JOINT ACTION OF SULFONYLUREA HERBICIDES AND MCPA

The joint action of mixtures of the sulfonylurea herbicides chlorsulfu ron, metsulfuron and tribenuron and MCPA as a dimethylamine salt was a ssessed on Stellaria media L. and Lamium purpureum L. Two statistical analyses, and appropriate experimental designs, were used-the additive dose model (ADM) and the principle of parallel dose-response curves. The ADM analyses revealed that mixtures of the sulfony-lureas and MCPA were less potent than expected on Stellaria while the parallel-line a ssay analyses showed that activity of the sulfonylurea herbicides on S tellaria and Lamium was reduced in mixtures with MCPA. The antagonism was found to be dependent on the ratio of the two herbicides. Activity decreased with increasing MCPA ratios in the mixtures. Sequential spr aying and selective placement of droplets revealed that MCPA only anta gonized the sulfonylureas when the herbicides were in physical contact with each other.The benefits of applying both statistical methods whe n studying the joint action of two herbicides both possessing activity on the test plants are discussed.