Developing strategies for reducing the risk from herbicide-resistant wild-oats (Avena spp.)

Abstract 

The objective was to characterise resistance in wild-oats (Avena spp.) and to develop strategies for the prevention, containment and control of herbicide-resistant populations. Laboratory, glasshouse, outdoor container and field experiments were conducted and this integrated approach proved very successful.

Resistance was confirmed on 65 farms in 19 counties of England, showing that resistance is widespread geographically. Resistant wild-oats have not yet been recorded in Wales, Scotland or Northern Ireland. Resistance was shown to occur in both species of wild-oats - Avena fatua (common wild-oats) and Avena sterilisssp. ludoviciana (winter wild-oats).

Cross-resistance studies showed that some wild-oat populations were only resistant to "fops" (aryloxyphenoxypropionates, e.g. fenoxaprop, fluazifop-P-butyl) but not to "dims" (cyclohexanediones, e.g. tralkoxydim, cycloxydim) or any other herbicide group. Other "fop" - resistant populations showed crossresistance to the "dim" tralkoxydim and to other herbicides, such as imazamethabenz-methyl and flamprop-Misopropyl. No resistance was detected to tri-allate, isoproturon, difenzoquat or cycloxydim.

Associated MAFF funded research identified two mechanisms of resistance. Enhanced metabolism results in herbicide detoxification and tends to confer partial resistance, but plants may be cross-resistant to several herbicide groups. Surprisingly, five annual applications of herbicides to which partial resistance already existed, did not result in any increase in level of resistance in a population (T/11) with enhanced metabolism. However, these studies also showed that resistance did not decline when herbicide use was reduced. Target site resistance blocks the site of herbicide activity and only affects "fop" herbicides in populations studied to date. Resistance tends to be absolute, and may develop faster than enhanced metabolism.

Correct timing and dose is critical to maximise control of partially resistant wild-oats. Full rates applied early (2- 3 leaves) were shown to be capable of good control. Later applications gave poorer control, especially when reduced rates were used. As dose was reduced and timing delayed, the risk of inadequate control increased, especially on resistant populations. Susceptible populations were well controlled, but highly-resistant populations were poorly controlled regardless of dose or timing.

The key "take home" messages are: monitor wild-oat herbicide performance and identify causes of poor activity; if resistance is suspected, have seed samples tested to establish the type and severity of resistance; contain the resistance problem and prevent it spreading; do not use "fop" or "dim" herbicides as the only means of grass-weed control in consecutive crops; consider herbicides with alternative modes of action; if resistance is suspected or confirmed, apply post-emergence herbicides when weeds are small (1-3 leaves) to maximise control of partially resistant wild-oats.