Abstract
Conflicting opinions exist regarding the effect of parity (lactation number) on mammary development and milk yield. The cumulative effect of successive pregnancies is difficult to distinguish from other effects accompanying advancing age, increases in body size, or digestive capacity.Pseudopregnancy in the rat, produced by mating with vasectomized males, causes an 11–12 day stimulation of the mammary gland similar to that of early pregnancy. To study the effect of prior mamary stimulation, rats were compared that had a) two prior pseudopregnancies, b) one prior pseudopregnancy, and c) no such previous mammary stimulation. These rats, of equal age, were subsequently bred to intact males and their reproductive and milk-producing performance determined.Rats stimulated by two pseudopregnancies showed a 15% increase in litter size, but this difference in reproductive preformance was not statistically significant. All litters were standardized at birth to ten young and weighed periodically to determine the lactational performance of the dam. Growth of the litters, number of young surviving, and dam weight at weaning were similar for all groups. This experiment thus indicates that the prior mammary stimulation afforded by pseudopregnancy did not increase milk production in the subsequent natural lactation.
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