New insights on an old pest: Six versions of downy brome ID’d in Northwest

It may all look the same, but it turns out one downy brome plant can be very different from another.

While other weeds, like prickly lettuce, show differences in their leaves from one plant to another, downy brome isn’t so showy. Instead, we now have evidence that the scourge of Pacific Northwest (PNW) wheat farming has considerable variability when it sets seed, evidence that may lead to better weed control strategies.

Downy brome was first reported in Washington in 1893. Although most of the expansion of brome in the West was accidental, it was deliberately sown at an experiment farm in Pullman in 1898 and subsequently sold across the West as a “100 day forage grass” in 1915. It has been a widespread problem in the PNW since 1925.

If downy brome emerges with wheat, it can cause yield losses approaching 90 percent. That’s why our research showing the weed’s variability in setting seed is so important.

Downy brome can reduce soil moisture to the permanent wilting point to a depth of 3.5 feet in fertilized fields. Because downy brome matures earlier than winter wheat, if it emerges with winter wheat in the fall, it often depletes soil moisture and nutrients when winter wheat is at a critical reproductive period—that is, when it’s heading. Thankfully, that seldom happens in our region’s fallow systems, where farmers plant much earlier than downy brome emerges.

More than 120 years after it was first identified in the state, downy brome is still a top pest. It continues to drive wheat farmers’ herbicide applications and university scientists’ weed biology research. Lately, we have been building on earlier research to understand more about how downy brome develops.

Two decades ago, Dan Ball, Oregon State University emeritus professor, quantified how long it takes for downy brome to produce mature seed after winter dormancy. He used growing degree days (GDD), and concluded about 1,000 GDD was a good estimate for when downy brome produces mature seed after coming out of winter dormancy. After that, it’s going to set seed no matter what it is treated with (GDD typically are associated with a base temperature, and for downy brome, we use 32 F). Unfortunately, as a decision tool, the GDD model wasn’t practical for planning herbicide applications.

Farmers typically treat winter wheat once in the spring with herbicides. It’s typical for both a grass and a broadleaf herbicide mixture to be applied simultaneously as weather permits. But timing of herbicide applications are often based on when fields are at their weediest and not on when herbicides will work best on a given species, including when a grass herbicide will work best on downy brome.

Results of our current approach have been variable. That includes variability in the efficacy of different herbicides and variability in timing of application. Both have combined to generate a lot of variability in downy brome control, along with some failures too.

It was complaints of inconsistency in response to herbicide applications that instigated our study to explore the variability in growth and development of downy brome. In work funded by the Washington Grain Commission and the Agricultural Research Service-supported Regional Approaches to Climate Change project, we collected downy brome from throughout the inland PNW.

Using next-generation genotyping tools—the same tools plant breeders use to breed and identify new wheat cultivars—along with old fashioned phenotyping techniques, we now know there are six types of downy brome in PNW wheat fields. Each of the six requires a different amount of GDD to produce mature seed.

We found one type begins flowering by 950 GDD on average, while others begin flowering as late as 1,350 GDD. In other words, an early type can be as much as 400 GDD ahead of a late-maturing version, which depending on the season, can be as much as four weeks! We also found that farmers are likely to have more than one type in a field.

We don’t currently use downy brome maturity to decide when to treat. Instead, we use growth and development of wheat (and opportunity), and wheat plants are frequently on a different schedule than brome. In other words, a spring herbicide application might fail to suppress downy brome that begins to flower by 950 GDD, but significantly suppresses downy brome that begins to flower by 1,350 GDD.

Our research is continuing, but in the meantime, keep your eye on that downy brome this spring. Although farmers are quite adept at identifying wheat as it approaches boot, it may pay dividends to watch downy brome just as closely. Treating the weed before it’s in the boot may be a better way to get a solid return on that herbicide investment.

• This story appeared in the November 2015 issue of Wheat Life, the official magazine of the Washington Association of Wheat Growers. Ian Burke is an Associate Professor of Weed Science in the WSU Department of Crop and Soil Sciences; Nevin Lawrence is a graduate student and Kendall Kahl is an associate in research in Crop and Soil Sciences.