SELECTION FOR REPRODUCTIVE TRAITS IN SWINE (RELAXED-SELECTION, LITTER SIZE, PUBERTY)

Direct and correlated responses were measured in lines selected for increased litter size (LS) and decreased age at puberty (first estrus, AP) and in a randomly selected line (relaxed selection, RS). Each line originated from a line which had undergone 9 generations of successful selection for increased ovulation rate. Increases in litter size relative to a contemporary, randomly selected control line (C) were noted in all high ovulating lines. Regressions of litter size, as deviations from line C, on six generations of relaxed selection for ovulation rate, were .19, .29 and .16 pigs/generation for the RS, LS and AP lines, respectively. Realized heritabilities, estimated by regression of cumulative response on cumulative selection applied, were .10 (+OR-) .18 for litter size and .39 (+OR-) .30 for age at puberty. Total response, measured relative to the RS line, was .38 pigs for five generations of selection for litter size and 7.9 days for four generations of selection for age at puberty. Decreased weight at puberty and increased fatness at constant weight were associated with selection for decreased age at puberty. An evaluation of ovulation rate, performed by counting corpora lutea (CL) on ovaries collected from 39 RS and 47 C line gilts slaughtered after their second estrous cycle, revealed a difference of 3.9 CL (RS-C), indicating that no slippage in ovulation rate followed relaxation of selection. Constant differences in ovulation rate and an upward trend in litter size indicate that embryonic survival has increased. The physiological mechanism of this increase is not obvious. Unintended selection for litter size may have been more effective in the high ovulating lines than in line C if ovulation rate selection caused changes in heritabilities and genetic correlations of the components of litter size