Genetic Implications for Beef Heifers
Livestock Update, October 2007
Dr. Scott P. Greiner Extension Beef Specialist, Virginia Tech
Genetics play a vital role in the successful development, reproductive performance, and future productivity of beef heifers. As the factory of the beef enterprise, profitable females have the following attributes:
Genetics play an important role in all of these factors. Therefore, the application of basic genetic tools that allows for balancing the large number of relevant traits is key. Not only do genetics provide the foundation for performance, the interaction of genotype with management, nutrition, and the environment have a profound impact on successful heifer development as well as cowherd profitability.
Genetics and Reproduction
Reproductive efficiency is the single most economically important trait for cow-calf producers. Unfortunately, limited tools are at our disposal to enhance reproduction through genetic selection due to the low heritability of reproductive traits and associated complexities involved in calculating EPDs. Capturing heterosis through the use of well-planned, structured crossbreeding programs provides the best genetic tool for enhancing reproduction. Maternal heterosis realized through the crossbred cow results in improvements in cow fertility, calf livability, calf weaning weight, and cow longevity. Collectively, these improvements result in a significant advantage in pounds of calf weaned per cow exposed, and superior lifetime production for crossbred females (see table).
With the lack of genetic predictors (EPDs) available to select directly for reproduction, heterosis is our best tool to genetically improve and maintain reproductive efficiency. Some level of heterosis is important, although maximum heterosis is not necessary. The use of artificial insemination or composite bulls are mechanisms to maintain heterosis while maintaining a sustainable crossbreeding plan.
Genetics contribute significantly to several traits which impact reproductive performance. Mature size and milk production both influence reproductive efficiency and are manifested through interactions with nutrition and the environment. Mature size and milk impact nutritional requirements, and therefore must be kept in balance with available feed resources to allow for optimum reproductive performance. The following table characterizes maternal performance of females sired by several breeds (Cycle VII Germplasm Evaluation Program, U.S. Meat Animal Research Center, Clay Center, NE).
This research reveals several important points. First, there are no significant differences in reproductive rate and calf survival among females sired by these breeds. Calving ease and birth weight were also similar among females sired by these breeds. Differences were noted among breeds for growth and milk production, although British and Continental breeds are more similar today than they were 30 years ago. Perhaps one of the most revealing findings is the fact that female mature size is essentially similar for all breeds, with the exception of Gelbvieh which are lighter than other breeds. Consequently, these breeds can be used in a complimentary fashion in crossbreeding programs without large swings in traits such as mature size and milk production based on breed of sire. Substantial variation within any breeds exists, and needs to be managed through proper use of EPDs for calving ease, milk, growth, mature size, and other traits.
Mature size is an economically relevant trait from several aspects. Mature size is measured in weight and/or height (frame score), and these two measures are highly correlated (genetic correlation = 0.86) (Bullock et al., 1993). Mature cow size influences nutritional requirements- at the same stage of production (90 days post-calving) and moderate milk production, 1200 pound cows (frame score ~ 5-6) have a 10% higher energy requirement and 7% higher protein requirement than 1000 pound cows (frame score ~ 4). As cow size increases to 1400 pounds (frame score ~ 7), energy and protein requirements increase 19% and 13%, respectively, compared to 1000 pound cows (NRC, 1996). These differences are due in large part to higher maintenance requirements of larger cows, as they simply have more body mass to maintain. Increased nutritional requirements result in higher cow carrying costs throughout the production cycle. Similarly, mature cow size impacts stocking rates and supplemental feed resource needs. Mismatches between cow size and nutritional resources may compromise reproductive efficiency.
Mature cow size also influences the uniformity of the calf crop produced- particularly in single sire herds. Variation in mature cow size frequently results in large differences in feeder cattle grade. This variation in frame size is contributes to differences in finished weight/carcass weight (when fed to a constant body composition) as well as quality and yield grades (with a constant time on feed) in the finishing phase. Long calving seasons that result in large differences in calf age also contribute to lack of uniformity.Mature size has a strong positive genetic correlation with birth weight (.64), weaning weight (.80), and yearling weight (.76) (Bullock et al., 1993). These relationships would suggest that selection for growth will result in a corresponding increase in mature cow size. Therefore, selection for extremes in growth traits can be detrimental. At the same time, small mature size- that may be advantageous in terms of costs of production, is associated with reduced growth. Therefore, optimization of growth and mature size within the boundaries established by production system and feed resources is key.
