Although the relationship between dairy cow nutrition and milk yield has been studied for more than a century, researchers have never pinpointed the mechanisms by which dietary nutrients determine the rates of milk synthesis in mammary glands.
Researchers at the University of Guelph, Ontario, Canada conducted a study to delve further into this complex relationship. Their work, published recently in the Journal of Dairy Science, created an intentional curbing of nutrients, and examined how the cows – and specifically their mammary tissue – responded.
In the study, 14 lactating Holstein cows were divided into 2 groups. Half of the cows were placed on an imposed fast with no feed access for 16 hours. The same group then was restricted to 60% of their previous TMR allocation for an additional 14 days.
The other half of the cows served as a control group with no feed restrictions and ad libitum access to the same TMR. The diet was a well-balanced ration containing approximately 45% dry matter.
Mammary biopsies and blood samples were collected from all cows at the end of the initial 16 hours. At the end of the 14 days, all cows were slaughtered and their mammary glands dissected.
Among the factors evaluated were dry matter (DM) and crude protein (CP) of the mammary tissue. While those typically sound like nutrition terms, they also have important bearing on mammary function.
DM refers to the non-water content of the tissue, including protein, carbohydrates, fats, and minerals. CP represents the total protein content of the tissue, which is necessary for milk protein synthesis.
The researchers were keenly interested in evaluating a specific signaling mechanism (rapamycin complex 1) that they believed “instructs” cells to change function based on systemic changes – in this case, decreased nutrition.
After the initial 16-hour fast, the restricted cows showed an acute increase in plasma non-esterified fatty acid (NEFA) concentration in their blood plasma. This indicates a faster mobilization of body fat reserves. In that same time window, milk production decreased precipitously, by about 14%.
Over the next 14 days, the restricted cows adapted to fewer nutrients by establishing a new baseline for milk production. Although their nutrients were cut by 40%, energy-corrected milk yield dropped by just 14% after the initial feed restriction, and stayed at that level as cows adapted to their “new normal.”
Dry matter in the mammary tissue of the restricted cows dropped 18%, and crude protein in mammary cells fell by nearly on third at 29%.
By the end of the two-week study, plasma NEFA concentrations in the restricted cows were no longer elevated, suggesting those cows had normalized milk synthesis and reached a new steady-state of nutrient flow at a lower milk yield.
After the initial fast, systemic signaling for protein synthesis, endoplasmic reticulum biogenesis, and cell proliferation in the mammary gland also quieted.
The researchers concluded that the initial, dramatic feed restriction created a nutritionally mediated downregulation in systemic signaling for a range of factors that support milk production. After establishing a new homeostatic setpoint in just 1-2 weeks, the restricted cows sustained milk yield at a decreased level from a reduced mass of mammary tissue.
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