Comparative effects of various dietary levels of Se as sodium selenite or Se yeast on blood, wool, and tissue Se concentrations of wether sheep

Abstract

The objective of this 60-week study was to determine the maximum tolerable level of selenium (Se) by feeding Se as sodium selenite or Se yeast at high dietary levels to wether sheep. Twenty-eight, 2-year-old, Rambouillet-crossbred wethers (62.3 ± 8.5 kg initial body weight) were utilized in a 2 × 4 factorial arrangement with 0.2, 20, 30, and 40 mg/kg dietary Se (as-fed) from sodium selenite or Se yeast added to a corn–soybean meal basal diet. Wethers were weighed at 8-week intervals and serum Se, whole blood Se, and wool Se were measured every 12 weeks. Samples of brain, diaphragm, heart, hoof, kidney, liver, and loin muscle, and serum samples for evaluation of albumin and enzyme activities were collected at the termination of the experiment. Wether body weight was affected by dietary Se level ( P < 0.05), source of dietary Se ( P < 0.05), and time ( P < 0.05). Average body weight decreased linearly ( P < 0.10) as dietary Se level increased, though most wethers gained body weight during the trial. Serum Se, whole blood Se, and wool Se concentrations were affected ( P < 0.05) by dietary level of Se and source of Se. Serum Se and whole blood Se ranged from 110 to 3922 μg/L and 392 to 6259 μg/L, respectively. Concentration increased in a quadratic fashion as dietary Se level increased ( P < 0.05). Wool Se ranged from 1.19 to 39.09 mg/kg and responded linearly ( P < 0.05) to increased dietary Se. Serum Se, whole blood Se, and wool Se concentrations from wethers receiving organic Se were higher ( P < 0.01) than those from wethers receiving inorganic Se. Selenium concentrations in brain, diaphragm, heart, hoof, kidney, liver, and loin muscle were affected ( P < 0.05) by dietary Se level, with higher Se concentrations generally observed in tissues from wethers receiving organic Se. Though Se concentrations in serum, blood, wool, and major organs at most times exceeded concentrations previously reported in livestock suffering from Se toxicosis, a pattern of clinical signs of Se toxicosis was not observed in this experiment. Histopathological, microscopic evaluation of liver, kidney, diaphragm, heart, and psoas major did not reveal definitive evidence of Se toxicosis in wethers on any dietary Se treatment. Wethers under our experimental conditions tolerated up to 40 mg/kg dietary Se for 60 weeks, though differences in Se source were observed. Contrary to previous thought, the range between optimal and toxic dietary levels is not narrow. The maximum tolerable level of Se, regardless of source, is much higher than the current estimate of 2 mg/kg diet.