CHLOROACETAMIDE RESISTANCE IN RIGID RYEGRASS (LOLIUM-RIGIDUM)

Metolachlor has been evaluated both as a herbicide for use in cultivat ed oats (Avena sativa L.) and for its potential as an alternative herb icide for the control of herbicide-resistant rigid ryegrass. Eight her bicide-resistant and two susceptible biotypes of rigid ryegrass were t ested for their susceptibility to metolachlor. Response to metolachlor was determined both in soil and an agar germination medium. The LD50 for metolachlor in agar for a susceptible biotype (VLR1) was 0.13 muM. Five biotypes, SLR5 (6.9 fold), SLR31 (5.2 fold), SLR10 (2.5 fold), N LR12 (2.1 fold) and VLR69 (1.9 fold), were cross-resistant to metolach lor when compared with VLR1. Relative response of the biotypes was sim ilar in both soil and agar, validating the use of an agar germination test to determine the susceptibility of rigid ryegrass biotypes to met olachlor. Biotypes cross-resistant to metolachlor also were cross-resi stant to alachlor (SLR5 6.7 fold, SLR31 5.9 fold, SLR10 2.4 fold, and VLR69 1.6 fold with the LD50 for VLR1 being 0.49 muM) and propachlor ( SLR5 7.2 fold, SLR31 7.2 fold, SLR10 3.0 fold and VLR69 2.5 fold with the LD50 for VLR1 being 0.47 muM) indicating that cross-resistance ext ends to other members of the chloroacetamide group. Cross-resistance t o chloroacetamides was observed in-biotypes that previously had been r eported as cross-resistant to other herbicides. In contrast, biotypes with limited herbicide histories were generally not cross-resistant to metolachlor. These results indicate that there is a high probability of chloroacetamide cross-resistance in populations of herbicide-resist ant rigid ryegrass.