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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 Sizes¹

Department of Dairy Husbandry, University of Minnesota, St. Paul

ABSTRACT

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 < .05). In second lactations the individuals in litters of 8 and 10 were not different from each other, but they grew faster than those with 12. During third lactations each increase in litter size resulted in a significant decrease in growth rates of individuals in the litters.

¹ Paper No. 5379, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota. Data taken from a thesis presented by R. R. Reddy in partial fulfillment of the requirements of the Ph.D. degree. University of Minnesota, June, 1964.