A MODEL TO DESCRIBE GROWTH-PATTERNS OF THE MAMMARY-GLAND DURING PREGNANCY AND LACTATION

Extensive proliferation and death of cells in the mammary gland occur during pregnancy and lactation. In this study, a mechanistic model was developed that yielded a single equation to describe the pattern of m ammary growth of mammals throughout pregnancy and lactation. The model contains a single pool, which is the cell population of the mammary g land; one influx, representing cell proliferation; and one efflux, rep resenting cell death. The parameters of the equation lend themselves t o direct physiological interpretation. The model fitted data on mammar y gland DNA adequately and can be related to current knowledge on fact ors and inhibitors of mammary gland growth. A unique definition of the parameters of the model can be difficult because of the high degree o f variation among animals, an improper number of observations, or timi ng, as indicated by analyses of simulated data. The model can also be applied to the study of the entire lactation curve. The widely applied gamma equation and the equation that was developed in this study were compared using weekly production data from dairy cows. The new model performed well, particularly when a sharp peak in milk production occu rred. The model has the advantage of providing, for the first time, a simple biological description of the lactation curve that can be used to discriminate changes in lactational performance that are associated with experimental treatments.