Output of selenium in milk, urine, and feces is proportional to selenium intake in dairy cows fed a total mixed ration supplemented with selenium yeast.

Fifteen rumen fistulated Holstein cows in late lactation and fed a total mixed ration offered ad libitum were supplemented with Se yeast to provide 0, 11, 20, 30, or 42 mg of supplemental Se/day to test the hypothesis that amounts of Se secreted in milk, excreted in urine and feces, and apparently retained in tissues would increase in direct proportion to Se intake. One-half of the yeast supplement was placed directly into the rumen through the fistula of each cow just before milking in the morning and again in the evening, and estimates of average daily excretion of Se were made using total collections of urine and feces from 25 to 31 d after treatments commenced. Amounts of Se secreted daily in milk and apparently retained in tissues increased linearly with average daily intake of Se. The amount of Se excreted in feces and total excretion of Se in urine plus feces increased curvilinearly with Se intake, such that proportionately less Se was excreted as the amount of Se fed increased. On average, total Se excretion accounted for 66%, Se secretion in milk accounted for 17%, and Se apparently retained in tissues accounted for 17% of total Se intake by cows. Thus, in herds fed large amounts of Se yeast, most of the Se will be excreted and retained on-farm. High concentrations of Se will be found where urine and feces accumulate (e.g., yards and effluent ponds), and effluent management practices must be tailored to avoid environmental issues.