Glyphosate is the most widely used post-emergence herbicide in the region for several reasons.
- Glyphosate is non-selective. This means it can be used to control most weeds when they are not resistant or naturally tolerant.
- Glyphosate is relatively inexpensive – compared to other herbicides.
- It is effective. Glyphosate is a systemic (translocated) herbicide that moves from the treated foliage to other plant parts, including the roots. In this way, glyphosate kills annual and perennial weeds.
- Glyphosate has little or no soil residual effect. It is rapidly bound by clay particles in the soil, rendering it inactive.
However, glyphosate is being closely studied as a possible human carcinogen and, even though the studies are not clear yet, some countries have started to put bans on it. Regardless of a potential glyphosate toxicity that could ban glyphosate in the future, farmers in the region could also lose glyphosate due to resistance problems.
In the past four years, I have been working with residual herbicides to control Russian-thistle resistant to glyphosate, but last year I also started to look for other post-emergence, non-selective herbicides that growers could use as an alternative. Particularly, I am concerned about the finding of some downy brome (also known as cheatgrass) populations resistant to glyphosate in the region. In my opinion, the glyphosate resistance in downy brome will not spread as fast as the resistance in Russian-thistle because of the difference in the seed dispersion between both weed species. However, farmers should be proactive and stay ahead of the potential problem by alternating or mixing herbicides to reduce the use of glyphosate.
If any of you are interested in knowing the results from the residual herbicides research to control Russian-thistle, you can watch the following YouTube video (5 min) titled, Who Wants Resistant Weeds?, read the PNW 492 titled, Russian-thistle: Management in a Wheat-Fallow Crop Rotation, or contact me to access the scientific publication that describes in depth the research. But, if you are more interested in knowing the results from last year’s trials to control grassy weeds with some potential alternative herbicides, two of the trials were described in the following YouTube video (9 min) titled, Weed Control with Alternative Herbicides to Glyphosate in Wheat Cropping Systems, and the results from the post-harvest trial can be found in Figure 1 of this post.
Figure 1. Russian-thistle control post-harvest in 2021 at the Columbia Basin Agricultural Research Center (CBARC) (Adams, OR) with glyphosate (Gly Star® 5 Extra), alternative herbicides to glyphosate (Reviton [a.i. tiafenacil], Vida® [a.i. pyraflufen], Forfeit® 280 [a.i. glufosinate], and Sharpen® [a.i. saflufenacil]), and some tank-mixtures. Whiskers above the orange bars indicate standard deviation of the data. Same letters above the whiskers indicate no significant difference among herbicide treatments.
One of the most interesting things from last year’s trials, in trying to find alternative herbicides to glyphosate, is that Reviton (a.i. tiafenacil) and Vida (a.i. pyraflufen), both group 14 herbicides, did not seem to have great weed control when they were applied alone. However, their activity was similar to glyphosate when they were applied in a tank-mix pre-seeding and post-harvest (unfortunately we did not have that treatment in the fallow trial).
Photo 1. Images of jointed goatgrass (Aegilops cylindrica) control in pre-seeding of spring wheat in 2021 at CBARC (Adams, OR) with a) glyphosate alone, with a tank -mix of b) tiafenacil + pyraflufen, c) tiafenacil + glyphosate, and d) pyraflufen + glyphosate, compared with an untreated plot (e).
Photo a.
Photo b.
Photo c.
Photo d.
Photo e.
Photo 2. Images of grassy weed control in a fallow trial at CBARC in 2021 where we tried different alternative herbicides to glyphosate. a) General view of the trial in April and b) a month later.
The main reason to bring this topic to the blog is that in our efforts to find alternative herbicides, we would like to put more trials out this year. If any of you have any tank-mix combination/product that you would like us to try, we would appreciate if you could answer to this blog or contact me via email at judit.barroso@oregonstate.edu. Looking forward to hearing some ideas, Thanks. Judit