Residual Feed Intake: Animal Performance, Carcass Quality and Body Composition

Feed efficiency has a major influence on the unit cost of beef cattle production. Gibb and McAllister (1999) have reported that a 5% improvement in feed efficiency has an economic impact four times greater than a 5% improvement in average daily gain. In addition, the cost of feed is second only to fixed costs in importance to the profitability of commercial beef operations (Basarab, 1999). The relative importance of the cost of feeding in beef operations is due to the fact that 70-75% of the total dietary energy cost in beef production is used for maintenance (Ferrell and Jenkins 1985; NRC 1996). However, genetic variation in maintenance energy requirement of cattle is moderately heritable (h = 0.22-0.71), suggesting an opportunity to select for more efficient cattle (Carstens et al. 1989; Bishop 1992). Selection for lower maintenance requirements is difficult and measures of feed efficiency such as Kleiber ratio and feed conversion efficiency are related to measures of body size, growth rate, composition of gain and appetite (Arthur et al. 2001a). A new concept called residual feed intake (RFI) is a feed efficiency trait that has been found to be independent of body weight and weight gain (Koch et al. 1963), and is defined as the difference between an animal’s actual feed intake and its expected feed requirements for maintenance and growth. The trait is moderately heritable (h = 0.29-0.46), implying that improvements could be made in feed efficiency without affecting body size, and thus maintenance requirements or growth rate (Archer et al. 1998; Arthur et al. 2001a). However, differences in efficiencies of growth may also be due to differences in composition of live weight gain (Pullar and Webster 1977; Ferrell and Jenkins 1998). For example, differences in rates of water and protein accretion have an influence on rate and efficiency of body weight gain, primarily because of the lower energy content of water and protein relative to fat (Ferrell and Jenkins 1998). Conversely, higher maintenance costs are frequently associated with greater visceral organ weights and increased feed intake (Ferrell and Jenkins 1998). In addition, higher maintenance costs are more associated with body protein than with body fat (Pullar and Webster, 1977). Thus, the negative consequences of selecting for RFI are uncertain, although selection for RFI has been associated with reduced carcass