Hardness of diet pellets and growth of pre-weaned mice: separation of direct effects on the young and indirect effects mediated by the lactating females.

The effect of pellet hardness on growth of pre-weaned mice was studied with the use of a specially constructed cage so that lactating females and their young could be fed separately. Diet pellets with identical ingredient composition but hardness of either 20 or 40 kp were prepared. Growth of the young not supplemented with feed was markedly depressed by the feeding of the 40-kp diet to the mothers, when compared with the 20-kp diet. If the young of mothers fed the 40-kp diet had free access to diet pellets with hardness of either 20 or 40 kp, their growth performance improved. When the breeding females were fed the 20-kp diet, feed supplementation of the young did not affect their body weight. This study suggests that the observed effect of diet pellet hardness on body weight of pre-weaned mice is mediated in part through the mothers. Free access to feed was found to stimulate growth of pre-weaned mice only if the mothers were fed extremely hard diet pellets.