In general, the more fertile females in the cow herd will conceive during the first heat cycle of the breeding season and thus, calve during the first 21 days of the calving season. As a result, the calves born in the first 21 days of calving season are generally healthier and heavier at weaning. Selection for replacements born during this time frame results in naturally selecting for more fertile females and increasing the likelihood of herd longevity.

Calf weaning weight should be directly correlated to mature cow size. A good thumb rule is at weaning a calf should weigh approximately 40 percent of the dam’s weight. However, continual selection for higher weaning weights also leads to larger mature size over time. While that heaviest heifer may be a favorite in the pen, she may not be the best candidate to maintain in the herd depending on the operation’s goals.

Don’t forget about pelvic area measurements and reproduction tract scoring 30 to 45 days prior to the initiation of breeding. Often, these measurements are utilized to identify potential problems such as odd-shaped or small pelvises or females that are slow to reach puberty and allows for marketing of these females sooner rather than later.

Keep in mind that pelvic area is a highly heritable trait, so if you have constant dystocia issues with a mature cow, chances are you will most likely have issues with her daughter too. Likewise, udder and teat conformation are moderately heritable traits, so remember which calves you had to help nurse last calving season. Heifers of those dams probably shouldn’t be the first of the list for the replacement pen. However, proper bull selection can aid in improving udder quality in one generation.

Typical replacement rates within a cow herd range from 15 to 20 percent each year. Producers should consider developing additional heifers (10 percent is suggested) to account for females that won’t make the cut. Evaluating replacement rate within your own herd is a useful tool to understanding how successful your heifer development program is. For assistance optimizing your heifer selection and development program, consult with your extension beef specialist, veterinarian and nutritionist.  

• Erika Lundy

• Extension Beef Program Specialist
• Iowa Beef Center - Iowa State University
• Email Erika Lundy