Following reports of poor weed control, the effectiveness of repeated applications of carbetamide and propyzamide in controlling Lolium rigidum Gaud. (annual ryegrass) was examined under field conditions. Repeated applications of carbetamide or propyzamide were less effective in controlling L. rigidum than an initial application. A single application of carbetamide or propyzamide was sufficient approximately to halve the efficacy of a subsequent treatment with the same herbicide. Laboratory incubations of soil linked the poor performance of carbetamide upon 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.
