The idea that dairy cows produce milk continuously is a common misunderstanding. A cow’s milk production is not a constant state but is directly linked to the lactation period, a biological cycle involving the secretion of milk from the mammary glands. This cycle always begins with one specific event and includes distinct phases of high production, declining production, and a necessary rest period.
The Biological Necessity of Calving
Milk production is fundamentally a reproductive function, initiated by calving. Hormonal shifts before and after delivery trigger lactogenesis, the initiation of milk synthesis. A sharp drop in progesterone combined with rising levels of prolactin and estrogen signals the mammary gland to begin producing milk.
Immediately following calving, the cow first produces colostrum, a thick, yellowish substance. This substance is rich in immunoglobulins and growth factors, essential for transferring passive immunity to the newborn calf. Since the bovine placenta prevents the transfer of protective antibodies during gestation, the calf must absorb them from the colostrum shortly after birth. Once the colostrum phase is complete, the cow transitions into producing marketable milk.
Phases of the Milking Cycle
The active period of milk production is tracked using the lactation curve, which charts daily milk yield over time. A typical milking cycle is managed to last approximately 305 days before it is intentionally halted. Production increases rapidly following the colostrum phase, reaching a peak yield usually between 40 and 70 days after calving.
During this early lactation period, the cow often experiences negative energy balance because the energy demand for milk production exceeds the energy she consumes through feed. After reaching its peak, milk volume declines for the remainder of the 305-day cycle. This natural decline occurs because the number and activity of the mammary secretory cells slowly decrease. The goal of management is to slow the rate of this decline, known as increasing the persistency of lactation.
The Dry Period
Toward the end of the 305-day milking cycle, the dairy cow is intentionally “dried off,” meaning milking is stopped. This cessation of production marks the beginning of the dry period, a required rest phase lasting between 45 and 60 days before the next calf is born. This period is necessary for two primary reasons: the regeneration of the udder tissue and the growth of the developing fetus.
During this rest, the mammary gland undergoes involution, allowing the secretory cells to repair and rejuvenate in preparation for the next lactation. Skipping the dry period can result in a significant reduction in milk yield, often 20 to 25% less, in the subsequent cycle. The dry period also provides time for the cow to replenish body reserves depleted during the intense energy demands of early lactation.