When it comes to the reproductive success of beef herds, the general recommendations are to maintain the cow herd at a body condition score of 5 to 6 by meeting the nutritional requirements.
For developing heifers, the recommendation is to develop heifers to 65% of mature body size at the time of breeding. With reproductive success estimated to be three to nine times more influential on operational profitability compared to other production traits, it makes sense that we would provide the best opportunities to maximize the number of cows we are able to get pregnant.
Reproductive success is measured by a female’s ability to reach critical production events, such as puberty onset as a yearling, first calf at 2 years of age, limited anestrus following calving, conception and maintaining pregnancy to calving. If all this can be achieved, it manifests in an increased number of calves weaned annually over the cow’s lifetime.
The reproductive failure of a beef cow is the most prominent reason for culling from the herd, as the retention of open females creates a financial drain on an operation. A beef cow must remain in the herd until an age where she will produce enough calves to offset the cost of her development and maintenance, as well as recoup the cost of other females in the herd who were culled prior to reaching this break-even point. Traditionally, this break-even point has been defined as a cow that remains in the herd until 6 years of age, but as the culling rate increases for a herd, so does the break-even age. Cows culled before reaching a break-even age result in a financial loss for an operation and jeopardize economic sustainability. Increasing the time a cow remains in the herd would decrease annual production costs associated with replacement heifers, increase the number of mature cows producing heavier calves and decrease the number of open cows to cull. Increasing the efficiency of a cow herd requires improvement of fertility in both cows and young heifers at both the management and genetic levels.
Genetic progress for fertility traits such as heifer pregnancy and cow longevity has been slow. The rate at which we can make genetic change depends on several factors, such as the heritability of the trait, how accurate our tools of selection are, our selection intensity (how choosy we are when selecting replacements) and the generation interval. A generation interval is defined as the average age of parents when offspring are born. Compared to other livestock species, beef cattle have a longer generation interval of about 5 years. Increasing our reproductive efficiency would result in cows remaining in the herd to an older age. As a result, the older the average age of our cow herd, the longer the generation interval and the slower the rate of genetic change.
Heritability is how much of the variation of a specific trait is due to genetic factors. The higher the heritability, the more genetic factors contribute to the trait and the faster the rate of genetic change can be made. For reproductive traits, these heritabilities tend to be low, indicating a sizable environmental influence on the expression of the trait. When it comes to reproduction, it isn’t surprising that the management or environment would greatly influence a cow’s ability to get pregnant and remain in the herd, but it also means a slow rate of genetic change.
Given the large economic influence for cows to maintain reproductive efficiency, the feeding of cows and heifers to target body condition or weight makes sense, but this practice may mask reproductive failures that would have otherwise manifested by the expression of females who are genetically inferior for fertility. This also creates a challenge for selection, especially since it removes the ability to increase the selection intensity for these traits. Reducing selection intensity and how choosy you are for selecting animals will slow genetic progress.
The challenges for making genetic progress for these traits do not necessarily suggest that genetic influence on the traits is less economically important than management inputs, or vice versa. The important thing to consider is that genetic selection for improvement in reproductive efficiency is attainable. The heritability for these traits may be low, but it is not zero. The rate of improvement for these traits would be slower compared to traits with higher heritabilities, such as mature weight and marbling, but given the economic influence of these reproductive traits, they should require more focus for genetic improvement.
Expected progeny differences (EPDs) for reproductive traits are published by several breed associations and are our best tool for making genetic improvement on traits such as cow longevity and heifer pregnancy. In economic indexes where the retention of replacement females is considered, reproductive traits have a large influence on the index. Genomic testing has increased our accuracy for EPDs. A challenge for measuring reproductive efficiency is that it is measured later in an animal’s life. Genomic testing provides information about the genetic potential of an animal at a younger age, and when incorporated with EPDs, it becomes a very powerful tool for genetic improvement.
