Effect of selecting for increased lean tissue growth rate in swine on low or high dietary protein levels

To determine the optimum nutritional status when selecting for increased daily lean tissue growth rate (LTGR) two selection lines of Swedish Yorkshire pigs were tested for responses and interactions when fed three diets that contained .70, .85, or 1.00% lysine and 13.9, 15.8, or 17.5% CP, respectively. Selection for increased LTGR was made while the pigs were fed diets with low CP (13.1%) and lysine (.64%) (LP line) or high CP (18.5%) and lysine (.96%) (HP line) concentrations. Two experiments were performed after three generations of selection, with pigs fed from 25 to 105 kg BW. The animals were fed restrictedly on an energy basis in relation to BW. A total of 236 pigs were tested. When testing main effects of selection line, the HP line had superior average daily lean tissue gain (P = .031), carcass lean percentage (P = .007), and lean to feed ratio (P = .028) compared with the LP line. No diet x genotype interaction was found for any of the traits tested. The selection in the HP line resulted in faster genetic progress because this line had best overall performance. This supports the theory that the most efficient selection is achieved in an environment that enables the animals to express their genetic capacity for the desired trait.