Selenium in some grass pastures of Natal and fertilizer-induced Se uptake by ryegrass (Lolium multiflorum)

Abstract

Samples of irrigated ryegrass pasture and natural veld from various parts of Natal were found, with one exception, to lack the amount of Se normally considered sufficient (0.1 mg kg⁻¹) for grazing animals. No soil properties could be identified by multiple regression as causing such a deficiency. Ryegrass was grown in pots filled with a Kranskop topsoil collected from a selenium-deficient pasture. Sodium selenate, selenite (mixed evenly with the soil or placed in a localized application), Selcote (selenate-based, slow-release granules) and fly ash (containing about 10 mg kg⁻¹ Se) were factorially compared at Se rates of 0.5, 1 and 2 mg kg⁻¹ in the presence of gypsum and lime. Sodium selenate and Selcote (to a lesser degree) increased foliar Se in ryegrass over a 15-week period to levels normally considered toxic to livestock (>4.0mg kg⁻¹). Gypsum and lime decreased Se uptake by one and a half to three times. Selenite boosted foliar Se to adequate (>0.1 mg kg⁻¹) but not toxic concentrations. Fly ash produced adequate foliar Se only at toxic and economically prohibitive concentrations of ash (20% of the soil). Selenite would be safer than selenate-based fertilizers for rectifying Se deficiency in pastures, but would be needed in larger amounts and would therefore be more costly.