Response of growing cattle to supplementation with organically bound or inorganic sources of selenium or yeast cultures

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Abstract
Bioavailability of organically bound and inorganic Se sources differ, the organic form being absorbed more efficiently. Dairy and beef calves (250 kg initial weight) were given a low Se diet alone or supplemented with sodium selenite (1 mg Se d−1; inorganic Se), a Se-enriched yeast (1.5 g yeast d−1 providing 1 mg of Se; organic Se), a live yeast culture (1.5 g d−1) or autoclaved yeast culture (1.5 g d−1). The latter treatments were included to determine the response to yeast per se. There were no treatment effects on rate of weight gain or efficiency of feed conversion by the calves. The Se souces had no effect on digestibility of the diet by sheep. The inorganic Se supplement increased the level of whole blood Se and glutathione peroxidase (GSH-Px) activity at a linear rate with time up to 3 mo from the start of supplementation. The effect of organic Se was sustained for 4 mo and both indices of Se status in blood were higher (P < 0.01) for calves fed the organic Se than for those fed inorganic Se after 4 mo (124 vs. 96 units of GSH-Px activity g−1 haemoglobin and 141 vs. 102 μg Se L−1 of whole blood). The values near the beginning of the trial were higher (P < 0.01) for Holstein calves than for beef calves but both groups showed the same response to source of Se. Subsequently, levels declined for Holstein and increased for beef calves given diets without supplemental Se. The results showed that Se supplementation for 4 mo had no effect on animal performance; that more than 4 mo are required to fully deplete or replete blood Se and GSH-Px; and, that supplemental organic Se gives higher (P < 0.01) blood Se and GSH-Px activity than does inorganic Se. Key words: Organic selenium, calves, glutathione peroxidase

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