By Dan W. Moser, Kansas State University beef cattle specialist
For many years, commercial cow/calf producers have used birth weight expected progeny differences (EPD) in sire selection to minimize calving difficulty. However, some producers have expressed concern that such selection over several generations might result in heifers and cows that have calving difficulty despite lower calf birth weights, as a result of the heifers being smaller at calving.
If calving ability of females is a concern, there's a simple but underutilized tool to avoid the potential problem you describe. It's Maternal Calving Ease EPD. MCE EPD (CEM in some breeds) describes the genetics of female calving ability, so if a sire did truly produce harder calving heifers and cows, that number would reflect it. Higher numbers mean easier calving daughters, so to minimize calving difficulty in a herd that raises its own replacements, pick bulls with higher EPD for both Calving Ease and Maternal Calving Ease.
It seems that fewer buyers of British breed bulls pay as much attention to MCE, but if they are planning on keeping daughters of a bull, it's nearly as important as direct calving ease. It's more widely used in the continental breeds, especially Simmental and Gelbvieh, probably because those breeds have provided Calving Ease and Maternal Calving Ease EPD for many years.
Recognize that birth weight is one of the more heritable traits in beef cattle. That means that the heifers with the heavier birth weights are more likely to have heavier calves themselves. When we calculate birth weight EPDs in seedstock, the calf's adjusted birth weight has quite a bit of impact because of this. So, to avoid future calving difficulty in a commercial herd, it would be wise to cull heifers born with difficulty, or with high birth weights if that information is available. It is especially important to cull big heifer calves that are born to first-calf heifers.
First-calf heifers tend to have lighter calves because of uterine environment and other age-related environmental factors. Some studies estimate that the same calf might be seven pounds heavier if born to a mature cow instead of a first-calf heifer, all else being equal. So, if she was big and out of a first-calf heifer, she would have been even bigger if she had been born to a mature cow, and probably has the genetics that reflect that.
Keep in mind, sire selection drives a vast majority of genetic change, not heifer selection or cow culling. This is especially true for traits like calving ease where powerful genetic evaluation information is available on the bulls you buy. While you might make a bad choice now and then, you won't hurt the cowherd selecting replacement heifers, if you are picking the right bulls to sire those heifers. You won't make great progress selecting heifers or culling cows either. There's much more variation among the bulls you can buy than there is among the heifers you keep or cull, and more information available to help you make the right choice.