Zinc concentration and distribution in mammary secretions of peripartum cows.

Mammary secretions were sampled from peripartum cows and analyzed for Zn concentrations. Subsamples of these secretions were fractionated into casein and whey, and the distribution of Zn was determined. Concentration of Zn was greater in the colostrum than in either prepartum mammary secretions or in milk. The high colostral Zn was reflected by high concentrations of Zn in both casein and whey fractions. Approximately 60% of the colostral Zn was in the casein fraction compared with nearly 90% of milk. Calcium, P, Fe, and Cu concentrations, but not Mn, also were elevated in colostrum. When milk was augmented with Zn, the added Zn was associated largely with casein. Although casein has a very large capacity for binding Zn, about half of the extra Zn in colostrum is found in the whey fraction.