An alternative measure of feed efficiency is residual feed intake (RFI) or net feed efficiency. This is defined as the difference between an animals actual intake and its predicted intake, calculated from its weight and rate of gain, with negative or lower values desirable. It is independent of growth and body size. Efficient animals eat less than expected and have a negative or low RFI value, whereas inefficient animals eat more than expected and have a positive or high RFI value. In other words, for the growing animal, RFI is the variation in intake that remains after accounting for the requirements for maintenance and growth. What makes up the variation in the remaining or residual portion is not fully understood. Estimates of the contributions of physiological mechanisms identified so far for animals differing in RFI are, animal body composition (5%), feeding patterns (2%), protein turnover, tissue metabolism and stress (37%), heat increment of fermentation (9%), digestibility (10%), activity (10%) and other (27%). However, these estimates are based on very few studies some of which have small animal numbers. 

If for example, we take two steers, the same age and weight and both gaining at the same rate but one is eating significantly less than the other, quite clearly, the animal consuming less feed is more desirable. Understanding why one animal is more efficient than another is also important if we are to further our progress.

Preliminary analysis carried out at Teagasc Grange on phenotypic data from pedigree breeding bulls at the National Bull Performance Centre, Tully from various breeds in Ireland showed that animals with low (bottom one-third) and high (upper one-third) RFI had similar growth rates, liveweight and weight per day of age (an indicator of mature size) but high RFI animals consumed approximately 11% more feed per day. Similarly, recent analysis on beef suckler cows from Teagasc Grange showed that cows with low (bottom half) and high (upper half) RFI did not differ in liveweight, liveweight daily gain and body condition score but those with high RFI consumed approximately 14% more. There was no effect of cow RFI on calving difficulty, colostrum yield and immunoglobulin concentration, calf immune status or on cow milk yield and calf daily gain. Likewise, in a subsequent study at Grange, cows with high RFI consumed 10% more than cows with low RFI but with no difference in output. These results indicate that bulls and beef suckler cows with lower residual feed intake are more energy efficient. 

Selecting herd replacements from low RFI animals should permit the production of energy efficient cows and progeny with lower maintenance requirements (i.e. consume less for a given bodyweight and growth rate) and therefore, with environmental and economical benefits for producers. Studies are required to validate the concept of RFI under Irish systems of production, as well as identifying any associated traits. Subsequently, if warranted, it may be incorporated into the national beef breeding programme. 

As part of the beef suckler cow research programme at Teagasc Grange, the concept of Residual Feed Intake as an alternative measure of identifying feed efficient cows and progeny will be examined. Evaluation will encompass divergent selection for the trait using the Simmental breed as a model. The objectives of the project are to demonstrate the magnitude of the benefit, determine some of the biological basis underlying the trait and identify any associated changes in selecting for it. The ultimate benefit for the beef producer should be the ability to select for a beef suckler cow and progeny with reduced feed intake for the same output.