Growth and development symposium: the molecular basis for feed efficiency.

Recent advances in our understanding of feed efficiency in livestock have moved the focus of much of the scientific community, possessing an interest in this important aspect of animal production, to a trait known as residual feed intake (RFI). This approach to understanding feed efficiency was first proposed as having potential for use in beef cattle by Koch et al. (1963), but is now being extended to swine, dairy, and poultry genetic selection programs. The RFI approach partitions the drivers of feed intake into 2 components: the feed intake expected for a given level of performance and a residual component. There are several reasons that interest in use of RFI is increasing: 1) there is considerable variability in RFI that can be exploited; 2) it is moderately heritable; and 3) it appears to be largely independent of other production traits and, thus, can be incorporated into a selection index aimed at simultaneously improving multiple traits. However, RFI is expensive to measure because it requires careful measurement of individual animal intake and BW gain. Nonetheless, interest from animal scientists and the livestock industries is driving this approach to improving feed efficiency into prominence. The high cost of accumulating RFI data and developing well-defined populations of animals is also driving an interest in understanding the molecular basis for feed efficiency. Scientists believe that developing technological approaches to characterize animals divergent for RFI will accelerate progress and ultimately reduce the cost of identifying superior animals. Thus, a symposium aimed at providing an overview of current knowledge of the molecular basis for variation in feed efficiency in beef cattle, poultry, and swine was convened at the annual meeting of the American Society of Animal Science on July 11, 2008 in Indianapolis, IN. Because variation in RFI is extremely complex and driven by multiple (probably hundreds of) genes, scientists are attempting to partition the processes that contribute to RFI, and it appears that more than 35% of the total variation is associated with efficiency of metabolic processes (Richardson and Herd, 2004). As energy (i.e., ATP) generation in mitochondria is a large contributor to these processes, mitochondrial function is a likely target for scientific investigation. All 5 of the papers presented at this symposium (Bottje and Carstens, 2008; de Lange and vanderVoort, 2008; Herd and Arthur, 2008; McDonald and Nielsen, 2008; Moore et al., 2008) addressed this area as having potential for improving our understanding of the molecular basis of feed efficiency. Two of these addressed this topic as a major focus (Bottje and Carstens, 2008; McDonald and Nielsen, 2008). Although there are likely to be important species differences in the molecular and genetic basis for variation in RFI, at a biological process level, the magnitude of response of these variables appears to be somewhat consistent across species. For example, in beef cattle, poultry, swine, and mice, mitochondrial-related variance and behavioral variance appear to follow similar and perhaps expected patterns. Moore et al. (2008) reviewed the potential for gene markers in beef cattle in the forms of SNP and QTL to add to our understanding of the molecular basis of variation in RFI. This is the technology of the future and has huge potential to rapidly uncover specific genes that are the drivers of this variation. Although this potential is being realized, the authors point out that there are at least 2 main barriers to progress: understanding of the genetic interactions of genes that are seemingly unrelated to feed efficiency and the limits imposed by small numbers of animals that are wellcharacterized for RFI. Bottje and Carstens (2008) provide a review of the mitochondrial-related processes that contribute to the variation in RFI in beef cattle and poultry, recapitulating the notion that at the biological process level, there Growth and Development Symposium: The molecular basis for feed efficiency