Time-dependent effect of selenium supplementation on the relationship between selenium concentrations in whole blood and plasma of sheep

The ratio between selenium (Se) concentrations in pairs of plasma and whole blood of sheep after commencement of Se supplementation was calculated from data obtained from 10 independent investigations. The diets in the studies consisted of a variety of ingredients, from the individual feeding of feedlot diets to grazing trials on Se-deficient pastures. Means from 51 treatments, derived from 179 collections at various stages after commencement of Se supplementation, were used to calculate the ratios. In one study it was found that plasma Se concentration reached steady-state within 4 days of commencement, while Se in whole blood reached steady-state only at the collection 60 days after supplementation commenced. Using the 179 pairs, the ratio of whole blood and plasma stabilized at about 50 days after commencement when inorganic Se was supplemented, and at about 60-70 days when the Se was in the organic form. The ratios stabilized when they were between 2:1 and 3:1, and remained practically constant from 50 days post onset of supplementation, when plasma Se constituted 0.445 of whole blood Se concentration for the inorganic Se source and 0.410 when the Se source was organic. It was concluded that after commencement of Se intake, plasma Se concentration remains relatively constant from within 4 days of onset, while whole blood Se concentration apparently reaches steady-state only after approximately 50 days. Before 50 days, whole blood Se seems not to be reliable to predict the Se status of an animal. In a situation where Se intake is stable and Se requirements do not fluctuate, a bloodplasma ratio of between 2:1 and 3:1 would be a guideline to indicate that whole blood Se and plasma Se concentrations could both be used to assess the Se status of sheep.