Relationship between milk yield and mammary gland development in mice.

Female mice from four lines exhibiting different nursing ability (as measured by mean 12-day weight of eight offspring within a nursed litter) were used to examine milk yield, body size, feed intake, and cellular development of mammary glands and liver. Three selected lines exceeded the control line in milk yield and mammary gland and liver weights. They had more total deoxyribonucleic acid, ribonucleic acid, and protein contents in both mammary gland and liver tissues than the control. Phenotypic correlations between milk yield and deoxyribonucleic acid contents of mammary glands and liver were .39 and .35, while correlations between milk yield and protein to deoxyribonucleic acid ratio of mammary glands and livers were .04 and .11. Milk yield was correlated closely with body size .57 and with feed intake .55. Milk yield was related more to cell number than to cell size of the mammary glands and liver, and large amounts of milk were produced by heavy dams with substantial feed intake.