ATRAZINE AND CYANAZINE ACTIVITY IN ONTARIO AND MANITOBA SOILS

Abstract

A growth room bioassay, with barley (Hordeum vulgare L. ’Fergus’) as the indicator plant, established that atrazine and cyanazine were nearly twice as active in five Ontario soils as in four Manitoba soils examined. In most soils, a higher rate of cyanazine application was required to establish activity equal to atrazine. Adsorption and desorption studies indicated that up to 85% of the initial application of either herbicide was potentially available in the solution phase in some Ontario soils, whereas only 20–50% was available in the solution phase of Manitoba soils at equilibrium. This two- to fourfold difference in sorption could be a major reason for the large differences between Ontario and Manitoba soils in either atrazine or cyanazine activity. Atrazine degradation was negligible in non-sterile soil over a 3-wk period, whereas over half of the cyanazine applied was degraded to two metabolites in this same time period. Thus, differences in degradation rates could contribute to the lower activity of cyanazine compared to atrazine observed in most soils.