EFFECTS OF SELECTION FOR HIGH AND LOW POSTWEANING WEIGHT GAIN IN RATS: CORRELATED RESPONSES IN GROWTH COMPONENTS, REPRODUCTION AND MATERNAL PERFORMANCE

The response to selection for increased (U line) or decreased (D line) 3-to 9-week gain on growth components, reproduction and maternal performance was evaluated after 34 generations of mass selection in rats. The study was divided into three phases. Phase I evaluated growth (ADG), feed intake (ADI), gain/feed (FE), body composition (BC), metabolic rate (AHL, per unit metabolic size), maintenance requirements and partial efficiency of weight gain (ADG/FG, FG = feed only for gain). An asymmetrical response was observed in ADG, compared to the C line. The ADI, was 6% higher for U line but only 3% lower in D line, compared to the C line. Both U and D lines showed a significant positive correlated response (+5 and -5%) in FE. Although both selected lines were fatter, only the D line was significantly fatter than line C. There was a tendency for both selected lines to be higher in AHL at 6 wk of age than C lines. The opposite was true at 9 wk of age. Maintenance requirements and ADG/FG were 5 and 7% and -4 and -7% for the U and D lines, respectively, compared with the C line. Phase II evaluated fertility, litter size (LS) perinatal mortality (PM), age (AVO) and weight (WVO) at vaginal opening, ovulation rate (OR), normal fetuses (NF), pre-implantation (PRI) and post-implantation (PSI) losses in females, as well as age (AFM) and weight (WFM) at first spontaneous mating, slaughter weight (SLW) and weight of testes (TWT) at 91 days of age in males. Selection for increased postweaning gain resulted in a decrease in AVO but no change in WVO. The D line females were similar in AVO to the C line, but reached puberty at a lighter weight. Litter size, OR, NF and PSI were significantly higher for the U line but PRI was not. Down line females were lower in LS. All lines were similar in AFM but the U line was higher than the C and D line males in WFM. A positive correlated response was also found for SLW on TWT. The third phase evaluated the correlated response in direct genetic and postnatal maternal effects on 12-day weight, dam size and maternal feed efficiency. A significant positive correlated response was observed in direct genetic effect for increased preweaning gain, but the opposite resulted from decreased gain selection. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM