After the peak yield in milk secretion, which usually occurs within a few days or weeks after parturition in most species, a decrease in milk secretion takes place. This decrease is primarily due to the involution of the mammary gland tissue.
Involution results in a decrease in the number of secretory cells in the mammary gland tissue.
Part of the reduction in milk yield after peak lactation may also be due to a reduction in secretion rate for each remaining cell. An almost complete involution of secretory tissue takes place after the cessation of milking or the suckling of young. This induced involution is a much more drastic form of involution than the process that takes place during lactation. After the cessation of milking or suckling of young, there is a very rapid disappearance of the secretory cells and a degeneration of the alveolar and lobular structures. If hormonal, nutritional, environmental, or other factors could be found to eliminate or reduce the involutional process in animals kept for milk production purposes, it would greatly affect the efficiency of milk production and change the dairy industry.