The effects of long-term molybdenum exposure in drinking water on molybdenum metabolism and production performance of beef cattle consuming a high forage diet

Fifty-four multiparous beef cows with calves were used to evaluate the effects of Mo source (feed or water) on reproduction, mineral status, and performance over two cow-calf production cycles (553 d). Cows were stratified by age, body weight, liver Cu and Mo status, and were then randomly assigned to one of six treatment groups. Treatments were: 1) Negative control (NC; basal diet with no supplemental Mo or Cu); 2) Positive control (NC + Cu; 3 mg of supplemental Cu/kg DM); 3) NC + 500 µg Mo/L from Na2MoO4·2H2O supplied in drinking water; 4) NC + 1000 µg Mo/L of Na2MoO4·2H2O supplied in drinking water; 5) NC + Mo 1000-water + 3 mg of supplemental Cu/kg DM; and 6) NC + 3.0 mg of supplemental Mo/kg diet DM from Na2MoO4·2H2O. Animals were allowed ad libitum access to both harvested grass hay (DM basis: 6.6% crude protein; 0.15% S, 6.7 mg Cu/kg, 2.4 mg Mo/kg) and water throughout the experiment. Calves were weaned at approximately 6 months of age each year. Dietary Cu concentration below 10.0 mg Cu/kg DM total diet reduced liver and plasma Cu concentrations to values indicative of a marginal Cu deficiency in beef cows. However, no production parameters measured in this experiment were affected by treatment. Results suggest that Mo supplemented in water or feed at the concentrations used in this experiment had minimal impact on Cu status and overall performance.