Fat is an essential component of all body cells and is an essential component of cell membranes. Fat cells are formed early in the development process but do not fill until excess energy is fed to the pig. Their initial formation is largely under genetic influence, but the filling of these cells is influenced by nutrition. When the pig consumes an excess amount of carbohydrates or fats, it is deposited as fat, largely in the fat cells, whereas other nutrients, when fed in excess, are largely excreted. Fat in the subcutaneous (backfat) area serves largely as the storage area for energy, but it also serves as a natural insulation layer for the animal. The pattern of subcutaneous fat development occurs in a defined sequence, from the shoulder to the mid portion of the back, and from the rear toward the mid portion of the body. Although the area from the 10th to the last rib has a lower backfat thickness than the rest of the topline, its depth at this site is highly correlated with total body fat. This value is subsequently used to evaluate and calculate lean gain and/or body condition.
The amount of body fat deposited is influenced by nutrition and genetics, but the type of fat deposited is influenced by the type and amount of fat fed. Although fat deposition occurs throughout the life of the pig, its rate of deposition accelerates most rapidly during the latter part of the finisher period. This generally coincides with the period when muscle formation declines (Figure 5). The quantity of fat deposited increases faster and earlier in fatter genotypes, whereas the leaner genotypes have a lower deposition of fat at lighter body weights. Because meat packers are currently slaughtering pigs at heavier body weights (250 to 280 lb.), they desire pigs with less fat at heavier body weights. Heavier-weight pigs with a greater genetic potential for muscle development and a lower rate of fat accretion will therefore have less total fat at 250 to 280 pounds body weight.