The concentration of lactose is low in colostrum and changes in an inverse manner to other constituents such as fat, protein and ash . A low level of lactose in early post-partum milkings, followed by a steady increase thereafter, until a normal level is reached, has been reported by several authors . Lactose concentrations as low as 1.2% have been reported in colostrum . In general, lactose concentration reaches a normal concentration within 7 days post-partum. lactose content of milk did not reach normal levels until 60 days post-partum; however, in this particular study, no samples were collected between 30 and 60 days post-partum.
Lactose is responsible for about 50% of the osmotic pressure of milk ; its production causes the movement of water from the cytoplasm of mammary epithelial cells into the secretory vesicles and subsequently into milk. This influx of water into milk regulates the volume of milk produced and the concentration of casein in milk . A low level of lactose results in the production of milk that is extremely viscous and contains little water due to the absence of the osmoregulator lactose.