Substantial hurdles exist in the quest to genetically design the beef female which is reproductively efficient, highly adapted to the environment and low-cost, produces a profitable calf with carcass and consumer acceptance, and does so with longevity. Capturing heterosis and breed complementarity through systematic crossbreeding serve as the foundation for accomplishing these goals. Proper application of existing and new selection tools (EPDs) within breeds are also key.
As noted previously, there are few tools available to directly select for reproduction although efforts are ongoing to make these tools available. The American Red Angus Association publishes a Heifer Pregnancy EPD which predicts the likelihood of a bull’s daughters to conceive to calve as two-year olds. This EPD could be used to exert genetic selection pressure on fertility. Similarly, Red Angus also publishes a Stayability EPD which predicts the likelihood of a sire’s daughters remaining in the herd until six years of age (longevity). Since a large proportion of cows leave the herd as a result of reproductive failure, the Stayability EPD indirectly identifies favorable reproduction genetics. Several breed associations are in the developmental phases for similar genetic prediction tools which may be available in the near future to utilize for direct enhancement of reproductive efficiency.
Optimum growth, maternal ability, and end product merit are also paramount in the genetic design of heifers. These traits are directly related to revenue generated through progeny in the commercial sector. Genetic selection tools in the form of EPDs are widely available for these economically important traits and known to be effective. Selection strategies should be focused on defining optimum EPD values that are compatible with management and nutritional resources. The unfavorable relationship between growth and mature size, and the potential consequences associated with increased maintenance and feed costs and potential reduced reproductive efficiency underscore the importance of optimizing growth. Similarly, milk production must be matched to feed resources to avoid complications with reduced body condition and lower conception rates if nutritional resources are not met. Frame scores are an objective tool which can be utilized to control mature size. Mature Daughter Weight EPDs are also available, and can be used in conjunction with growth EPDs to keep cow size in check while allowing for genetic progress in weaning and yearling weight. In a similar fashion, maternal calving ease EPDs should be included as selection criteria for making replacements.
The beef industry also has recently introduced selection tools to enhance our capability to identify genetics which are favorable for reducing costs of production. Two examples include the Cow Energy Value EPD ($EN, American Angus Association) and Maintenance Energy EPD (Amercian Red Angus Association). Both of these EPDs are associated with genetic differences in cow energy requirements, and can be used to enhance efficiency.
The final consideration involves carcass traits. The economic relevance of carcass merit needs to be evaluated, and will be dependent upon individual marketing systems. To date, there has been no evidence that selection for marbling is detrimental to traits associated with cow productivity and therefore improvements in quality grade can be made in concert with other traits described above. However, enhancements in yield grade may be best accomplished through breed complementarity (blend of British and Continental genetics). Due to the antagonisms between reproduction and reduced fatness, selection for extremes in cutability may be detrimental to cow productivity.
Fortunately, for many of the antagonistic relationships that have been discussed (growth and calving ease, growth and mature size), the associations are relatively small. Genetic correlations that are small enhance the likelihood that animals exist in the population that have a desirable combination of genes for these traits. This underscores the value of using predictable, proven genetics through AI in breeding programs.
There are a number of relevant traits for which we do not have objective genetic tools for selection. Examples include udder quality, feet soundness, fescue adaptability, hair coat, and fleshing ability. All of these traits arguably are related to cow productivity and profitability within given environments and therefore warrant attention in selection strategies.
Genetics have a pronounced impact on a number of cow traits, and genetics directly influence profitability through relationships with both revenue (production) and costs (management and nutritional requirements). Since reproductive efficiency is the single most economically important trait related to beef production, advantages in reproductive efficiency attained through heterosis need to be captured. Concurrently, traits such as mature size and milk production need to be matched to the environment. Selection of individual sires through EPDs allows for the design of genetically superior replacement females for economically important traits. In short, crossbred females out of genetically superior sires create the opportunity for a productive and profitable herd.
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