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In two experiments, calves were fed milk replacer containing 40, 200, 500, 1000, or 5000 ppm Mn or 40, 200, 500, 700, or 1000 ppm Zn in DM, from 3 to 38 d of age, to estimate the minimum toxic concentrations of Mn and Zn. Starting at 1000 ppm Mn, weight gains and feed efficiencies were decreased slightly; none of the calves fed 5000 ppm Mn survived the 5-wk experiment. Liver and bile showed the largest increases in Mn concentration. In the Zn experiment, only at 700 and 1000 ppm Zn were weight gains, DM intake, and feed efficiency reduced. Largest Zn increases were in liver, kidney, and plasma. Thus, performance of the preruminant calves was not affected adversely by 500 ppm Mn or 500 ppm Zn in milk replacer, concentrations that are markedly higher than the NRC recommendations of 40 ppm Mn and 40 ppm Zn. However, Mn and Zn concentrations increased in some tissues, and toxicities might have arisen if the trial had been continued. Evidence was not obtained indicating that the calf benefits from Mn or Zn intakes above the NRC recommendations.

In vitro and in vivo experiments demonstrated that rumen bacteria were capable of metabolizing inorganic 75Se and incorporating the element into the microbial protein. The fixation of 75Se into bacteria in vitro was inversely proportional to the previous dietary intake of selenium by the host sheep. In sheep fed a purified diet low in selenium and vitamin E, selenium supplementation caused a marked alteration of the rumen microorganisms. Characterization of the 75Se-containing compounds in the rumen bacteria protein hydrolysates revealed the presence of 75Se-selenomethionine and 75Se-selenomethionine selenoxide.