Effects of genetic type and protein levels on growth of swine

A total of 1,588 pigs representing six genetic lines were included in this study. These lines were selected to represent a wide range of biological types for appetite, rate and composition of gain, and meat quality traits. Each line was fed four different diets differing in lysine content. Barrows and gilts were used in the experiment, and pigs were randomly assigned to come off test at one of three weights (113.6, 131.8, or 150 kg). Rates of increase in weight, backfat thickness, and longissimus muscle area were essentially linear. Genetic type and diet influenced (P < .05) rates of change in weight, backfat thickness, and longissimus muscle area and 10th rib fat depth, carcass longissimus muscle area, and dressing percentage. Genetic type × diet interactions were significant for weight change and 10th rib fat. As off-test-weight class increased fat and longissimus muscle area increased in linear fashion. In general, interactions associated with diet resulted from feeding the diet lowest in lysine. There was little evidence of genetic type × diet interactions. If those interactions that tested significant are real, they are a result of extremes in both genetic type (high fatness) and lysine level (low). Slaughtering pigs at heavier weights results in no change in rate of gain over the feeding period and linear increases in longissimus muscle area and backfat thickness.