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Milk Secretion Rate


Cows walking to the barn.

Milk yield is dependent on (1) the amount of secretory tissue and (2) the rate of milk secretion (per unit of time). Secretion rate is affected by the accumulation of milk in the alveolar lumen. Accumulation of milk in the lumen increases the intra-mammary pressure (see figure below). Once the intra-mammary pressure reaches a certain level (probably about 8 to 10 hours after the last milking int he dairy cow), secretion rate declines. If the pressure increases enough (in the cow at about 70 mm Hg), then secretion stops and milk starts to be resorbed. In the dairy cow, secretion rate stops (reaches zero) at about 35 hours after the last milking. Instruments have not been available to measure intra-alveolar pressure. Generally the intra-mammary pressure has been measured in the teat cistern using a teat cannula. So these intra-mammary pressure estimates reflect total gland pressure from accumulation of milk and not directly the intra-alveolar pressure. These types of studies are done with great care to assure that milk ejection is not stimulated, which would also increase intra-mammary pressure.

Diagram of realtionship between milk secretion rate and intra-mammary pressure after milking.

[Adapted from Schmidt, G.H., 1971, Biology of Lactation, W.H. Freeman and Co., p. 150.]

The inhibition of milk secretion that accompanies increasing intra-mammary pressure is probably caused by a chemical inhibitor (FIL) rather than the increased pressure of the fluid itself (see above).

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