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 ef fects of the interrelationship between genotype, sex, and dietary lysi ne on growth performance and carcass composition in a 2 x 2 x 2 factor ial arrangement. Genetic comparisons were made between pigs characteri zed with either a high or medium potential for lean tissue gain. Withi n 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 reache d 104 kg. One pig per pen was slaughtered to determine carcass charact eristics and chemical composition. From 104 to 127 kg, dietary lysine was lowered to .75 or .55% for pigs fed .90 or .70% dietary lysine, re spectively. When the pigs' mean weight met or exceeded 127 kg, both pi gs 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. N o 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 po orer (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 increa sed CP accretion (P < .01) and lipid accretion (P < .05) compared with medium-lean pigs. Gilts had larger longissimus muscle area and less b ackfat (P < .01) than barrows. Similarly, gilts had increased (P < .01 ) CP accretion and decreased (P < .01) lipid accretion compared with b arrows. Cumulative ADG (44 to 127 kg) was greater (P < .05) in high-le an pigs, in barrows, and in pigs fed the .90/.75% lysine regimen. Aver age daily feed intake was increased (P < .05) in barrows compared with gilts. Differences between genotypes were greater at 127 kg; high-lea n pigs had larger (P < .01) longissimus muscle area and longer carcass es than medium-lean pigs. Gilts also had increased (P < .05) longissim us muscle area, greater CP accretion, and decreased backfat thickness compared with barrows. Crude protein accretion was greater in high-lea n pigs than in medium-lean pigs; high-lean gilts had the greatest magn itude of response to increased dietary lysine.