The first sign of a cobalt deficiency in cattle is depressed appetite. Normally, iodine must be supplemented to a feedlot diet. This compound is added for its red color and not for its iron content since iron oxide is very low in bioavailability. Other iron sources are needed if there is a deficiency. Ferrous sulfate is high in bioavailability while ferrous carbonate is moderate in bioavailability.
Most selenium compounds are quite volatile.
Have a good air control system and use a gas mask when handing and mixing concentrated selenium pre-mixes. Bulls and steers may have more problems with a zinc deficiency than heifers and some strains of Holstein-Fresian cattle will have higher zinc requirements.