Influence of Dietary Cobalt on Fiber Digestibility and Serum Cobalt and Cobalamin Concentrations in Horses

The 2007 Horse NRC reduced cobalt (Co) requirements from 0.1 ppm to 0.05 ppm in dietary dry matter, though preliminary research suggested increasing dietary-Co enhanced fiber digestion in horses. This study aimed to evaluate fiber digestion with varying dietary Co amounts, as well as to compare serum Co concentrations with dietary Co content. Four geldings (mean body weight [BW] 503 ± 37 kg) were studied in a 4 × 4 Latin square design consisting of four 5-week periods, each comprising a 2-week washout phase followed by a 3-week treatment phase. Supplementing 0.0, 5.6, 16.8, or 28.0 mg of Co daily resulted in total Co dietary concentrations of 0.06, 0.7, 2.0, and 3.0 ppm, respectively. Feces and urine were collected to evaluate fiber digestibility and Co balance. Co supplementation did not affect neutral detergent fiber, acid detergent fiber, or lignin digestibility. Co balance was negative for the control diet but positive for the other three treatments (<i>p</i> = 0.04). Serum cobalamin concentrations slightly decreased (6%) with increasing dietary Co intake (<i>p</i> = 0.003), while folate concentrations were unaffected. Serum Co concentration differed (<i>p</i> < 0.01) with a mean of 4.7 μg/mL for the high treatment and 0.8 μg/mL for the control. Results suggest that serum Co concentrations above regulatory limits are likely due not to higher levels of Co in fortified feeds, but rather to parenteral administration of Co solutions.