Climate Change and Downy Brome in Pacific Northwest Dryland Agriculture

 Q&A with Weed Scientist Dr. Ian C. Burke

By Doug Finkelnburg, Area Extension Educator, Cropping Systems, University of Idaho Extension

In the book “Advances in Dryland Farming in the Inland Pacific Northwest”, the common weed downy brome or “cheatgrass” is identified as potentially problematic for wheat producers as the climate changes. Downy brome is projected to head earlier in the season and expand its present occupied acreage. Such changes are happening concurrently to broader herbicide resistances being found in Pacific Northwest downy brome populations, a combination that puts increased pressure on weed managers. Curious how these issues interacted, I asked Dr. Ian Burke, Washington State University Weed Scientist and lead author of the Advances chapter “Integrated weed management” about how climate change and herbicide resistance will affect downy brome management.

Downy brome is a winter annual and facultative spring annual grass. It develops early, not only competing for nutrients, light and space but also typically dropping seed before harvest, avoiding removal with harvested grain. It is controlled with crop rotation, cultivation and fall and spring herbicides. In-crop herbicides for wheat are limited to Group-2/B ALS-inhibitors (pyroxsulam, sulfosulfuron, imazamox, etc.) with limited exceptions.

DF:  What challenges will earlier-heading downy brome populations in the wetter agro-ecozones pose for weed management in Pacific Northwest dryland farming?

 ICB: Earlier heading will alter the ability for a farmer to apply spring treatments prior to the brome setting seed. We currently see brome heading start in late April and proceed through May. If it advances a month, then we are will have active flowering in March! That’s all moot, though, as it is very likely that, by the time sufficient change has occurred, most of the brome will be resistant to post-emergence herbicides anyway.

DF:  At the time this chapter was written, the authors noted early-flowering downy brome populations resistant to Group-2/B ALS-inhibitors would be increasingly likely to spread from drier to wetter regions; basically from central Oregon and Washington to eastern Oregon, eastern Washington and northern Idaho. Does this remain the principal concern for herbicide resistance spread from downy brome populations related to climate change or are there other herbicide resistances occurring in these areas to add to the ALS-inhibitors?

ICB: Spread, as I think you are using it here and we may have used in our chapter, is probably not the right word. What we have found is that brome, which is an obligate selfing species (no pollen is shared among plants), has sufficient variation within fields to adapt. It’s routine to find early downy brome growing with late downy brome in the same field, often right next to each other. So ‘spread’ doesn’t have to happen, there’s already locally adapted brome. As climate change occurs, it’s likely to favor the earlier brome, and they’ll become the dominant type.

Will they also carry ALS resistance? It’s highly likely. With the CoAxium system increasing in use, we are likely to see a very rapid evolution of resistance to the herbicide used, quizalofop. And then there is glyphosate resistance, which is uncommon but likely increasing.

DF:  What are we continuing to learn about Pacific Northwest herbicide resistance and climate change? Where is the research focused today and how can someone learn more about this topic?

ICB: Herbicide resistance research is an ongoing component of my program, and Drew Lyon and I work to distribute our findings through the WSU Extension Dryland Cropping Systems Team’s Wheat and Small Grains website through reports, publications, and podcasts.

Climate change research has continued, although not so purposeful in my own program. I’m really interested in understanding how we can use the very adaptive nature of brome against it through rotation and selection. We know how to select for early types, and there are clearly late types, but can we come up with a system that allows us to limit a population to just late types, and then rotate in such a way to select against that population? Can we put the ability to test the brome for earliness or lateness into the hands of a farmer? These are some of the questions we are working on.

What have we learned? Earlier heading downy brome will shift herbicide application to a time when adverse weather is more likely, complicating effective applications. Herbicide resistance in downy brome is increasing, also complicating effective management as resistant populations occur with increasing frequency. Mitigation solutions may be found in research aimed at better targeting crop rotation, and integrated weed management in general, into more effective weed management tools. Weeds are weeds because they are superior adapters to changes in their environment. As the climate changes we must expect problematic weeds, such as downy brome, to adapt to that too and plan accordingly.