Excretion is the discharge into the lumen, ducts, cistern, teat and final harvest by the milking person. As the collecting ducts begin to fill with the accumulated excretion of the alveolar cells, they experience difficulty in ridding themselves of milk. When the pressure in the udder rises, the cells and lumina of the alveoli become distended, compressing the small collecting ducts. This prevents a pressure overload on the teat sphincter and any leaking of milk. There is little milk production going on while milking is being done.
Udders can extend themselves greatly to hold high volumes of milk.
Milk let-down results in a dramatic increase in mammary pressure which will gradually subside, even if no milk is removed. However, stress and mastitis incidence can be reduced significantly by equal milking intervals or by 3-times milking for very high milkers. 26 Rapid removal of milk after stimulation of letdown is essential for complete milking. There is always some residual milk left in the udder after milking, but it is normally less in goats than in cows. Regular and complete milking is one of the requirements for continuance of lactation.