Maternal performance over three parities in rats selected for rate or efficiency of protein gain.

Direct, maternal and heterosis effects were estimated for preweaning traits over three parities, with unstandardized litters, in rats selected for 14 generations for rate (LG) or efficiency (LE) of postweaning protein gain. In generation 25, after 11 generations of relaxed selection, 60 males and females from each of the selected lines and the control (C) line were mated in a 3 X 3 diallel. Two more parities were produced by remating the females at each weaning. Loss of female breeders due to infertility and death was higher in the selected lines (LG 43%, LE 40%, C 27%). Rebreeding intervals in parities 2 and 3 were 3.1 (P less than .01) and 3.0 (P less than .10) d longer in LG compared with C, while LE was similar to C. Heterosis was significant only for litter size born and it was negative. Because the estimate included all line interactions, negative heterosis could arise from incompatibilities between sperm and ova or embryo and uterus. Both selected lines, but especially LE, had positive direct effects and negative maternal effects on litter size born and weaned. Maternal effects of LG on average pup weight were similar or slightly better than C, while lactational performance was lower in LE.