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* Department of Animal Science, Colorado State University, Ft. Collins, CO 80523 and
and
USDA, ARS, U.S. Meat Animal Research Center, Clay Center, NE 68933
1 Correspondence: 209 Animal Science (phone: 970-491-7833; fax: 970-491-5326; E-mail: djohnson{at}agsci.colostate.edu).
The development of energetic efficiency concepts followed a recognized pattern of knowledge evolution that began with novel insights leading to creative new concepts. The second phase integrated concepts from other fields to create new applicable principles. The third phase was the adoptive or dissemination phase, yielding solutions to industry or societal problems. It is our contention that animal energetics has been in the adoptive phase for approximately 100 yr. Concepts developed during the early phase of nutritional energetics included the concept that life is a combustion process, the laws of thermodynamics, and the Law of Hess. Subsequent efforts established relationships between gas exchange and heat production and established the concept that food not only functions as fuel, but also as a body-building material. Much of the research effort for the last 100 yr has been to 1) devise bases for evaluation of foods that could be related to energy requirements and energy expenditures and 2) establish causes of energy expenditures. Much of the effort has focused on general and broadly applicable processes (e.g., mice to elephants) of biology or broad-based populations within species. Little effort has been focused on the amount or causes of individual variation in efficiency of energy utilization by cattle, even though differences among individuals have long been recognized. Observed maintenance requirements and energetic efficiencies, for example, have not been substantially altered during the last 100 yr of intensive beef production. Reasons for the lack of change in energetic efficiencies include a lack of a consistent selection goal, loose and inconsistent definitions of efficiency, concentration on output characteristics, and emphasis on population similarities rather than individual variation. It is time to assess new or different tools and concepts to enhance efficiency of dietary energy use by beef cattle. Application of older (e.g., residual feed intake) and newer (e.g., QTL, gene expression microarray) technologies offers the potential to realize improved maintenance and system energetic efficiency through identification of individual animal phenotypic and genomic uniqueness.
Key Words: Beef Cattle • Energy Metabolism • Genome Analysis • Maintenance • Ruminants
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