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Data from two selected lines of pigs in closed herds of a breeding company were collected over a 5-year period. The lines were selected for growth rate and leanness and were housed together in the same environment. Animals were performance tested from 35 kg for about 58 days, and were fed individually according to a scale determined by time on test; the boars were more liberally fed than the gilts. At the end of test, average daily gain (ADG) values were calculated and ultrasonic measures were taken of fat depths at the shoulder (SF), loin (LF) and last rib, 4·5 cm (CF) and 8 cm (KF) off the mid line.
Estimates of heritabilities, phenotypic and genetic correlations were obtained from paternal half-sib analyses. All heritability estimates were significantly different from zero and ranged from 0·18 to 0·42 for ADG and from 0·21 to 0·59 for fat depths. The genetic correlations between ADG and fat measurements in boars of the two lines were not significantly different from zero, averaging 0·08; in the gilts they were negative, averaging –0·39.
After long-term selection, appreciable genetic variation remained in the closed lines studied here for growth rate and fat depths. Although sex and feeding level were confounded, it is likely that the genetic correlation between growth rate and fat depth was influenced by the level of feeding and associated with partitioning of energy between lean and fat deposition.
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