| Summary: | Direct and correlated responses to selection for weight gain were investigated after nine generations of within family selection. Four selection criteria were used: gain between 28 and 38 or 48 and 58 days of age, and under two feeding regimes, ad libitum consumption or restricted to 80% of control lines. Two lines consisting each of twenty pair matings were selected under each of these criteria. Two unselected control lines were kept. Carcass composition analyses were performed in generation nine at the beginning and end of each selection period and at 100 days of age. === Direct responses to selection in the first period were greater in the ad libitum lines while in the second period direct responses were greater in the restricted lines. Direct responses and realized heritability estimates were significantly different between the sexes; males had greater direct responses and higher heritabilities in all selected lines. Body weights before the selection periods decreased in all lines as a result of selection. Body weights after the selection period were not different from controls in the ad libitum lines while restricted lines remained smaller animals. Correlated responses in feed efficiency and feed consumption in the ad libitum lines were positive in the first period and negative in the second period. Restricted lines had a positive response in feed efficiency and negative response in consumption in both periods of selection. Changes in body composition in the first period reflected the changes in body weights through a lower crude protein percentage at the start of the period and a lower ash percentage at the end of the period. Body composition at the start of the second period was not altered by selection, while at the end of the selection period ad libitum lines had higher dry matter percentages and restricted lines had lower fat percentages. Body composition at 100 days of age was not affected by selection except for dry matter percent, that was lower in the restricted lines. === Correlated response in fitness was evaluated through litter size. In the first period lines selected under ad libitum feeding were not affected by selection for increased weight gain while selection for weight gain under restricted feeding caused a significant decrease in litter size.
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