by Lilly Platts
Developing heifers is one of the biggest investments in the beef business, and ensuring that heifers are set up for a successful breeding season is essential.
Recent research from the Iowa State University explores the effects of different management strategies, and reveals potential indicators of heifer success that could be especially useful during times of restriction. Dr. Randi Culbertson, Cow-calf Extension Specialist and Beef Geneticist for ISU, shared about this research during Fall Focus 2025.
Heifer development has a major economic impact, and the effects ripple throughout the cow herd for years. “The reproductive success of a heifer leads to the improvement in reproductive efficiency for the entire cow herd,” Culbertson said.
The ability to identify females that are not going to breed quickly can cut down on this cost. “If you can identify that a heifer isn’t going to get pregnant, she can go to the feedlot and you can recoup that cost,” Culbertson explained.
Developing heifers is expensive, especially as input costs rise, and the earlier a female breeds, the faster she will recoup that cost. Research shows that females that reach puberty earlier as heifers breed faster later in their life, calve earlier, have more time to recover, wean bigger calves, and consequently are more productive long-term.
“Heifers that breed early tend to stay in the herd longer. Understanding the factors that affect puberty can ultimately affect that heifer pregnancy rate,” Culbertson said. How quickly an animal reaches puberty depends on several factors, including genetics, environment, and nutrition. “We need to set these heifers up so they are cycling by the time we get to breeding season,” Culbertson explained.
Culbertson shared that the recent ISU study was inspired in part by research out of Nebraska suggesting that the standard advice, which is to make sure heifers weigh at least 65% of their eventual mature weight at breeding time, might not apply in every situation. Generally, producers are encouraged to develop heifers with the goal of reaching 60–65% of their mature weight by breeding time, but the research out of Nebraska makes the argument that heifers at 55% of their mature weight were equally fertile. Often, heifer development depends on available resources and overall environment, and if heifers can be developed with fewer inputs, producers could potentially save money during drought, or other restricted periods. “We want to match that development program to the type of cattle you have, and your production system,” Culbertson said.
Culbertson has observed the differences in management in her home state of New Mexico, where heifers are usually turned out on winter pasture. In the Midwest, where land is hard to come by and feed is plentiful, heifers are typically developed in drylots. Heifers on pasture tend to weigh less at breeding time, while heifers in drylots are usually heavier. Weight and Body Condition Score (BCS) are good indicators of maturity, but are not a surefire way to tell if a heifer is going to breed and stay pregnant. Other factors, like how these heifers put on fat, or how efficient they are, can also affect fertility.
Weight and Body Condition Score are good indicators of maturity, but are not a surefire way to tell if a heifer is going to breed and stay pregnant. Other factors, like how these heifers put on fat, or how efficient they are, can also affect fertility.
To further study the effects of management on heifer fertility, Culbertson and her team utilized ISU’s McNay research herd, which has a long history of focusing on carcass traits. This lent well to the study, as Culbertson was interested in the development factors that can’t be seen, like intramuscular fat (IMF), and muscle, in these two different management scenarios.
The team selected 124 Angus heifers from this herd for the study. The females were split into two groups: “restricted” and “non-restricted.” The development of each group was controlled with feed and average daily gain, with the goal of getting the restricted group to 55% of their target mature weight, and the non-restricted group to 65% by breeding.
Every 60 days, the team took carcass ultrasound measurements on each female to track muscle development and fat deposition. Body weight and BCS scores were also tracked. Throughout the development process, the team observed lighter weights, smaller ribeye area, less backfat, and less IMF in the restricted group.
Prior to breeding, each heifer was given a reproductive tract score. The restricted group had noticeably smaller pelvic measurements between the two groups. Females were culled from both groups due to reproductive tract scores and other factors.
At breeding time, an identical synchronization protocol was used for both groups. In the non-restricted group, 100% of the females responded and were cycling. In the restricted group, 25% of the heifers did not respond to the synchronization protocol, indicating that they had not reached puberty. These heifers were also culled. After breeding both groups by AI, a cleanup bull was turned out for 45 days. The total pregnancy rates between the two groups were very similar.
Culbertson and her team then evaluated the data collected throughout the study. They found that IMF and ribeye area were not strongly associated with heifer pregnancy. Backfat measurements were associated, however, indicating that the females in the restricted group that made it past culling and got pregnant may be more efficient. “If you think of two heifers, one in the restricted group and one in the non-restricted group, if they have the same backfat measurement, the heifer in the restricted group is more likely to get pregnant. She is more efficient,” Culbertson explained. “If she doesn’t have backfat, her odds of being open are going to be significantly higher.”
An unintended observation in the study was “grass crash,” or the shock that many heifers experience after being turned out on grass post-breeding. When heifers are developed in a dry lot, producers often turn them out on grass as soon as breeding is over. Conversely, if heifers are on grass but then held in dry lots to streamline breeding, their system can also be shocked. “We tend to see that the sudden change can be a problem,” Culbertson said.
After the study, when heifers were turned out on grass, the restricted group gained weight, and the non-restricted group lost weight. Management while transitioning heifers from the dry lot to pasture, or vice versa, is important. “Heifers losing weight can be problematic for pregnancy,” Culbertson explained.
The team’s observation that ribeye area and IMF didn’t have any effect on pregnancy rates is positive considering the industry’s push for carcass traits. Culbertson clarified that despite that observation, the non-restricted heifers in this study were not over-developed, or fat.
Culbertson concluded that more research is needed to determine if backfat is a solid indicator of heifer fertility. Within a group of females managed under the same protocol, it could help producers identify which are the most efficient, or thrifty, and the most likely to get bred. This could be especially helpful during times of drought or tight resources. “I really believe we should be developing heifers to their target body weight, but sometimes the environment steps in and we have scenarios where we are restricted. If this occurs, how do we select the heifers that can really be adaptive and perform in these environments?”
