Lactation Studies. V. Effect of Litter Size and Number of Lactation upon Milk Yields in Sprague-Dawley Rats with Observations on Rates of Gains of Young in Litters of Various Sizes

Summary The relative milk yields of 32 rats were estimated between the 8th and 18th days of their lactations by daily weighings of their litters. There were 32 first, 27 second, and 22 third lactations involved. The young born to each rat remained with her until the eighth day, at which time the numbers of young in the litters were reduced to 3, 6, 8, or 10 for 1st, and 8, 10, and 12 for 2nd and 3rd lactations. As the number of young in litters (size) increased, the weight gain of the litter increased, indicating that milk yields increased also. Milk yields in the second lactations were similar to those in third lactations and these were, in turn, higher than yields in first lactations. In first lactations the average growth rates of young in litters were not statistically different until the litter size exceeded ten; with a litter size of ten they grew significantly slower (P