Lifetime parental productivity in twenty-seven crosses of mice. II. Weaning traits reflecting reproduction and lactation.

Twenty-seven crosses of mice derived from three random-mating, unselected strains (SWO, J. PGH) with 16 matings/pureline or cross were cohabited continuously and monogamously for a 365-d reproduction period to evaluate lifetime reproductive performance of the parents. Data from 3,249 litters born (2,792 surviving to weaning) were analyzed to study the effects of parity (number of litters) and type of cross (pureline, two-way, backcross, three-way) on certain traits measured at weaning (21 d). The traits evaluated were 1) weaning to weaning interval (WWI), 2) litter size at weaning (LSW), 3) fraction alive at weaning (FAW)-the ratio of LSW to number born alive, 4) total litter weight at weaning (LWT), 5) pup weight at weaning (PWT), 6) weaning weight per pup observed alive at birth (WPP), 7) parental feed consumption per day (FPD) and 8) parental costs (PCT). Cost per live weanling animal was estimated as the ratio of cumulative parental costs to cumulative number weaned. Cost per unit of weaning weight was defined as the regression of cumulative parental costs on cumulative litter weight. Parity effects were important (P less than .01) for all traits measured. Type of cross effects were noted for LSW, LWT, PWT, WPP, FPD (P less than .01) and PCT (P less than .05). A parity X cross-type interaction was expressed for WWI, LSW, PWT, WPP, FPD (P less than .01) and FAW (P less than .05). Three-way crosses were more productive than backcrosses, two-way crosses and pureline matings for all traits studied. Three-way crosses displayed the smallest cost per animal weaned and smallest cost per unit of weaning weight. The influence of parity on traits measured at weaning was less than for traits measured at birth. Weight per pup born alive (WPP) was found to be a better indicator of reproduction and lactation ability than LWT because WPP accounts for preweaning mortality.