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Research on the optimum dietary levels of manganese and magnesium for growing swine is limited. Johnson (1940) fed pigs diets containing 0.3 to 100 ppm Mn and observed no difference in growth rate. Similarly, Liebholz, Speer and Hays (1962) found that 0.4 ppm Mn in the diet of baby pigs was sufficient to support maximum growth. A level of 4,000 ppm Mn was not toxic but growth rate of the pigs was reduced.