One of the aspects of my job I truly enjoy is analyzing pregnancy diagnosis data with students. Every fall ambulatory clinicians at Kansas State University bring us a treasure trove of data from their herd visits — gestational length, body condition scores, breeding groups and other data — that helps paint a picture of what was going on in clients breeding pastures over summer. 

When I was in private practice, I would try to spend as much time as possible at the end of every pregnancy check looking over the results and categorizing the pregnant cows into the different heat cycles based on their estimated gestational length. I found this exercise came naturally for some producers, though not for all. This effect was highlighted when a producer would have upwards of 90% of their cows bred overall, but relatively few bred in the first couple of cycles. 

To be clear, when healthy, actively cycling cows are mated with fertile bulls, approximately 2/3 of the nonpregnant cows should become pregnant every estrous cycle. In a herd of 100 cows, this would translate to an average of 66 bred in the first cycle, 22 bred in the second cycle and eight bred in the third cycle (96% overall). When I would point out the problematic nature of failing to approximate some form of this pattern to my clients, if the overall pregnancy percentage was good, too often I would get shrugs or blank stares. Other times, I would feel both internal and external pressure to identify a pathogen or nutritional deficiency to explain the poor reproductive performance.

Now that I am able to assess this type of data from a more distant perspective, it has become clear to me that failure of a herd to load the “front end” of their calving season is a major source of lost revenue for cow-calf operations. The most common cause of poor reproductive performance is rarely a pathogen or nutritional deficiency, but more often the failure to set cows up for success early in their production cycles. The keys to setting cows up for success often come down to a couple of factors: heifer development and understanding postpartum anestrus.

Setting them up for success

Bos taurus cattle in general, and Angus cattle in particular, have an amazing genetic potential for early onset of puberty. However, whether a heifer meets her genetic potential depends upon many factors. We are constantly learning more about the effects of nutrition, disease and environmental factors on the reproductive development of heifers both in the womb and after birth. Fetal programming is a very real, though incompletely understood, phenomenon wherein heifers exposed to disturbances in the womb may alter the way their genes are expressed. Outside of the womb, there is solid evidence weaned heifers will accelerate their reproductive development when placed on a high plane of nutrition, though the timing of that nutrition matters.

One study found 100% of crossbred heifers reached puberty by 13 months of age when fed a high energy diet ad libitum between 4-6 ½ months of age, compared to 60% of heifers reaching puberty at the same time point when restricted between 4-6 ½ months then fed ad libitum thereafter, and 30% of heifers reaching puberty when fed a restricted diet [i.e., a forage-based diet targeting approximately 1 pound (lb.) gain per day] throughout the study period. 

An interesting wrinkle in this study was that one of the groups fed ad libitum between 4-6 ½ months, were then on a restricted diet between 6 ½-9 months, fed ad libitum again between 9-11 ½ months, then restricted again until 14 months. This is called a stair-step method, and aside from more efficiently utilizing feed resources to develop heifers’ reproductive potential, it has the added benefit of allowing heifers to grow into their frames and develop their udders without developing excessive body condition. Additionally, this study showed that the period immediately following weaning is a critical period for reproductive development of heifers.

Regardless, the hardest pregnancy isn’t usually the first one, it’s the second. If heifers don’t calve sufficiently early in the season relative to the rest of the cow herd, it can be difficult for them to recover in time for the subsequent breeding. 

Of all the reproductive problems I saw in practice, 3-year-olds coming up open at pregnancy check was by far the most common. Even with optimum feeding and development, most of the challenge associated with getting 3-year-olds to breed back comes down to postpartum anestrus, or the time it takes for a female to start cycling again after calving. Under optimal nutritional conditions, it takes a mature cow about 60 days on average to have her first heat after calving; whereas for a first-calf heifer, it takes an average of 80 days. There are outliers; some cows will start cycling as early as 40 days postpartum, and some heifers will start as early as 50. 

However, if the goal is to maximize the number of replacement heifers that stay in the herd, it is best for heifers to start calving about one estrous cycle ahead of the cows.

There are other factors to consider when developing heifers. As a rule, growth implants have a disruptive effect on reproductive development and should be avoided in heifers that may become replacements. Although, just because a heifer was implanted as a calf does not necessarily mean she won’t get bred. Vaccination and parasite control are also important measures for preparing heifers to enter the workforce as mama cows, especially when it comes to protecting against fetal infection with bovine viral diarrhea virus. 

Opportunities ahead

Maybe one of the most important aspects of heifer development is understanding that not all operations are set up to do it well. Providing appropriate nutrition usually requires access to feed, facilities and equipment. Pair heifers with the right bulls may require expertise and resources that aren’t feasible for all operations to invest in. If a herd’s current calving distribution is overly dispersed into the later cycles, it may also be difficult for that operation to retain sufficient numbers of heifers with a reasonable chance of becoming pregnant in time to calve early.

All of these factors indicate a unique opportunity for seedstock producers to market bred heifers as replacements. Potential buyers may balk at the initial investment, but if the heifers are set up for success, the sellers can be confident their clients may gain returns in improving calving distribution and herd genetics. Whether a producer is developing heifers to sell, or for their own herd, reproductive success will almost always hinge upon whether they are built for tomorrow.  

Editor’s note: Todd Gunderson is a clinical assistant professor, beef production medicine, at Kansas State University in Manhattan, Kan.