You're out looking at your heifers on a cold January afternoon. You notice “mobs” of heifers as they begin showing signs of sexual maturity. And, this activity seems to be picking up.
You conclude that your heifers have reached puberty, and you're just a bit ahead on your development program.
Seeing this cycling action tempts you to back off the heifers' rations and maybe save a little money on concentrates. Chances are, though, those heifers are just acting like the teenagers they are.
What you're likely seeing is one of two possible physiological events in the life of the heifer — either precocious puberty or non-puberal estrus (NPE).
Precocious puberty describes heifers that show signs of reproductive maturity at an early age — usually less than 300 days of age. Occasionally, precocious puberty, which is a fertile heat, can even occur as early as six or seven months of age.
NPE, on the other hand, is a non-fertile heat in which the heifer may or may not ovulate. If she does ovulate, there's not enough progesterone in her system to maintain the pregnancy.
Either estrus is simply part of the animal's maturation process leading to her mature heat cycles. But, it's something to be aware of — and manage for prior to the breeding season.
Unpredictable intervals sum up the problem of NPE and precocious puberty. Rancher and DVM Barney Cannon, Winnemucca, NV, has observed and recorded first estrus and subsequent heats in his replacement heifers for years.
Cannon's replacement heifers start cycling at 9 months old, weighing far less than his target breeding weight of 700 lbs. His records reveal that heifers showing outward signs of standing heat usually don't cycle again on the traditional 18- to 21-day interval.
“Instead, they cycle for the second time anywhere from seven to 30 days,” Cannon says. “Some heifers were not detected cycling again at all.”
Cannon was likely seeing NPE, says Bob Bellows, Bozeman, MT. He's a reproductive physiologist recently retired from USDA's Livestock and Range Research Lab (LARRS) Miles City, MT.
Whatever he saw, Cannon did not let his heifers breed on this sign of heat. More importantly, however, neither did Cannon let the visual signs of estrus slow his development program.
He starts his artificial insemination program in April when his heifers have reached target weight and have had three or four cycles. By then most are cycling on a regular basis.
“I've had great success getting these young heifers bred at this point,” he explains. “This strategy also boosts success in getting the second conception. It just sets them up to succeed.”
Scientists at LARRS have shown that 18-30% of English and English-cross beef heifers will exhibit NPE. And, Texas A&M University research indicates up to 60% of Brahman-cross heifers will exhibit NPE.
As ranchers continue to select for bigger, early maturing cattle, they're bound to see more problems associated with precocious puberty and NPE, explains Bellows.
“Research clearly shows the first or puberal estrus cycle is less fertile than later cycles,” explains Bellows. “It's essential for these reasons that heifers reach target breeding weight and be cycling early.”
“Before you abandon your development program, closely observe your heifers morning and evening,” advises Cannon. “Determine which heifers are in standing heat and which are just riding.”
He and Bellows suggest recording this information for a month or more.
“What you think is a pile or mob of heifers bulling may just turn out to be one or two of these NPE heifers in standing heat for 24 to 48 hours,” says Cannon.
Often, especially with heifers, cattle will hang around the “action” even though they are not actually in heat themselves.
“They will ride the hot cows but not stand to be ridden themselves,” adds Bellows. “They are not in heat themselves, just curious and bored.” This is true especially in confinement situations such as a winter feed ground.
The moral to the story cannot be over-emphasized — don't back off on your heifer development program and the goal of attaining the target breeding weight of 65-70% of mature weight.
“Seeing a few bulling heifers early is a good sign,” Bellows adds, “but it's not the end of a heifer development program.”