Effect of SO4‐S Fertilization on Se Concentration of Alfalfa ( Medicago sativa L.) 1

Selenium is not required for plant growth, but is necessary for the prevention of white muscle disease and other low‐Se related animal disorders. Areas in the Pacific Northwest that produce forages low in Se are nearly identical to those known to be S deficient. While SO 4 ‐S has been shown to inhibit SeO 4 ‐Se uptake by plants, the effect of S fertilization on Se uptake by plants growing on low‐Se soils where other forms of Se may exist has not been evaluated. This was examined on eight alfalfa ( Medicago sativa L.) experimental sites where S fertilization was a variable in southern Idaho. Yields were measured, and plant samples were taken from the first harvest of alfalfa (0.1 bloom) and analyzed for total Se and S, and Se uptake. Sulfur fertilization significantly reduced forage Se concentrations at four of the eight experimental sites. Forages at seven sites contained less than 0.1 ppm Se before S fertilization, which was further reduced by the S fertilization. The decrease in Se concentration mainly reflected a dilution effect caused by a growth response to the S fertilization. No direct relationship was apparent between forage Se and S levels. This study showed that S fertilization of S‐deficient, low‐Se soils to increase forage production may increase the incidence of Se deficiency in animals. Thus ranchers, cattlemen, and agronomists should become aware of this potential problem and provide protective measures against Se deficiency in their livestock.