REDUCED EFFICACY AND ENHANCED DEGRADATION OF CARBETAMIDE AFTER REPEATED APPLICATION IN AUSTRALIA

Following reports of poor weed control, the effectiveness of repeated applications of carbetamide and propyzamide in controlling Lolium rigi dum Gaud. (annual ryegrass) was examined under field conditions. Repea ted applications of carbetamide or propyzamide were less effective in controlling L. rigidum than an initial application. A single applicati on of carbetamide or propyzamide was sufficient approximately to halve the efficacy of a subsequent treatment with the same herbicide. Labor atory incubations of soil linked the poor performance of carbetamide u pon repeated application to an enhanced degradation rate. We conclude that the repeated application of carbetamide or propyzamide results in less effective weed control under field conditions and that this can be partially avoided by rotation between carbetamide and propyzamide.