Heifer selection (Proceedings)
The heifer selection program is a vital aspect of a commercial cow-calf operation. Since replacement heifers represent the future potential of the herd, successful replacement programs are a high priority item requiring careful attention by the ranch manager. Unfortunately, for many producers, the heifer selection process is reduced to a ten second decision at weaning before the calf is released from the squeeze chute. The beef cattle veterinarian can be an integral part of the production team by focusing management on crucial areas.
A successful heifer selection programs begin after the producer has specific written goals in place for the operation. For example, if producing calves that excel on a carcass grid is a primary goal, then a terminal cross program may be optimal. With this objective, a producer would have to derive replacement females from outside sources. If maintaining a purebred operation is important, then crossbreeding for hybrid vigor would not be an option. Helping your clients define achievable production goals is the first step in the replacement heifer selection process. Once the operation has specific objectives in place, then the current herd can be evaluated and a selection program implemented to help attain the goals.
The replacement heifer program should select for females that have desirable genetic traits, meet phenotypic specifications and will be profitable for the operation. Proper attention in these areas will assure a good replacement heifer program.Genetic selection
The dairy cow's very defined purpose has enabled the dairy industry to experience rapid advances within their genetic base. The dairy producer selects almost exclusively for milk production, a trait that has both high heritability and reflects the purpose of the dairy cow. In contrast, genetic selection in beef cattle is primarily based on performance of the calf. While these traits do have good heritability, they are not as well defined as milk production because there is often no clear picture of what the final product should be (weaning weight, yearling weight, rib-eye area, marbling, etc.). The confounding problem is that calf performance is actually not the purpose of the beef cow. The beef cow should be selected to produce and wean calves. Relevant traits include fertility, longevity, mothering ability and health. These traits are best achieved through heterosis with a planned crossbreeding program.
Many producers limit genetic selection for maternal traits to calving ease (CE) and milking ability, which are really not maternal traits. Caving ease is primarily related to birth-weight, which is actually a growth trait. By selecting for calving ease, long-term production is lowered because we are actively selecting against growth. Milking ability, similar to dairy cattle, is a growth trait. Its purpose in beef cattle is to increase weaning weight of calves, which is measurement of growth or calf performance. Too much emphasis on increased milk production may result in negative affects on calves and cows. For the lactating female, milk production is an obligatory drain on the system that may lead to poorer reproductive performance due to a high-energy demand and resulting deficit in body condition. In general, most females on range operations are limited in milk production from a nutritional base rather than their genetic potential.
One trait that can be selected for and result in positive returns is puberty. Early puberty females are more likely to conceive in the first part of the breeding season, therefore calving at the onset of the calving season. These heifers will then have a longer postpartum period before the breeding season and will be more likely to continue to settle early throughout their reproductive tenure. Age at puberty is positively correlated with pregnancy percentage and milk production and also enjoys a slightly positive correlation with growth traits. Presently, this trait is probably the best proxy for fertility. A veterinarian that understands the reproductive and production consequences of genetic traits can help clients focus their genetic program.