Comparative performance and body composition of control and selection line large white pigs 3. Three low feeding scales for a fixed time

Growth performance and body composition differences between Large White control (C) and index-selected (S) pigs were evaluated on feeding scales calculated to give very low, low and medium daily growth rates of approximately 450 g, 550 g and 650 g respectively. Starting at 30·3 (s.e. 0·32) kg, 72 boars were penned in groups of six, and one C and one S boar was fed on each of the three feeding scales for 84 days. The feeding scales started at 1·1, 1·2 and 1·3 kg per pig per day for the intended very low, low and medium growth rates with weekly increments of 0·025, 0·050 and 0·075 kg per pig per day. The 34 C and 35 S boars which completed the trial were slaughtered, their carcasses dissected and the whole empty bodies minced and chemically analysed. In no case was the interaction between line of pig and feeding treatment significant. S boars grew faster than C boars on all three levels of feeding. S boars also grew lean tissue faster, deposited less total fat, and had smaller backfat depths than C boars. Similar differences between lines in chemical composition were also apparent for whole body crude protein and lipid. Although the index selection at Newcastle was based on ad libitum performance tests, improvements in the lean content and lean tissue growth rates of the selection line were apparent even at very low levels of feeding.