Influence of High Levels of Dietary Zinc on Zinc in Milk, Performance and Biochemistry of Lactating Cows

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Abstract
Twenty-four lactating dairy cows were fed either a) 0, b) 500, c) 1,000, or d) 2,000 ppm of supplemental zinc (as ZnO) in the concentrate portion of the diet for 6 weeks. Silages were the only source of forage and constituted 43% of dry matter intake. Average Zn contents of the total dry matter of the four groups were: a) 44, b) 372, c) 692, and d) 1,279 ppm. The level of supplemental Zn did not significantly affect milk production; FCM production; fat, SNF, protein, or Mg content of milk voluntary forage dry matter intake; body weight changes; blood hemoglobin; packed cell volume; blood serum lactic dehydrogenase; blood serum alkaline phosphatase; or apparent animal health. The addition of the supplemental Zn materially increased the Zn content of the milk. However, the higher levels had progressively less effect with the milk from cows given 1,000 ppm supplemental Zn containing as much Zn as those fed 2,000 ppm. The average ppm of Zn in the milk of each of the four groups was: a) 4.2, b) 6.7, c) 8.0, and d) 8.4. The blood plasma Zn concentration increased with each increase in supplemental Zn. These data suggest that the udder is discriminating against Zn at the higher dietary and blood levels. Recoveries of Zn in the milk as percentages of dietary Zn were: a) 12.5, b) 2.2, c) 1.6 and d) 0.8 for those fed the four levels of Zn.