feed can be limited, this is not an ideal situation.
“The other options for selection involved reproductive tract scoring at breeding or keeping a large number of replacements,” Carpenter said. “This would require keeping those replacements for selection after the first breeding season to see who is pregnant.”
Numerous studies found heifers can breed when they are at 50%-57% of their mature body weight, but what is mature body weight?
“Mature body weight is essentially a guess, because we truly don’t know,” Carpenter said. “Because the variability is so great, getting heifers to 60%-65% of their estimated mature weight gives us some insurance for the guessing game.”
Frequent and consecutive drought is common in the many regions, but mostly in the Southwest. Seasonal forage changes and timing of breeding according to those changes are also major factors in heifer development.
“Lining up our calving and breeding seasons with the best-quality forage is a strategy we need to think about more,” Carpenter said, while also keeping in mind the good years are not the ones producers should worry about; the bad ones need their attention most.
In several consecutive studies involving Carpenter, the timing of calving was related to lifetime production abilities. Calving date for a first-time heifer determines their patterns of calving, longevity and lifetime return on investment (ROI).
“Those calving earliest typically have higher weaning weights throughout their lifetime,” Carpenter said. “This also alludes to how long these cows will breed back easily.”
Earlier-calving heifers produced calves who performed better from a carcass standpoint, as well, said Carpenter. It all starts when the calf hits the ground. It determines a majority of their lifetime performance.
Carpenter said he is not satisfied with this guessing-game approach to heifer selection. He is in the middle of a project looking at an alternative way to determine pubertal maturity of first-calf heifers.
“We took a pregnancy blood test on Day 40 after letting bulls out on the heifers,” explained Carpenter, expanding on this protocol that showed if heifers were pubertal from the results of the test. Receiving a prostaglandin shot on days 7-12 caused them to either cycle naturally or respond directly to the shot.
Carpenter said this could be a way to select for puberty and fertility even when a producer cannot get to his cattle at his leisure.
For more detail on Carpenter’s presentation — including the proceedings, PowerPoint and video of his presentation — visit the Newsroom at www.appliedreprostrategies.com.
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