Hardness of diet pellets and its influence on growth of pre-weaned and weaned mice.

The hardness of 3 commercial diet pellets for laboratory rats and mice was studied. Hardness was measured as the force (kp) required to crush a diet pellet. Between-batch variation, expressed as coefficient of variation, ranged between 12 and 70%. In one type of diet hardness of the pellets varied between 4 and 50 kp. Hardness did not change during storage for a period of 8 weeks. To investigate the effect of hardness on growth performance of young mice experimental diets were prepared with identical ingredient composition but with hardness of either 15 or 45 kp. Female mice provided with hard pellets (45 kp) were not able to raise their litters. Feeding the experimental diet as 15 kp pellets resulted in a slightly retarded growth when compared with the experimental diet in the form of meal. Weaned mice were fed with the experimental diet in 4 different forms: meal, 15 kp, 45-kp and ground 45-kp pellets. Male mice, unlike females, fed the 45-kp diet grew slower than their counterparts fed the other diet forms. It is concluded that batches of extremely hard pellets may be occasionally delivered, and that such diets have undesirable effects on performance of young mice.