Methemoglobinemia occurs when nitrite is absorbed from the rumen into the blood of the animal. Ferrous iron (Fe2+) in hemoglobin is transformed into ferric iron (Fe3+), rendering the hemoglobin molecule (now called methemoglobin) incapable of transporting oxygen to the tissues (Ozmen et al., 2005). Nitrite is formed in the rumen as an intermediate in the reduction of nitrate to ammonia. In animals not previously adapted to nitrate in their diet, the reduction of nitrate to nitrite occurs at a higher rate than the reduction of nitrite to ammonia, resulting in an accumulation of nitrite in the rumen and subsequent absorption (Bruning-Fann and Kaneene, 1993).