[Optimal zinc requirement of lactating dairy cows based on various dose-response relationships]

Dose-response relationships were established by multiple regression analysis of data from a zinc depletion-repletion study conducted with five first-lactation cows in order to estimate optimum zinc requirement. Zinc supply in the various experimental periods ranged from 6 to 436 ppm in the dry matter of the semisynthetic diet. The response criteria were the zinc concentrations in serum and milk and the activity of the alkaline phosphatase in serum. They correlated best with the reciprocal of the dietary zinc concentrations. Serum zinc also showed a close negative relationship to daily milk production (9-24 kg) and milk zinc to stage of lactation (2-21 weeks post partum). The response of the activity of the alkaline phosphatase, which was sufficiently described by simple regression to the inverse of dietary zinc concentration, was limited to a distinctly lower zinc supply range than the response of the serum and milk zinc level. The optimum zinc requirement of the cows was estimated to have been met with about 35 ppm Zn in the ration on the basis that 90% of the maximum response of the serum and milk zinc concentration was reached at this supply level. In the practical feeding situation, dairy cows should, however, be allowed 50 mg zinc per kg dry matter intake according to current recommendations in order to make certain that the physiological capacity of milk to bind zinc is sufficiently fulfilled even at highest milk production.