The search for the optimum beef cow is ongoing. Finding her is only somewhat less challenging than defining her. Cow efficiency has been described, researched, and discussed in many different forums, and has taken on numerous definitions. Recent discussions have focused on cow size (mature weight), and the importance and relevance of this trait on profitability and sustainability of beef production systems. Cow size is a relevant component to measures of cow biological and economic efficiency, however it is important to note that a number of other components also impact these important measures.
Biological efficiency has historically been defined as pounds of calf weaned per cow exposed, pounds of calf weaned per cow exposed per unit of cow weight, as well as pounds of calf weaned per cow exposed per unit of energy consumed. Factors affecting biological efficiency include cow maintenance, gestation, and lactation requirements, and reproductive performance, along with calf maintenance and growth requirements, and calf weight. Through assignment of the input and output costs associated with these factors, one can arrive at economic efficiency. Biological and economic efficiency, while related, are not necessarily one and the same. It is possible to have high economic efficiency and relatively low biological efficiency. As an example, cows with low biological efficiency as a result of high inputs relative to calf weaning weight may have relatively high economic efficiency when feed costs are low. Similarly, cows producing high value progeny may compensate to some extent for low biological efficiency (lower calf weights or high feed inputs). This underlines the basis that the search for the optimum cow must optimize costs of production with potential calf income. Intuitively, the goal would be modest size cows with high reproductive rates and low input costs which produce high-value calves.
Cow Size, Milk, and Growth
Energy consumption during the cow-calf portion of the production cycle represents 72% of energy utilized from conception to harvest (Ferrell and Jenkins, 1982), and 70-75% of the total energy consumed by the cow herd is used for maintenance (Ferrell and Jenkins, 1985). Research has demonstrated that high-maintenance (energy requirement per unit of body weight) cows are characterized by high milk production potential, high organ weight, and high lean body mass (low fat mass). Conversely, low-maintenance cows have low milk production potential, low organ weights, and low lean body mass/high fat mass.
Cow intake, energy and protein requirements are influenced by mature cow size. As mature cow size increases from 1000 to 1400 pounds, intake, energy, and protein requirements increase 23%, 19%, and 13%, respectively for cows 90 days post-calving. Bigger cows simply require more feed inputs, in part due to larger body mass to maintain. Similarly, cows with higher milk production have additional costs associated with protein and energy requirements. The energy status of the cow impacts reproductive performance (Short and Adams, 1988), and energy status is a function of nutrient intake and availability relative to requirements. Hence, severe restrictions in nutrient intake relative to requirements impact body condition and rebreeding success.
The widespread use of genetic tools such as EPDs have resulted in tremendous gains in performance as measured in growth and milk production. Mature size has a strong positive genetic correlation with weaning weight and yearling weight (0.80 and 0.76; Bullock et al., 1993). Therefore, genetic trends for increased growth over time also reveal a corresponding increase in mature cow size. The combination of larger mature size (maintenance) and increased production (growth and milk production) influence total energetic needs of the cow herd.
The above discussions have related to differences in cow size and levels of production as they relate to differences in energy requirements. It is important to note that these measures are not measures of cow efficiency. In fact, cow size alone is a poor indicator of biological efficiency- although it is inherently correlated with costs of production (primarily nutritional inputs). These input parameters must be put in context with outputs such as number and weight of calf weaned (or slaughter weight) to derive biological efficiency, and further combined with costs and income parameters to arrive at economic efficiency. Research has demonstrated the interaction between environment (nutritional resources) and mature size, milk, reproduction, and growth at various levels of dry matter intake (Jenkins and Farrell, 1994). The most biologically efficient cow in a restricted feed environment is smaller in mature size and lower in milk production. These advantages change as feed becomes more available, and is contrasted by the large, high-milk cow being the most efficient when feedstuffs are abundant. Therefore, matching growth and milk production to the environment (feed resources) is a key component in defining efficient cows.
Differences in cow efficiency are profoundly affected by differences in reproduction, irrespective of other factors such as feed consumption and calf weight. Efficient cows are those that produce calves regularly, those that do not will not be efficient. Successful reproduction is the constant variable defining cow efficiency, while the relative importance of other variables may change with fluctuations in production environments and prevailing market conditions (Notter, 2002).
Tools for Enhancing Efficiency
As has been described, cow efficiency is a complex, multi-trait measure that is variable depending on differences in production environment and management system. Hence, the most efficient cow is likely not the same for every enterprise. However, tools exist which can be applied to enhance cow efficiency. Some of these tools have been at our disposal for more than thirty years, while others have become available very recently.
Reproductive success is paramount to cow efficiency, however genetic improvement through direct selection for reproduction has been limited 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.
Research has demonstrated that economic efficiency is most improved in systems which exploit both individual and maternal heterosis, and the use of terminal sire crossbreeding systems is an effective way to ameliorate the potential antagonisms between increased lean growth and mature size with maternal performance (Tess and Davis, 2002). These systems which take advantage of sires selected for post-weaning performance and end product merit, mated to cows of moderate size and adapted to the production environment offer additional advantages worthy of consideration. Among these include potentially more simplified management schemes and concentration of resources for small herds (replacement females outsourced, fewer management groups, fewer specifications for sire selection, etc.).
Relatively new EPD tools are now available to allow for direct selection on traits impacting cow efficiency. Heifer Pregnancy EPDs predict 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. Selection tools directly related to cow size include Mature Daughter Weight EPDs (Angus) are also available, and can be used in multiple trait selection to influence cow size while allowing for selection pressure in other traits.
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 (Red Angus Association of America). Both of these EPDs are associated with genetic differences in cow energy requirements, and can be used to enhance efficiency.
Profitable and sustainable beef enterprises of the future are likely to successfully optimize the potential association between higher levels of production and increased costs. This may be accomplished through adherence to the low-cost producer philosophy while concurrently taking steps to add value to the calf crop. Cow size and efficiency are critical components in this endeavor. Changes in cow size and efficiency will be dependent on accurate record-keeping which will enable producers to make informed decisions for their enterprises. Determination of current efficiency measures is a necessary first step in positioning for the future.
By Scott P. Greiner, Ph.D., Extension Animal Scientist, VA Tech
Livestock Update newsletter (April 2009) - Virginia Cooperative Extension