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Two hundred and four Holstein cows and heifers were randomly assigned to mineral supplement groups 30 d prior to expected calving. Supplement treatment groups were Cu, Mg, Cu plus Mg, and no mineral supplement. The total diet of supplemented groups averaged 15 mg/kg of Cu and .30% of Mg. Pastures consisted of bermudagrass, bahiagrass, and millet during the summer and oats and ryegrass mixture during the winter. Corn and sorghum silage were also fed. Blood samples were taken just prior to initiation of mineral supplementation and at wk 1, 2, 4, 6, and 8 postpartum. Hemoglobin and packed cell volume were determined and plasma was assayed for Cu and Mg. First service conception rates were 57% for the Cu plus Mg treatment and 27, 38, and 33% for treatments 1, 2, and 4, respectively. Ninety-two percent of the cows in the Cu plus Mg-group conceived by 210 d postpartum as opposed to an average of 75% for the other groups. Plasma Mg was different among cows grouped on a fertility basis and hemoglobin was correlated with days to conception. Plasma Mg was correlated with hemoglobin. Both were inversely related to postcalving infection and uterine involution. In summary, cows supplemented with both Cu and Mg showed improved fertility, whereas those supplemented with Cu or Mg alone did not.

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