Selection for postweaning gain in rats: II. Correlated response in reproductive performance.

Reproductive performance was evaluated on lines of rats selected 34 generations for up (U) and down (D) 3- to 9-wk weight gain and from a control (C). Direct response to 3- to 9-wk weight gain was 16% in U and -8% in D lines. Inbreeding was similar among lines. Both selection lines were poorer in percentage of fertile pairs than the C line, especially so in D (P less than .10). The lines differed (P less than .05) in litter size born with means of 11.4, 10.4 and 9.6 for U, C and D, respectively. The significant positive correlated response in litter size corresponded to that expected from intraline association with female body weight. Selection for increased postweaning gain resulted in 5% earlier age but little change in weight at vaginal opening. The D line was similar to C in age, but 7% lighter in weight at vaginal opening. The U line was significantly higher (14, 12 and 69%) for ovulation rate, number of fetuses and post-implantation losses, but not for pre-implantation losses. The D females were similar to C females in all components of litter size. Males of all lines were similar in age at first mating, but U males were 22% heavier than C males in weight at first mating. A positive correlated response in weight of testes at 91 d of age disappeared when adjusted for intraline association with body weight.