Effect of copper and cobalt addition on digestion and growth in heifers fed diets containing alfalfa silage or corn crop residues.

Copper and cobalt were added to the diet of heifers in excess of NRC recommendations for these minerals. In Experiment 1, the rate of DM disappearance from dacron bags placed in the rumen was increased by the additional dietary Cu and Co for alfalfa hay (14.6 vs. 8.4%/h) and corn cobs (5.8 vs. 3.1%/h) but did not affect the DM disappearance rate for corn crop residue silage (3.6 vs. 3.4%/h). In Experiment 2, additional Cu and Co increased DM disappearance rate of corn crop residue silage (6.2 vs. 3.4%/h) but did not influence the rate of alfalfa hay (8.6 vs. 7.6%/h). In Experiment 3, a random design with a 2 x 2 factorial arrangement was used. The diet consisted of 45% corn crop residue silage, 35% alfalfa silage containing 60 or 40% DM, 18% high moisture corn, and 2% vitamin and mineral mix with or without added Cu and Co, respectively. Each of the four diets was group-fed to 9 light and 11 heavy Holstein heifers. Growth rate was lower with Cu and Co supplementation, but DM digestibility was improved by the additional Cu and Co. These results indicate that addition of Cu and Co above the NRC requirements may aid in digestion of low quality forages.