Herbicide Cross-Resistance in Annual Ryegrass (Lolium rigidum Gaud)

Currently, biotypes of some 100 weed species have been documented as herbicide resistant. In all but two cases the resistance is confined to an individual chemical type and mode of action. Cross-resistance to a range of dissimilar herbicides has now been documented in biotypes of the prominent grass weeds Alopecurus myosuroides and Lolium rigidum . Biotypes of Lolium rigidum are cross-resistant to a range of aryloxyphenoxyproprionate, cyclohexanediones and sulfonylurea herbicides and the dinitroaniline herbicide trifluralin. Studies to identify the mechanism of cross-resistance in Lolium rigidum indicate that resistance is not the result of any barrier to herbicide uptake or translocation. There is no major difference in the sensitivity of acetyl coenzyme-A carboxylase, the enzymic target site for the aryloxyphenoxypropionate and cyclohexanedione herbicides. Similarly there are no differences in acetolactate synthase, the target site of the sulfonylurea herbicides. Activity of the conjugating enzyme, glutathione-S-transferase, is similar in the resistant and susceptible biotypes. Cross-resistance in Lolium rigidum may be related to enhanced metabolic de-toxification of a range of dissimilar herbicide types.