Abstract
Twenty-four Angus steers were used to determine the effects of dietary zinc (Zn) level and source on Zn metabolism and ruminal volatile fatty acid (VFA) patterns. All steers were fed a low Zn diet (18.8 mg Zn per kg) for 42 days prior to assignment to dietary treatments. Treatments consisted of: (1) control (low Zn diet), (2) Zn sulfate (ZnSO 4), (3) Zn methionine complex (ZnMet) and (4) Zn glycine complex (ZnGly). The Zn sources were added to provide 20 mg of supplemental Zn per kg diet. A 5-day total collection of feces and urine was conducted beginning on day 14 of the study. Liver biopsy samples were obtained on days 0 and 42 for Zn analysis, and ruminal fluid was collected on day 42 for VFA and ruminal soluble Zn determination. Plasma samples were obtained on days 0, 2, 19, 21, and 42 for determination of plasma Zn and alkaline phosphatase activity. Urinary excretion of Zn was higher ( P<0.05) in steers supplemented with ZnMet or ZnGly than in controls. Apparent absorption and retention of Zn were not significantly affected by treatment, but tended to be higher in steers receiving ZnGly. Liver Zn concentrations were higher ( P<0.05) on day 42 in steers supplemented with ZnGly, compared to steers receiving control, ZnMet or ZnSO 4 treatments. Plasma Zn concentrations were higher ( P<0.05) for the ZnGly and ZnMet treatments compared to the control on day 42. Plasma alkaline phosphatase was not affected by treatment. Ruminal soluble Zn concentrations were higher ( P<0.05) in steers fed ZnMet compared to those fed the control or ZnSO 4 diets. Total VFA concentrations were higher ( P<0.05) in steers fed the control and ZnSO 4 treatments than in those receiving ZnGly or ZnMet. Steers supplemented with ZnMet had a higher ( P<0.05) molar proportion of propionate and lower ( P<0.05) molar proportions of butyrate and valerate than controls. Compared to the control treatment, valerate was the only VFA affected by ZnSO 4 or ZnGly supplementation. Results suggest that Zn from ZnGly was more bioavailable than ZnSO 4 or ZnMet. However, ZnMet supplementation resulted in the highest ruminal soluble Zn concentrations and altered ruminal VFA proportions to the greatest extent of the Zn sources evaluated.
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