Developing Integrated Herbicide Strategies With the WSU Herbicide Resistance Screening Program

Contributed by Marija Savic, Washington State University

The Herbicide Resistance Screening Program is an integral part of the weed science program at WSU and a useful farmer-oriented service for developing integrated herbicide strategies. The goal of the program is to help farmers determine if the lack of efficacy of herbicides in their fields is due to heritable traits in the weeds (that is, resistance).

Since 2013, we have received and processed between 30 and 50 samples each year. Major troublesome grass species in dryland cropping systems include downy brome (cheatgrass), wild oat, Italian ryegrass, and jointed goatgrass. Many of these submitted grass samples are found to be resistant to one or two different herbicide site of action groups. Frequently received broadleaf species include mayweed chamomile (dog fennel) and prickly lettuce.In the past several years, the majority of the resistant species identified appear to be grasses – downy brome, wild oats, and Italian ryegrass. The problem arises from the fact that these plants belong to the same botanical family as wheat–Poaceae (grass family)–which means that herbicides selective to wheat that will control grass weeds are limited, and the genes conferring tolerance to the wheat are likely present within these weeds. A limited number of active ingredients for our crop rotations create ideal conditions for resistance to develop in weeds. Downy brome and wild oat are mostly found to be resistant to Group 1 (ACCase inhibitors), Group 2 (ALS inhibitors), or often both Groups. Occasional samples are glyphosate resistant. Groups 1 and 2 herbicides are commonly used to manage grassy weeds in cereals or other crops.

Figure 1. Downy brome genotypes collected in 2022 appear to be resistant to Maverick and PowerFlex HL and sensitive to Roundup.

Resistance to Group 2 in downy brome is widely spread across eastern Washington in wheat cropping systems. It is not unusual for downy brome to develop a cross-resistance–resistance to two or more herbicides from the same Group. For example, we found that downy brome resistant to imazamox is often also resistant to mesosulfuron-methyl (Osprey) and propoxycarbazone-sodium (Olympus) but sensitive to pyroxsulam (PowerFlex HL). The cross-resistance development can make it difficult to develop a management strategy. Some of the Group 2 herbicides with notable and increasingly common resistance in grass weeds in eastern Washington are pyroxulam (PowerFlex HL), imazamox (Beyond), sulfosulfuron (Outrider), mesosulfuron-methyl (Osprey), and propoxycarbazone-sodium (Olympus), while Group 1 herbicides include pinoxaden (Axial XL) and quizalofop-P (Assure II or Aggressor). Mayweed chamomile and prickly lettuce samples we’ve screened are also mostly resistant to Group 2 herbicides since these herbicides are ideally used to selectively control broadleaf species in cereals. Every year the program receives a number of downy brome samples with suspected glyphosate resistance. However, only 3 of the 20 samples received in 2022 have been confirmed to have developed glyphosate resistance.

Part of Herbicide Resistance Screening Program is a large-scale project with the goal to map downy brome resistance in wheat growing regions of Washington. We are collecting downy brome seeds and screening samples from farms and natural areas to the main herbicide Groups used in wheat-fallow systems.

For additional information, contact Marija Savic (marija.savic@wsu.edu) or Dr. Ian Burke (icburke@wsu.edu).