Milk Production | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Mammary Glands and Udder

• Milk is produced in glands grouped in an organ called the udder.
• Udder weight varies (11-27 kg) with a positive correlation to milk production.
• Udder suspended by strong ligaments from the rear abdomen.
• Udder divided into fore and hind quarters, each with an independent gland and teat.
• Forequarter secretes 20%, rear quarter secretes 30% of total milk.
• Milk cisterns above teats collect milk, averaging about a pint in capacity.

Alveoli and Milk Production:

• Alveoli are key centers responsible for milk production.
• Each alveolus lined with milk-secreting cells.
• Numerous alveoli connected to ducts draining into a gland cistern.
• Small muscle fibers around alveoli contract to facilitate 'let down' of milk.

Teat Structure:

• Teat cistern connected to streak canal, a narrow tube.
• Streak canal has a muscular sphincter preventing milk leakage until milking.
• Sphincter also prevents entry of bacteria and contaminants into the teat.

Mammary Development

Start of Development:

• Mammary gland development in bovine fetus begins 4-6 weeks after conception.
• Initial development from a single layer of cells behind the umbilicus.
• Cells converge into two mammary lines along each side of the midline in the inguinal region.

Formation of Mammary Buds:

• Each mammary line develops into mammary buds.
• Buds form solid cores of cells, primary sprouts, giving rise to teat and gland cisterns.
• Primary sprouts give off secondary and tertiary sprouts forming the duct system of the udder.

Underlying Tissue Development:

• Underlying mesenchyme tissue gives rise to fat tissue.
• Basic vascular, lymphatic, and nervous systems of the udder develop.
• Most fetal development completed by six months after conception, except for further fat deposition.

Development of Mammary Glands in Cows

At Birth:

• Calf is born with defined teats, teat cisterns, and gland cisterns.
• Secondary sprouts develop from gland cistern, some rudimentary.

Post-Birth to Puberty:

• Growth from birth to puberty mainly involves fat deposition in the gland.
• Little udder growth till two to three months old.
• Between three months and puberty (8-10 months), mammary gland grows 3.5 times faster than the body.
• Reaches peak pubertal development by twelve to fourteen months.
• Limited growth after puberty until the cow becomes pregnant.

After Puberty:

• After puberty, recurrent estrus cycles occur.
• Mammary gland undergoes cyclic changes, growing during the estrogenic phase and regressing during the progestational phase.
• Cumulative growth in the first four to five cycles, with minimal gain until pregnancy.

During Pregnancy:

• Largest growth of mammary gland occurs during pregnancy.
• First three months: Duct system develops.
• From the third month onwards: Rapid growth of secretory tissue (lobule-alveolar system).
• Secretion of colostrum starts before calf's birth, leading to significant udder enlargement.
• Considerable growth of secretory tissue continues during early lactation.

Hormonal Influence on Mammary Gland Growth

Estrogen and Progesterone:

• Estrogen stimulates duct growth, while a combination of estrogen and progesterone stimulates lobule-alveolar development.
• For growth comparable to late pregnancy stages, prolactin and growth hormone are also needed.
• Major hormones involved: prolactin, estrogen, and progesterone.
• Adrenal and thyroid hormones indirectly affect mammary development.

Placental Role:

• Placenta produces hormones (estrogens, progesterones, prolactin-like) crucial for mammary development in later pregnancy.

Hormonal Control of Milk Secretion

Terminal Pregnancy Phase:

• Epithelial cells of mammary gland start secreting milk.
• Sudden increase in secretory activity at or after birth.
• Hormonal changes during pregnancy make mammary gland less responsive to prolactin and adrenal hormones.

Parturition:

• Rise in prolactin and adrenal corticoids, decline in estrogen and progesterone initiate lactation.

Milk Biosynthesis

Alveolar Cells:

• Milk made in alveoli from blood constituents.
• Some components taken directly from blood, while others synthesized by alveolar cells.
• Small droplets of milk migrate within alveolar cells and are ejected into alveolar lumen.

Blood Flow:

• About 500 volumes of blood flow through mammary gland for each volume of milk synthesized.

Post-Partum Period:

• Milk production peaks in 2-4 weeks, then gradually declines.
• Suckling or milking stimulates hormone release essential for lactation.
• Maintaining optimal intra-mammary pressure is crucial for consistent milk production.

Hormonal Requirements for Lactation

Prolactin:

• Main hormone for maintaining lactation.
• Growth hormones can enhance established lactation.
• Adequate levels of adrenal hormones are necessary for milk precursor maintenance.

Thyroid Hormones:

• Not essential, but thyro-active substances can increase milk production when provided with sufficient nutrients.

Parathormone and Insulin:

• Parathormone for optimal calcium levels.
• Insulin regulates carbohydrate metabolism.

Milk Ejection

• Continuous synthesis of milk between milkings within alveolar cells.
• Majority of synthesized milk stored in alveoli and smaller ducts.
• Milk ejection occurs when there is a sudden increase in intra-mammary pressure, allowing drainage of stored milk.

Milk Ejection Process

Stimulus for Milk Ejection:

• Actions like calf suckling or preparing the cow for milking generate sensory impulses.
• Impulses reach the hypothalamus in the brain.
• Hypothalamus triggers release of oxytocin from the pituitary gland.

Oxytocin Effect:

• Oxytocin reaches the udder within a minute.
• Causes contraction of myoepithelial cells around alveoli.
• Contraction forces milk into collecting ducts and gland cistern, ready for removal.

Duration of Oxytocin Effect:

• Oxytocin's effectiveness lasts for only 3-5 minutes.
• Milking should be completed within this time.
• Stress or disturbances release adrenaline, partially or completely blocking milk ejection.

Composition of Milk

Major Constituents:

• Water, fat, protein, lactose, and ash (minerals).
• Minor constituents include phospholipids, sterols, vitamins, enzymes, and pigments.

Average Chemical Composition (Cow vs. Buffalo):

• Water: 86.6% (Cow) vs. 84.2% (Buffalo)
• Fat: 3.5-5.5% (Cow) vs. 6.0% (Buffalo)
• Protein: 3.1-3.6% (Cow) vs. 3.9% (Buffalo)
• Lactose: 4.5-5% (Cow) vs. 5.15% (Buffalo)
• Ash: 0.7-0.8% (Cow) vs. 0.8% (Buffalo)

Changes in Milk Composition

With Age and Lactation:

• Fat percentage increases up to three years, then declines slightly.
• Solids other than fat decline more with age.
• Total protein remains stable, but casein decreases while whey proteins increase.

After Parturition:

• Fat percentage high in the first few weeks, then declines for 3-4 months.
• Fat content influenced by cow's condition at parturition.
• Other solids increase around the second month, with a sharp rise near the seventh month.

Milk Lipids:

• Comprise triglycerides, phospholipids, cholesterol, fat-soluble vitamins, squalene, free fatty acids, and more.
• Fat globules are small, with an average size of 3-4 micrometers.

Milk Minerals:

• Major minerals include calcium, phosphorus, sodium, potassium, and chlorine.
• Smaller amounts of magnesium and sulfur.
• Trace amounts of other minerals like aluminum, boron, copper, iron, etc.

Vitamins in Milk:

• All known vitamins present.
• B-complex vitamins synthesized by rumen microorganisms.
• Vitamins A, D, and E dependent on diet; Vitamin K synthesized in rumen and intestines.

Contaminants in Milk:

• Drugs, antibiotics, and substances in feed can pass into milk.
• Contaminants cause off-flavors and odors in milk.
• Volatile substances introduced through the lungs have a more noticeable impact on milk quality than those ingested.