Sources of variation in preweaning weight of mice.

The effects of body weight of dam after parturition (D), litter size at birth (N), and birth weight (B) on the weight of mice at 12 da of age (M) were examined in 3 experiments under different conditions: Experiment 1, cohabitating of breeders at 3 wk of age, a sexually immature stage; Experiment 2, cohabiting of breeders at 9 wk of age, a mature stage; and Experiment 3, cohabiting of breeders at 9 wk of age plus cross-fostering at birth. In all experiments, litter size was standardized to 8 at birth. Standard partial regression coefficients of M on D, N, and B were .325 +/- .036, .298 +/- .038, and .483 +/- .034 in Experiment 1; .204 +/- .045, .161 +/- .044, and .370 +/- .043 in Experiment 2; and .375 +/- .098, .074 +/- .097, and --.137 +/- .287 in Experiment 3. The variation in M attributable jointly to all independent variables (D, N, and B) was 49, 22, and 17% in Experiments 1, 2, and 3, respectively. It was concluded that when sexually mature mice at nearly the same age are cohabited for reproduction, the effects of D, N, and B on an indirect measure of milk production (M) are not large. The potential for genetically improving milk production under the condition of Experiment 3 was discussed.