Comparative growth and body composition of index selected and control lines of large white pigs

Slaughter and metabolism trials were conducted on the Newcastle control (C) and index selected (S) lines of Large ’.’hite pigs. The index combined hackfat depth, feed conversion ratio and growth rate, and pigs were compared on ad libitum performance.tests. The two fixed time (84 day) slaughter trials, conducted after ten generations of selection, used a time-based ration scale and to-appetite feeding. Rations in the metabolism trials ranged from maintenance up to appe­tite. Carcass dissection and whole body chemical analysis were employed in the slaughter trials, and nitrogen retention was measured by metabolism trial. Entire males were used for all trials. After feeding a fixed, time based ration from about 27 kg live weight for 84 days (Trial 1), S boars were heavier than C (96,2 v 93.8 kg,P < 0,01), having converted feed more efficiently (0,392 v 0.378 kg gain/kg food, P < 0.01). Their carcasses contained more lean (34.1 v 31.8 kg, P < 0.001) and less sub-cutaneous fat than C (12.5 v 13,7 kg, P < 0.001), but while their bodies contained less chemical lipid than C (17.4 v 18.8 kg, P < 0.001), protein contents were similar for the two lines (13.8 v 13.4 kg, P < 0.05). When individually fed to appetite for 84 days (Trial 2), C boars ate 9 kg more than S in total (202 v 193 kg, MS), of which nearly 7 kg was during the first half of the trial (85.5 v 78.3 kg, P < 0.01).This difference in feed intake emphasised between-line carcass and body composition differences, which were, for S and C boars respect­ively: lean (kg) 36.5 and 33.5 (P < 0.001), dissected fat (kg) 19.6 and 22.6 (? < 0.01), chemical lipid (kg) 20,8 and 25.1 (? < 0,001), protein (kg) 15.4 and 14.2 (P < 0.01), Results from the metabolismvi 11 trials did not give significant line differences, but S boars tended to retain more K than C on high levels of feeding and less than C on low levels. Ten generations of index selection at Newcastle have markedly reduced backfat depth and increased lean tissue growth rate, but these improvements were accompanied by lower appetite and greater energy loss as heat. Future pig improvement may increase lean growth by increasing voluntary feed intake, assuming backfat controlled