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Absorption and Tissue Zinc Content in Lactating Dairy Cows as Affected by Low Dietary Zinc^{1, 2,}

University of Georgia, Athens 30602

Abstract

A High beet pulp, low-zinc diet (16.6 ppm) was fed without and with supplemental zinc (39.5 ppm total zinc) as ZnO to 10 first-lactation Holstein cows for 6 weeks. Following initiation of the low-zinc diet, cows made several adjustments within the first week, including a 50% increase in zinc absorption, a decrease in milk zinc and a decline in fecal zinc excretion. Zinc content of body tissues, based on analyses of samples which represented 84% of the total body (fresh weight) were 23.1 and 22.1 ppm for cows fed the control and low-zinc diets, respectively. Of 25 tissues analyzed, only rib cartilage and rumen wall from cows fed the low-zinc diet had significantly reduced zinc. Even though gastrointestinal (GI) tract digesta of cows fed the low-zinc diet contained considerably less zinc than controls, GI tract tissue zinc was not significantly different. Thus, the homeostatic control mechanisms of these cows were effective in preventing marked zinc depletion from their tissues.

¹ This study was supported in part by Public Health Service Grant No. AM–07367–NTN from the National Institute of Arthritis and Metabolic Diseases.

² The authors thank Kraft Foods Company of Garland, Texas. for the dried whole whey and Dawe's Laboratories, Inc., of Chicago, Illinois, for vitamins A and Da. Appreciation is extended to P. Randall Fowler for procuring the cows and to Tack C. Brown. Susan M. Fowler, Donald M. Futch, Albert L. Glenn. Jr., Sidney O'Steen, Kenneth R. Roberts, Ralph C. Watson, Jr. and Harley R. Young, Jr. for technical assistance.

³ Department of Dairy Science.

⁴ Department of Medicine and Surgery.

⁵ Agronomy Division, Cooperative Extension Service, University of Georgia.