Whether purebred or commercial, reproduction has long been a major limiting factor in beef-cow operations. And, one of the most common frustrations is the failure to get first-calf heifers rebred.
The first-calf heifer has a few things working against her, says Tom Geary, of the USDA Ag Research Service in Miles City, MT. First, she's not mature, so she has to find enough energy for growth, maintenance and lactation all at once. Plus, she's often asked to do these things at a time of year when forage quality is the poorest. Is it any surprise the pregnancy rate in two- and three-year-olds is frequently the lowest in the herd?
The two-year-old female is the most expensive and valuable animal in the herd. After all, she hasn't yet generated any income, but considerable money has been invested in her. In fact, estimates say it costs $950 to develop a replacement heifer and get her to the point of her first calving, Geary says.
So, if a heifer fails as a two-year-old, that's a significant financial loss. In most herds, a replacement female won't actually pay for herself until she's five years old — after weaning her fourth calf.
For all those reasons, it's often logical to invest a little extra in getting her rebred than to start all over with another animal, he adds.
Geary says many research studies have looked at identifying and addressing the rebreeding troubles of first-calf heifers. One result has been the discovery of differences in the postpartum interval (PPI) between heifers and their older counterparts.
Cows need 40-60 days to recover from calving and to overcome the resulting negative energy balance. Once that's done, a cow will return to regular estrous cycles and be ready to rebreed.
But, two- and three-year-olds may require as much as 70-90 days. Genetic selection for increased productivity has tended to worsen the problem when that female's genetic potential gets “out of synch” with the production environment. After all, high-performance animals have higher nutritional requirements.
Research suggests the key to increasing pregnancy rates, especially among young cows, is to shorten PPI, he says. This increases the number of times she can conceive in a given breeding season — and increases her fertility early in the breeding season.
Some producers try to get around the heifer's longer PPI by breeding heifers three weeks before the rest of the cow herd. The intent is to allow them more time to recover before the next breeding season begins.
But, studies show this practice can backfire due to seasonal effects related to changes in light. In fact, cows that calve earlier in the spring may actually have longer PPIs, Geary says.
In addition, those heifers have an even longer wait after calving for green grass. If just-calved heifers don't get sufficient nutrients, they'll be even further behind at the next breeding. Postpartum nutrition affects fertility primarily, but deficiencies during that period can also lengthen PPI.
More Successful Strategies
Geary says research suggests other strategies can more successfully shorten a heifer's PPI. One is to ensure heifers have sufficient energy stores before calving. It's very difficult, or impossible, to make those up afterward.
In fact, prepartum nutrition, especially during the 50-60 days before calving, is the primary controller of PPI length. She should be in a body condition score of 5-6 at calving.
Also, five different studies suggest feeding ionophores after calving shortens PPI in cows an average of 18 days, if adequate energy is also available. It will increase feed costs by less than 2¢/day.
Heifers that calve late as two-year-olds often fail to rebreed, or they calve later as three-year-olds. So, having them calve early in the calving season is critical. That means they must be cycling at the beginning of the breeding season. Research offers several suggestions:
Weight: The old rule that heifers must be at least 65% of their mature weight at the start of breeding season is still true. What's different is mature weight; it used to be around 1,000-1,100 lbs., so heifers needed to weigh 650-700 lbs.
Now, mature cows weigh 1,250 lbs. or more, so heifers must be at least 800 lbs. to be at 65% of their mature weight. Selecting replacement heifers from older calves will help get them there.
Synchronization can be a helpful tool for any heifer development program — even with natural service — to increase the number of heifers that calve early. Synchronization can be as simple as feeding MGA in pellets for 14 days, then turning in bulls two weeks after the final feeding.
Calving difficulties (dystocia), which are known to increase PPI and delay rebreeding, are more common among first-calf heifers. That fact has made artificial insemination (AI) for heifers popular. It allows producers to be certain of using only proven calving-ease sires.
One study showed that heifers experiencing dystocia were 35% more likely to be culled than herd mates. Future reproductive failure is most often the cause.
If calving assistance is needed, it must be given early. After a heifer has spent 1.5 hours in Stage 2 labor (with hooves visible) every 30-minute delay in getting her help meant a six-day-longer interval to her next pregnancy.
Estrus induction can be done in several ways. One is to expose heifers, ideally 55 days after calving, to sterile bulls or androgenized cows. A bull pheromone is what makes this technique work; it requires 30 days of exposure and a ratio of one bull or cow to 20 heifers.
Estrus can also be induced with hormones used for synchronization, though neither a normal nor a high dose of MGA worked in studies. But, a CIDR inserted into the vagina for seven days releases progesterone. In early postpartum cows treated with the CIDR method, 60% were in estrus within four days. The fertility of the estrus, however, was not tested.
Another hormone, GnRH, can be injected to induce estrus. It causes the release of progesterone for five to seven days, initiating a short cycle. The estrus that follows has been shown to be very fertile. Either hormonal induction can be used 30 days after calving.
Early weaning: Short-term calf removal effectively induces estrus in postpartum cows, but doesn't work as well in first-calf heifers. However, early and permanent weaning holds more promise for improving reproductive efficiency in that group than all other methods combined.
The demands of lactation are a critical factor affecting PPI, especially in first-calf heifers. To affect reproduction, the calf should be removed, preferably, before the beginning of the breeding season. Thus, it may mean weaning calves less than 60 days old.
But, studies have shown that even 40-day-old calves can outperform suckled calves if fed a highly palatable and high-energy ration, Geary says.
Extra labor, management and expense may be necessary to make a difference in reproductive efficiency for first-calf heifers. But, it can be worth the effort.
This article was written and produced by the National Association of Animal Breeders (NAAB) as a service to U.S. beef producers. For more information about NAAB, visit www.naab-css.org  or call 573/445-4406.