EFFECTS OF THE INTERRELATIONSHIP BETWEEN GENOTYPE, SEX, AND DIETARY LYSINE ON GROWTH-PERFORMANCE AND CARCASS COMPOSITION IN FINISHING PIGS FED TO EITHER 104 OR 127 KILOGRAMS

One hundred twenty pigs (initially 44 kg BW) were used to determine effects of the interrelationship between genotype, sex, and dietary lysine on growth performance and carcass composition in a 2 x 2 x 2 factorial arrangement. Genetic comparisons were made between pigs characterized with either a high or medium potential for lean tissue gain. Within genotype, barrows and gilts were separately fed either a .90 or .70% lysine diet until the mean weight of pigs in each pen of three reached 104 kg. One pig per pen was slaughtered to determine carcass characteristics and chemical composition. From 104 to 127 kg, dietary lysine was lowered to .75 or .55% for pigs fed .90 or .70% dietary lysine, respectively. When the pigs' mean weight met or exceeded 127 kg, both pigs were slaughtered to determine carcass characteristics and chemical composition. Carcass length, longissimus muscle area, average backfat thickness, and 10th rib fat depth were measured 24 h postmortem on the chilled carcasses. The right side of each carcass was then ground and chemically analyzed to determine protein and lipid accretion rates. No interactions were detected from 44 to 104 kg; therefore, main effect means will be discussed. At 104 kg, high-lean pigs had increased ADG (P < .01) and gain:feed ratio (G/F; P < .05) compared with medium-lean pigs. Barrows had increased (P < .05) ADG and ADFI but exhibited a poorer (P < .01) G/F than gilts. Pigs fed .90% lysine had improved (P < .01) ADG compared with pigs fed .70% lysine. High-lean pigs had increased CP accretion (P < .01) and lipid accretion (P < .05) compared with medium-lean pigs. Gilts had larger longissimus muscle area and less backfat (P < .01) than barrows. Similarly, gilts had increased (P < .01) CP accretion and decreased (P < .01) lipid accretion compared with barrows. Cumulative ADG (44 to 127 kg) was greater (P < .05) in high-lean pigs, in barrows, and in pigs fed the .90/.75% lysine regimen. Average daily feed intake was increased (P < .05) in barrows compared with gilts. Differences between genotypes were greater at 127 kg; high-lean pigs had larger (P < .01) longissimus muscle area and longer carcasses than medium-lean pigs. Gilts also had increased (P < .05) longissimus muscle area, greater CP accretion, and decreased backfat thickness compared with barrows. Crude protein accretion was greater in high-lean pigs than in medium-lean pigs; high-lean gilts had the greatest magnitude of response to increased dietary lysine.