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Role of Milk Removal in Galactopoiesis


Cows walking to the barn.

Milk removal is required for maintenance of lactation!!!

Milk removal from the lactating mammary gland is the major factor in maintaining milk secretion. Removal of milk from mammary alveoli results in a range of effects on mammary gland function manifested as immediate, short and long term effects (Wilde et al., 1987). Immediate effects of milk removal occur within seconds to minutes of stimulation of the gland and removal of milk. As milk is secreted from the mammary epithelial cells into the alveolar lumen milk components accumulate in the lumen. In addition, an autocrine factor, termed feedback inhibitor of lactation (FIL; Wilde et al., 1995; Knight et al., 1998), is secreted from the alveolar epithelial cells and accumulates in the alveolar lumen. The FIL then feedsback and inhibits further milk secretion from the alveolar epithelial cells. Removal of milk at milking or suckling removes FIL and allows for continued milk secretion. Milk accumulation in the gland also causes an increase in intramammary pressure, which reduces blood flow to the tissue. Furthermore, the stimulus during suckling or massage of the gland by the piglets causes increased prolactin concentrations in the sow's blood (Bevers et al., 1978; Kendall et al., 1983; Algers et al., 1991; Spinka et al., 1999). Short term effects of milk removal are thought to occur through the enhancement of epithelial cell differentiation (Wilde et al., 1987; Wilde and Knight, 1989). This work has been done primarily in ruminants and is based on increasing frequency of milk removal. Higher frequency of milking in ruminants results in greater activities of milk synthesizing enzymes per cell in lactating tissue. Long term effects of more frequent milking in ruminants are thought to occur over a period of weeks or longer and occur in response to increased cell proliferation (Henderson et al., 1985).

Stimulation Intensity :

In the case of a dairy cow that is milked by a milking machine, the intensity of milking is controlled by the machine and stays mostly constant throughout a lactation. However, in nature there may be variable numbers of offspring suckling at any given time, the nursing offspring may be of different sizes and have different levels of aggressiveness in nursing. The greater the nursing intensity the more mammary growth and the more milk produced. Nursing intensity means the number of nursing young (litter size), especially in litter-bearing species such as the pig. Although it is less well documented, this effect of stimulation intensity also probably means the vigor with which the young nurse, perhaps involving the degree of gland emptying which occurs at each nursing or the intensity of stimulation of the nipple. (What is the effect of litter size on mammary growth during lactation? If you do not remember, then see the Mammary Development section?).

The pig will be used as the example in this section. Factors which contribute to suckling intensity in pigs include litter size, suckling interval (frequency of milk removal), piglet size, limitations of milk letdown, piglet behavior, and a number of maternal characteristics (Hartmann et a., 1997; Brooks and Burke, 1998; King, 2000). All of these factors will contribute to the overall process of milk removal. It is reasonable to expect that the cell proliferative effects of very frequent milk removal occurring in the sow may provide a primary stimulus for mammary growth observed during lactation. While the relationship between milk removal from the sow's mammary glands and cell proliferation in those glands has not been demonstrated directly, there is considerable indirect evidence for such a relationship.

Litter size is related to total milk production by a sow and extent of postpartum mammary gland growth, as discussed above. The more piglets suckling a sow, the more total milk is removed. Size of the suckling piglet is positively correlated with sow milk production and average daily gain of the piglet (Pluske and Dong, 1998; King, 2000). More effective milk removal is expected when a larger piglet is suckling a gland. Several studies have demonstrated that shorter suckling intervals result in greater total milk removal (Sauber et al., 1994; Auldist et al., 1995; Brooks and Burke, 1998). Maximal refilling of the gland occurs within no more than 35 min after a suckling bout (Spinka et al., 1997). Unlike the ruminant, which has large cisterns and non-alveolar milk storage areas, the sow has little non-alveolar milk storage volume. Milk accumulation in the sow's mammary gland after a suckling bout should rapidly diminish the rate of further milk secretion. Nonnutritive suckling bouts occur frequently in domestic sows and are part of the natural behaviour of these animals (Brooks and Burke, 1998). While not resulting in milk removal, nonnutritive suckling bouts probably stimulate further prolactin secretion and contribute to maintenance of lactation. The total number of nutritive sucklings per 24 h period is an important determinant of total milk removal from the mammary glands. Small differences in suckling intervals could result in significant differences in total milk removed from the gland within a day.

Health and aggressiveness of the suckling piglet, sow mothering behaviour, and perhaps teat size and shape are among a range of other factors which could directly or indirectly affect milk removal (Fraser, 1990; Brooks and Burke, 1998). Age, parity and sow body condition all affect milk production in sows (King, 2000) and would be expected to affect milk availability and ultimately milk removal.

Some other observations related to milk removal and galactopoiesis:

There is no evidence for nerves directly controlling secretory activity. Mammary glands which have been transplanted to the neck of goats can still secrete milk.

Nursing stimuli which occurs without milk removal (ie. when the nipples are ligated) in lactating rats results in a slower rate of involution, but does not quantitatively prevent losses of mammary cells or metabolic activity.

In rats, repeated replacement of litters with new foster litters can prolong lactation, for up to a year.

In cows, if you do not milk 2 quarters for 2 weeks (but continue milking the other 2 quarters) then start milking the unmilked quarters again, they will recover milk secretion, but the yield is typically lower than in control quarters (the quaters milked throughout the period). However, throughout the next lactation, the experimental quarters produce more than the control quarters. These types of experiments indicate that local, intra-mammary factors are responsible for the observed effects. Each gland is exposed to the same blood or the same systemic stimuli.

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