I don’t know when I first heard someone reference culling the 3 O’s, but I mention it a lot in extension presentations. This refers to producers considering culling cows that are open, old, and ornery. As a general rule, I can’t argue with considering these three categories of cows as culling candidates. But, I also like to mention two other categories of cows to consider when one is looking at potential cows to cull.
First, I encourage producers to look at their late calving cows. Producers that capture weaning weights will likely notice these cows as they will tend to wean smaller calves, simply due to the calves being younger at weaning time. But, the lost value is really even more significant as there usually will be fewer of these late born, lighter calves. This means that when the calves from these cows are sold, they will be sold in smaller groups and take an additional discount for that reason. The number of calves that are sold in one group (often referred to as lot size) has a huge impact on price. The combination of weaning a smaller calf and having that calf sell in a smaller group can greatly impact the revenue associated with a late calving cow.
Secondly, I like for producers to consider the size of the cow when they look at their weaning weights. Again, records are key to being able to do this, but the concept is what is most important. Most costs are going to be higher for larger cows, which means they have to wean larger calves to offset those additional costs. Sometimes comparing cows based on the weaning weight of their calves, as a percent of their body weight, can provide a bit more perspective on which cows may be candidates for culling.
Source : osu.edu