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30 MAYLAND ET AL. Linseed oil meal has been found effective in reducing the toxicity of Se in rat diets (Palmer et al., 1980). The protective substances were cyanohydrin glycosides, which upon hydrolysis yield HCN. Conceivably, cyanide released from the glycosides might react with some of the Se compounds to form selenocyanates, which could be excreted by the animal. Therefore, excess levels of Se can be detoxified by feeding or injecting cyanide (Palmer & Olson, 1979), since Se and cyanide tend to counter the toxicity of the other. FECAL AND URINARY SELENIUM Urine is the primary route of Se excretion by monogastric animals, regardless of whether the Se is given orally or injected. The main route of Se excretion in ruminants, though, is a function of the method of adminis- tration and the age of the animal (NAS-NRC, 1983). When Se is ingested by ruminants, most of it is excreted in feces. In contrast, Se that is injected either intravenously or subcutaneously into ruminants is excreted mostly in urine. Lambs, and presumably calves, that have not developed rumen func- tion can excrete 66 to 75% of the orally ingested Se in the urine. Since most exogenous fecal Se is likely Se that has been reduced to an unavailable form such as elemental Se (Langlands et al., 1986), rumen organisms undoubted- ly contributed to this age effect. Nearly all of the Se excreted in the feces of ruminants is in an unavaila- ble form and very little is available for uptake by plants. Peterson and Sped- ding (1963) showed that <0.3% of the Se taken up by three pasture species originated from the Se contained in sheep manure during a 75-d study. The manure was obtained from sheep dosed orally with isotopically labeled selenite. Trimethylselenonium ion (TMSe+) is an important urinary Se metabo- lite. When added to nutrient solutions, TMSe + was readily absorbed and translocated to leaves and stems, but not to the grain of wheat (Olson et al., 1976a). However, large differences were observed in Se uptake by barley, wheat, and alfalfa when TMSe + was applied in a soil-pot study in the green- house. The authors noted that very little of the Se from TMSe + was ab- sorbed by plants, and some of the absorbed TMSe+ was even lost to the atmosphere through volatilization from the plant. Therefore, TMSe + ex- creted in animal urine likely contributes little biologically active Se to plants. A portion of the Se added to soil as TMSe + was also biologically volatilized, and this loss was increased by liming the soil at rates to increase the pH from 5.45 (initial) to values of pH 7.05 (Olson et al., 1976b). In ad- dition, 30 to 50% of the Se added as TMSe + to several different soils was sorbed to the soil particles during a 21-d period. The biologically inactive TMSe + in urine plus the stimulation of plant growth by the N and S added in the form of urine may explain the lowered Se contents in grass growing on urine patches (Joblin & Pritchard, 1983).