Education Physiology


Mammary System, Structure and Function:

What is milk and where does milk come from?

         The mammary system (udder) is designed to produce milk from nutrients from the cow's reserves and from the nutrients that the cow ingests on a daily basis. After the mammary system has been activated, the milk is produced continuously and is stored in the mammary system until it is removed with milking or nursing.  The four quarter mammary system is suspended by the lateral and median suspensory ligaments from the pelvis of the cow.                                                                            

The suspensory ligaments act as stablizing shock absorbers for the mammary system (udder) which can be very heavy when full of milk. This suspensory structure is very important to keep the mammary system suspended properly off the ground. A strong suspensory is influenced by genetics and proper and adequate nutrition. 

        Each of the four (4) quarters of the mammary system will act independently in its milk production and storage. The mammary glands or quarters on the same side have some interconnection of blood supply but none with the opposite side.  The shape, position, length, angle, and the muscularly controlled sphincter or streak canal of the teat are all genetic and of extreme importance in preventing leaking, invading bacteria from entering the mammary gland, and in milking efficiency.

        The alveoli are glands where the milk is made, and where the arterial blood and lymphatics bring the raw nutrients needed for milk production to the alveoli. The secretory cells, once activated with calving, will produce milk continuously dumping the finished product into the lumen of the alveolus. The lumen is squeezed by a muscular layer (myepithelia) which forces the milk into the major ducts to deliver it to the gland and teat cisterns. This circulation is very important as approximately 500 liters of blood must be pumped through the gland inorder to deliver the necessary raw materials and hormones needed for continuous peak milk production.  The secretary cells lining the lumen of the alveoli are highly specialized and are responsible for the amount of milk and percent of milk componets produced. These secretary cells are controlled by genetics. Proper and adequate nutrition, envionment and milking practices will also have an effect on the amount of milk and the percent of milk componets produced.








Lactation process in more detail:

        As parturition (spring, freshening, birthing or whatever reference one chooses to use) nears, the fetus' rising adrenal cortical hormones gradually loosens the placenta attachment, initiates a mobilization of raw material for the production of the milk components (protein, carbohydrates, fats, lactose, solids and colostrum).  The appropiate hormonal signal from the fetus signals the anterior pituitary gland of the cow, which begins a cascade of hormonal changes both bringing about the delivering of the new calf and the preparation of mammary system for continuous milk production on demand.  Milk let down for nursing, hand or machine milking is triggered by a combination of nervous and hormonal reactions. Seeing, hearing, or tactile stimulus signals the cows posterior pituitary and a release of the oxytocin hormone which is carried by the blood circulation to the myoepithelial cells covering the alveoli causing a muscular contraction and milk being squeezed into the lumen. Negative stimulus such as unpleasant, unfamilar noises, sound, environment and pain will trigger adrenaline hormone which will reduce or stop milk let down totally.








        Milk production and secretion will continue as long as the environmental conditions are favorable, adequate fresh water is always available, nutrition is adequate and milk is timely removed from the mammary system (udder). When this process ceases, the secretory cells begin to undergo involution and milk production gradually stops.

Let's take a closer look at the synthesis of milk componets. Milk componets are used for many consumer dairy products.

        The mammary gland secretion (milk) has several components (lactose, minerals, albumen, immunoglobulins, colostrum, casein, protein, fats); each contribute to the taste, over all nutritional value of milk, and have individual and specific consumer purposes such as smooth and curdled creams, ice cream, butter, cheese and puddings.  The various milk componets of milk are transported or metabolized in the secretory cells of the mammary gland. Genetics, nutrition, and environment control their production level.  Synthesis of lactose in milk begin with the fermentation of forage in the rumen to form volatile fatty acids (VFA). Primarily the volatile fatty acid (VFA) propionic acid is metabolized in the liver to form glucose. The glucose supplies energy to the secretory cells of the mammary gland for the production of lactose, protein and fat. Shortages of glucose production and key ingested minerals will not only deprive their volume and contents in the milk secretion, but they could lead to serious metabolic imbalances in the cow such as "Ketosis" and "Milk fever". The synthesis and secretion of lactose into the alveoli causes an increase of water to be drawn into the alveoli because of an osmolarity difference and ultimately an increase in milk volume. Cows with a genetic high blood growth hormone level will produce more milk volume by causing a redistribution of nutrients in the cow's body and better utilization.

Synthesis of protein in milk is produced in the mammary secretory cells from amino acids brought in by the blood flowing through the mammary gland. The maxium protein synthesis is predetermined by genetics, environment and proper nutritional availability.

The exceptions to this rule are for albumin, immunoglobulins which are made elsewhere and transported to the secretory cells by the blood and colostrum made elsewhere and transported to the secretory cells only during terminal pregnancy.

 Synthesis of fat in the milk:

        Plant fatty acids derived from the diet make up the majority of fat content in the milk. These plant fatty acids are often deficient in hydrogen atoms and therefore must be furnished by the rumen before entering the blood traveling to the mammary secretory cells. Upon arriving at the mammary secretory cells they must be broken down to glycerol as they are too large to cross the cell membrane. Therefore rumen health and efficient fermentation is essential to milk fat yield. Milk fat is influenced primarily from rumen health, genetics, environment, proper and adequate diet.  Secretion of minerals in the milk consist of many trace minerals. Calcium, phosphorus, potassium and magnesium are the most important minerals found in milk and they will be found bound to the amino acids in the casein fraction of milk. These important minerals will only be present in milk if the cow's intake exceeds their body needs. Sodium , potassium and chloride are found in lesser concentration in milk. They have an osmotic effect on increasing water uptake by the mammary gland and complementing lactose in increased milk volume. These minerals will only be present in milk if the cow's intake exceeds their bodily needs.

        It should be clear now why milking cows are so thin as the milking energy demand is very high and often the forage diet, nutrients, water and environment, struggle to meet caloric needs. This is also why the food value of milk and all dairy products will only be fulfilled if proper and adequate daily nutrients are available to the dairy cow.

         As you can now clearly see, our Raw Jersey milk, relies on proper, adequate and continuous daily nutrition. - Quality before Quantity.