Several conversations with cattle breeders over the past few weeks while attending bull sales led to addressing the topic of selection for extremes; particularly, as it relates to calving ease.
Selection for extremes refers to choosing animals that have the maximum or minimum genetic values for a specific trait. While this approach can lead to rapid genetic progress in a targeted area, if taken too far, it can lead to biological extremes and economic risks due to genetic antagonisms, environmental sensitivity and nature’s resistance to biological extremes.
Throughout the recorded history of beef cattle breeding, selection for extremes usually results in a needed “correction” in breeding programs to improve cattle resilience and profit potential. A few examples follow.
The selection for extremely “compressed,” early-maturing cattle from the 1940s resulted in the 1960s discovery of dwarfism and carcasses that didn’t fit the industry standards for red meat yield once grain finishing became the norm. In the 1970-80s, intense selection pressure was applied to increasing frame size and growth which resulted in excessively tall, late-maturing cattle that came at a cost in the form of maintenance requirements and fleshing ability. Selection for extreme milk levels leads to increased maintenance requirements and excessive rebreeding intervals in the cow herd.
This leads to the question: Does selection for extreme calving ease come with a downside? In short, yes, it can. Selection for extremely low birth weights does reduce calving difficulty. Those smaller calves can also have less vigor, higher mortality and reduced growth potential. In short, most studies show the highest survival rate of newborn calves is at intermediate birth weights. Calves that are extremely small, or extremely large, when born are less likely to survive. Birth weight, like most traits, has an “intermediate optimum.”
Calving ease is best managed (and selected for) as a threshold trait. Once we reach the desired threshold, in this case, a live, vigorous calf born unassisted, we have achieved the goal. No one wants to assist heifers during calving season. That being said, abnormal presentations (a non-genetic, random event) occur about 5% of the time. Accordingly, monitoring heifers during calving season is a prudent management practice.
EPDs for Calving Ease Direct and Birth Weight are important selection tools to consider when selecting bulls to use on virgin heifers. Since EPDs within a breed are comparable across time and geography, past production records and reviewing the EPDs of the sires of past calf crops can serve as an excellent guide to how to apply selection pressure for calving ease in future bull purchases. — Mark Z. Johnson, Oklahoma State University Extension beef cattle breeding specialist
