Many Oklahoma ranchers choose to breed the replacement heifers about a month ahead of the mature cows in the herd. In addition, they like to use a shortened 45 to 60-day breeding season for the replacement heifers. The next logical step is to determine which of these heifers failed to conceive in their first breeding season.
Therefore now would be an ideal time to call and make arrangements with your local large animal veterinarian to have those heifers evaluated for pregnancy. By two months after the breeding season ends, experienced palpaters should have no difficulty identifying which heifers are pregnant and which heifers are not pregnant (open). Those heifers that are determined to be "open" after this breeding season, should be strong candidates for culling. Culling these heifers immediately after pregnancy checking serves three very useful purposes.
1) Identifying and culling open heifers early will remove sub-fertile females from the herd. Lifetime cow studies from Montana indicated that properly developed heifers that were exposed to fertile bulls, but DID NOT become pregnant were often sub-fertile compared to the heifers that did conceive. In fact, when the heifers that failed to breed in the first breeding season were followed throughout their lifetimes, they averaged a 55% yearly calf crop. Despite the fact that reproduction is not a highly heritable trait, it also makes sense to remove this genetic material from the herd so as to not proliferate females that are difficult to get bred.
2) Culling open heifers early will reduce winter costs. If the rancher waits until next spring to find out which heifers do not calve, the winter feed expense will still be lost and there will be no calf to eventually help pay the bills. This is money that can better be spent in properly feeding cows that are pregnant and will be producing a salable product the following fall.
3) Identifying the open heifers shortly after (60 days) the breeding season is over will allow for marketing the heifers while still young enough to go to a feedlot and be fed for the choice beef market. The USDA grading system has an impact on the merchandising of culled replacement heifers. "B" maturity carcasses (those estimated to be 30 months of age or older) are very unlikely to be graded choice. As a result, heifers that are close to two years of age will suffer a price discount. Therefore, it is imperative to send heifers to the feedlot while they are young enough to be fed for 4 to 5 months and not be near the "B" maturity age group. Auction barn order buyers will be especially leery of heifers that may be near 20 to 24 months of age, because of the risk of "B" maturity beef that receives a considerable discount when harvested at the packing plant.
Certainly the percentage of open heifers will vary from ranch to ranch. Do not be surprised, if after a good heifer development program and adequate breeding season, that you find that 10% of the heifers still are not bred. These are the very heifers that you want to identify early and remove from the herd. It just makes good economic business sense to identify and cull non-pregnant replacement heifers as soon as possible.
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist