Gastrointestinal Sites of Absorption and Endogenous Secretion of Zinc in Dairy Cattle

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Abstract
Three mature dairy cows and 3 calves ranging in age from 5 to 12 months were dosed orally twice daily for 10 days at feeding time with Zn65 plus Ce144Cl3 and Cr2O3 as non-absorbed markers. They were fed hay and concentrate in pre-determined amounts that would be consumed within an hour. In addition, 2 new-born calves fed whole milk received the radioisotopes 7 days. At the end of the dosing periods, the animals were slaughtered and the gastrointestinal tracts removed and divided into the rumen, omasum, abomasum, 6 equal sections of small intestine, cecum, two sections of colon, and the rectum. A multichannel gamma spectrometer was used to determine Zn65 and Cel44 in the contents of each segment. Results were expressed as the ratio of the percents of daily administered Zn65 to Cel44 or Cr2O3. Absorption or excretion of Zn from a particular segment was indicated by a decrease or increase of the Zn/indicator ratio in relation to the preceding segment. Zinc was absorbed from the abomasum and lower small intestine whereas absorption or excretion below the cecum was negligible. Secretion of zinc took place in the upper small intestine. Apparent net absorption of Zn65 administered daily averaged 12% in mature cows, 20% in older calves, and 55% in week-old calves. Values determined using Ce144 and Cr2O3 as markers were in good agreement.