Better Model, Better Weed Resistance Predictor

John DavisHerbicides

riceA better simulation model will give producers a better predictor of when weeds will develop herbicide resistance. An article in the journal Weed Technology talks about how the model will predict when rice fields’ biggest scourge, barnyardgrass, will become resistant to two widely used herbicides – propanil and quinclorac.

[The] model that can analyze the simultaneous evolution of resistance to herbicides that inhibit the acetolactate synthase (ALS) and acetyl-CoA carboxylase (ACCase) enzymes. The model assumes the use of Clearfield rice, a commercial brand of rice cultivars that is resistant to certain ALS-inhibiting herbicides.

Using data from three locations in the rice-growing Mississippi Delta region of eastern Arkansas, the computer simulation model creates weed management scenarios across 1,000 hypothetical rice fields. The model takes into account three stages of growth—dormant seedbank, emerged seedlings, and mature plants—and extends over a 30-year period.

The simulation shows that if an ALS-inhibiting herbicide was used alone in three annual applications, weed resistance would develop within 4 years. Weed management that used both ALS- and ACCase-inhibiting herbicides reduced the risk of barnyardgrass developing a resistance to the ALS-inhibitor, but found a higher risk for the weed developing resistance to the ACCase-inhibitor by year 14. Resistance to both of these herbicide types when used together was predicted by year 16.

An important discovery during the study showed that if farmers don’t stop using an herbicide soon after resistance evolves, the resistance can be accelerated for the next herbicide alternative employed even if it provides a different mode of action. The researchers suggest focusing on minimizing the weed seedbank and diversifying management techniques with timely applications of herbicide.