ROWTH rate and efficiency of feed utilization are two of the most economically important characters in the production of meat animals. In these days of world shortage of both plant and animal protein, and with an energy crisis looming on the horizon, the necessity both to reduce maintenance costs by speeding up growth rate and to improve the efficiency of growth by improving the efficiency of conversion of plant feed to animal protein, has become more urgent than ever.

Many studies have been reported on efficiency of feed conversion, dealing with the physiological parameters which affect it and which could conceivably show genetic variability. These parameters include the efficiency with which ingested nutrients are absorbed through the gut (Digestibility), the energy required to maintain body weight (Maintenance), and the efficiency with which absorbed nutrients are converted into a given body tissue, principally protein and fat, and the relative efficiencies of the deposition of these two. Perhaps the most important influences treated have been the differences in feed intake (appetite), and the ratio of fat to lean in the tissue gained. Many individual studies have also been carried out on growth rate and body composition, but few have attempted to relate these to each other or to the physiological factors affecting efficiency. The present paper attempts to synthesize our knowledge by treating the physiological characters mentioned above and the interrelationships among the three characters of growth rate, feed consumption, and composition of the body, along with the “synthetic character” generally called feed efficiency, the ratio of gain in weight of body to the amount of feed consumed over a given period of time. A review of published results will be presented and compared to the results from our own mouse laboratory in Colorado. Projections from mice to large animals will be made periodically to see what insights may be offered on an economic scale.