To understand the reason that F/G becomes worse as pigs mature, it is important to understand the physiology of growth. During the growth cycle of a pig, the pounds of feed required per pound of gain increases, or said another way, the feed efficiency becomes worse as the pig’s weight increases. Because it is more efficient to build muscle than fat, the efficiency of converting feed to live weight gain is best for young pigs and declines as pigs grow larger and older. Early growing pigs convert dietary feed at less than 2:1 F/G while finishing pigs convert feed at over 3:1 F/G. The poorer feed efficiency is caused by two major factors. First, the feed required for maintenance relative to lean growth increases as the pig gets larger. Second, the composition of gain shifts from primarily lean growth to a larger segment of lipid accretion as a pig approaches mature weight. Therefore, the mature frame size of the genetics is an important factor in feed efficiency near market weight. Early maturing pigs enter the less efficient fattening phase at a lighter final weight. Modern high lean genetics typically stay in the lean growth phase longer.