Saturday General Session
Biological Basis for Variation in Energetic Efficiency of Beef
Abstract:
Rising feed, fuel and fertilizer input costs prompted by growing global demand for animal protein and the implementation of biofuel policies have renewed interest in selection programs to improve the conversion of feed energy into marketable product. Considerable genetic variation for feed efficiency, both among and within beef cattle populations, exists, but the expense of measuring feed intake and lack of an appropriate trait for use in selection programs has limited genetic progress in feed efficiency.
Residual feed intake (RFI) is a feed efficiency trait that quantifies inter-animal variation in feed intake beyond that expected to meet energy requirements for maintenance and production — efficient animals are those that eat less than expected for a given body weight and level of production. Unlike ratio-based efficiency traits (e.g., feed:gain) that are influenced by growth and maturity patterns, RFI is phenotypically independent of the production traits used to compute expected intake, so favorable selection for RFI will improve feed efficiency with minimal affects on growth or mature size. Consequently, RFI better reflects inherent variation in metabolic processes related to energetic efficiency to support the discovery of biologically relevant mechanisms associated with inter-animal differences in feed efficiency.
Residual feed intake has been linked to heat production, methane production, composition of gain and digestibility, suggesting that more efficient RFI phenotypes are leaner, have lower maintenance energy requirements and methane emissions, and improved diet digestibility.
Herd et al. (2004) estimated that approximately one-third of the biological variation in RFI of growing calves could be explained by inter-animal differences in digestion, heat increment of feeding, composition of gain and activity, and that the remaining two-thirds was likely associated with cellular processes like ion pumping, protein turnover and mitochondrial function. Mitochondria are the “energy powerhouse” of cells responsible for capturing greater than 90% of the energy from nutrients in the form of adenosine triphosphate (ATP). Thus, differences in energetic efficiency of cattle with divergent RFI phenotypes are likely associated with variation in the generation of mitochondrial ATP or cellular utilization of ATP.
Although not surprising, it is readily apparent that numerous biological processes are responsible for genetic variation in RFI. While our understanding of RFI in growing cattle has advanced, we have limited knowledge regarding the associations between RFI measured during the early postweaning phase and the biological efficiency of mature cows. Moreover, little is known about the impact of selection for RFI on other economically relevant traits (e.g., reproduction, meat quality), or the effects of diet quality or stage of growth on genetic rank for RFI.
A more complete understanding the biological mechanisms responsible for inter-animal variation in RFI will provide insight into anticipated genotype-by-environment interactions associated with feed efficiency, and will drive the search for novel biomarkers and DNA variants associated RFI to facilitate early, cost-effective and more accurate identification of cattle with favorable phenotypes for feed efficiency.
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About the speaker:
Gordon Carstens is associate professor of animal nutrition in the Department of Animal Science at Texas A&M University. After receiving a bachelor’s degree in animal science from Iowa State University, Carstens worked for a major pharmaceutical company prior to pursuing his advanced degrees. He then completed his master’s degree and doctorate at Colorado State University.
In addition to teaching animal nutrition courses, Carstens conducts research on energy metabolism and growth and development in ruminants. Specific research areas include the regulation of growth and composition of carcass and mammary tissues by nutritional control and the use of externally administered (exogenous) growth regulators. Recent research has focused on methods to increase the ability of newborn calves to produce heat and fight off cold stress, and the influence of genetic and nutritional components on this ability.
Professional memberships include the American Society of Animal Science, the American Dairy Science Association, Gamma Sigma Delta and Sigma Xi.
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