Effects of early pregnancy on postnatal maternal performance of mice
SUMMARYThe influences of age at first pregnancy and number of suckling young on maternal performance of mice were studied. Mean mating ages for early-pregnant and normal-pregnant females were 27 and 52 days respectively. Litter size from parturition (day 0) to day 12 of lactation was standardized to either 8 or 12 pups. The more rapid (P<0·01) growth rate and feed conversion efficiency of early-pregnant dams from mating to parturition continued from 0 to 12 days of lactation but occurred primarily in the first 3 days of lactation, so that there were no differences in body weight or feed intake from days 3 to 12 of lactation. There were no age at pregnancy effects on total mammary gland DNA, RNA or RNA/DNA ratio at day 12 of lactation or on individual progeny weights or feed conversion efficiency of litter growth from 3 to 12 days of lactation. Dams suckling eight young had a lower (P<0·01) feed intake, gain and feed conversion efficiency from 0 to 3 days of lactation. From 0 to 12 days of lactation, these dams were smaller, consumed less feed and their individual progeny weights were larger (P<0·01). Dams suckling eight pups had smaller (P<O01) mammary glands, less (P < 0·05) mammary gland DNA and a higher (P<0·05) RNA/DNA ratio. Interactions between pregnancy age and number suckling were not significant. Feed intake of the dam from 0 to 12 days of lactation was influenced more by litter weight gain (34%) than either dam weight gain (5%) or metabolic body size (6%). Maternal performance of mice, based on pup growth and mammary gland growth and function, was not affected by early pregnancy.