Effect of dietary vitamin C on concentrations of ascorbic acid in plasma and milk.

The addition of exogenous ascorbic acid to milk reduces the development of oxidized flavor. This experiment was conducted to determine whether feeding ascorbic acid to cows influenced vitamin C concentrations in milk. Thirty-two midlactation Holstein cows were fed a basal diet of 56% forage, 36.6% concentrate, and 7.4% roasted whole soybeans (dry basis) that was top-dressed with a premix that provided 0, 3, 16.5, or 30 g/d of L-ascorbic acid (provided by ascorbyl-2-polyphosphate) for 28 d. Supplementation had no effect on milk yield or composition or dry matter intake. Treatment linearly increased plasma concentrations of ascorbic acid (19.8, 22.3, 21.9, and 25.7 mumol/L, respectively) but had no effect on plasma dehydro-L-ascorbic acid (DHAA). Concentrations of ascorbic acid (103.7 mumol/L) and DHAA (9.5 mumol/L) in milk were not affected by treatment. Secretion of ascorbic acid into milk appeared to follow Michaelis-Menton kinetics, with a Vmax of 3.92 mmol/d and a Km of 3.59 mumol/L. Milk flavor as evaluated by a panel was normal for all samples after 1 d of storage. After 7 d of storage, the average flavor score was 2.5 (moderate oxidized flavor), but no differences among treatments were observed. Supplemental dietary vitamin C did not increase vitamin C concentration in milk, probably because the maximum potential secretion of the vitamin was occurring in unsupplemented cows.