The Optimum Cow

Livestock Update, October 2008

Scott P. Greiner, Ph.D., Extension Beef Specialist, VA Tech

The Optimum Cow- defining, identifying and propagating her has proven to be a challenge over time. Perhaps part of our challenge has been that she can be a moving target, as the pressure points affecting revenue and input costs change as a result of many factors influencing our industry.  As a an example, escalating costs of inputs (feed, fuel, fertilizer) have brought our search for the optimum cow to the forefront and focused much attention on issues such as mature size, and maintenance costs.

So how do we define the optimum cow?  Many measures have been utilized and debated.  Measures of cow efficiency are commonly expressed as pounds of calf weaned per cow exposed, or pounds of calf (or carcass weight) per unit of energy consumed by the cow and progeny.  Common attributes of optimum cows identified through various measures include the following:

• Reach puberty early, calve at 2 years of age, and then annually thereafter with minimal calving difficulty
  
  
• Wean a calf annually which fits demands of marketplace and meets consumer expectations
  
  
• Highly adapted to environmental and managerial resources
  
  
• Optimizes revenue vs. costs over a long, productive life

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.

Maternal Heterosis:

Maternal heterosis has proven to be our best tool to genetically improve and maintain reproductive efficiency.

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 most recent evaluation of beef breeds conducted at the U.S. Meat Animal Research Center (Cycle VII Germplasm Evaluation Program) revealed no significant differences in reproductive rate and calf survival among females sired by Angus, Hereford, Charolais, Gelbvieh, or Simmental bulls.  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.

Maintenance energy costs represent approximately 70% of the total energy costs in beef production.  The majority of this energy cost is associated with the cow herd.  Mature size is an economically relevant trait from several aspects, including its association with nutritional requirements.  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 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.  Heifer Pregnancy EPDs 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.  The Stayability EPD 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.

Simultaneously optimumizing growth, maternal ability, and end product merit are also paramount.  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 (not maximizing) 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.  Mature Daughter Weight EPDs are a tool that can be used in conjunction with growth EPDs to keep cow size in check while allowing for genetic progress in weaning and yearling weight. 

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.

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