Measurement of Milk Yield in the Lactating Rat from Pup Weight and Weight Gain

A new method was developed that enables milk yield to be estimated from pup weight and pup weight gain in well-nourished rats. Daily milk yield, body weight, and daily weight gain were measured over 4-5-day periods in 38 suckling pups aged 3-13 days postpartum. Each 24-h period was divided into consecutive 6-h cycles, during which dams and pups were separated for 4 h and then reunited for 2-h suckling intervals. Daily milk yield per pup was calculated from pup weight gain during the four suckling intervals for each 24-h period, and corrected for insensible weight loss of the pups. The following equation, relating pup milk yield to pup weight and weight gain, was obtained by stepwise forward multiple regression analysis: yield = 0.0322 + 0.0667 (weight) + 0.877 (gain); where yield is daily yield per pup (g/pup/day), weight is pup weight (g), and gain is pup weight gain per day (g/day) (n = 174; F = 552.3; p less than 0.001; r2 = 0.866). Per litter milk yield can be obtained by multiplying per pup yield by litter size. Using this equation, dams fed a stock diet and nursing eight pups were estimated to have produced 29.5 ml of milk on day 10 of lactation. This value is similar to yields measured by more laborious methods. SEs of yield estimates calculated with this method represent 1-3% of predicted yield values. The new method gives useful estimates of rat milk yield during the first 2 weeks of lactation from measurements that are more easily obtained than is the case with other methods.