For a good many years, it was assumed feed efficiency was positively related to feed consumption. The animal that consumed the most, in relation to its body weight, gained more and did it on less feed/lb. of gain.
The theory was that the more an animal ate, the more that was left for production, after taking care of body maintenance. Also, there was thought to be little difference in efficiency of utilizing feed above maintenance.
In recent years, those theories have been challenged with the concept of residual feed intake (RFI). RFI is the difference between actual feed intake and what would be expected based on the animal's body weight and growth rate over a specified period.
Positive RFI animals eat more than expected in relation to their weight and gain, so they are less efficient.
Researchers from the University of Alberta-Edmonton, and the Lancombe Research Center in Alberta, recently reported on relationships between RFI and other factors. High, medium, and low RFI steers were selected from a group of 306 that had just completed a feedlot test. The average body weight for the RFI groups were 495.6 kg for high RFI; 529.1 kg for medium RFI; and 501.2 kg for low RFI.
The selected steers were then used in digestibility and calorimetry trials, using corn- or barley-based concentrate diet in years 1 and 2, respectively. During the trial, researchers found feed intake was not significantly different among the three RFI levels.
Residual feed intake was correlated with daily methane production and energy lost as methane. Also, the low RFI steers produced significantly less methane — 28 and 24% — compared to the medium or high RFI steers, respectively. Interestingly, low RFI steers made significantly fewer trips to the feed bunk and spent less time eating than medium RFI steers, which made significantly fewer trips and spent less time than high RFI steers.
Residual feed intake tended to be associated with the digestibility of dry matter (DM) and crude protein (CP). Researchers found that digestibility of DM and of CP was significantly greater for the low RFI steers compared to the high RFI steers.
They concluded these results have practical implications for the selection of animals who eat less at a similar BW and growth rate (J.D. Nkrumah, et al. J. Animal Sci. 2006. 84:145-153).
— January 2006 Texas A&M Beef Cattle Browsing newsletter