For a period of 20 days post parturition the cow is called a fresh cow. Milk production quickly increases during this phase but milk composition is also significantly different from the rest of the cycle. This first milk, called colostrum, is rich in fats, protein, and also maternal immune cells. This colostrum is not usually commercially sold, but is extremely important for early calf nutrition. Perhaps most importantly, it conveys passive immunity to the calf before its immune system is fully developed.