PSIV-10 - Effects of chronic molybdenum exposure in drinking water or feed on molybdenum and copper status and production performance of gestating and lactating beef cattle consuming a low-quality forage diet.

The objective of this experiment was to evaluate the effects of Mo source (feed or water) on performance and mineral status of cows and calves fed a forage-based diet for two years (DM basis: 6.6% CP; 0.14% S, 6.7 mg Cu/kg, 2.4 mg Mo/kg). Fifty-four cow-calf pairs were stratified by cow age, BW, and liver Cu and Mo status, and randomly assigned to one of six treatments. Treatments consisted of: 1) Negative control (NC; no supplemental Mo or Cu); 2) Positive control [PC: NC + Cu (3 mg Cu/kg diet DM from CuSO₄·5H₂O)]; 3) NC + 500 µg Mo/L from MoNa₂O₄·2H₂O in drinking water (Mo 500-water); 4) NC + 1000 µg Mo/L of MoNa₂O₄·2H₂O in drinking water (Mo 1000-water); 5) Mo 1000-water + 3 mg Cu/kg diet DM from CuSO₄·5H₂O (Mo 1000-water+Cu); and 6) NC + 3 mg Mo/kg diet DM from MoNa₂O₄·2H₂O (3.0 Mo-diet). Cattle were allowed ad-libitum access to feed, water, and a protein supplement throughout the experiment. During the winter months animals were housed in three replicate pens per treatment and during the summer months animals were housed in separate pastures by treatment where cow and calf feed and water intake could be measured separately. Data were analyzed as a completely randomized block design. Throughout the experiment no signs of molybdenosis were observed. There was a treatment x time (P < 0.01) interaction for cow liver Cu concentrations. Cows not receiving supplemental Cu had lower (P < 0.01) liver Cu concentrations than cows receiving supplemental Cu. Cow and calf performance, calf mineral status, cow liver Mo, and plasma and milk Mo and Cu concentrations were similar across treatments. These data indicate that Mo supplemented in water or feed at concentrations used in this experiment had minimal impact on Cu status and overall animal performance.