- Depending upon the year of screening, up to 75% of the populations screened had some level of resistance to group 14 herbicides. This includes not only the POST herbicides – fomesafen, lactofen, acifluorfen – but also the residual herbicides – flumioxazin, sulfentrazone. Resistance to the residual herbicides is not manifested as a complete lack of control, but rather reduced longevity of control so that there is a loss of overall effectiveness. Growers trying to control waterhemp in nonGMO or RoundupReady soybeans will have to use group 14 herbicides for POST control, and should take steps to determine whether group 14 resistance is present before counting on these to work. The abundance of Enlist, XtendiFlex, and LibertyLink soybeans has reduced our use of POST group 14 herbicides overall, which may have slowed the development of additional resistance.
- We observed some level of variable response to atrazine, mesotrione, s-metolachlor, and 2,4-D among the 38 total populations we screened from 2019 and 2020. About half the populations had a reduced response (less than 80% control) to foliar application of atrazine at 1 lb ai/A, although most of them were still controlled at 4 lbs/A. About half of the 2019 populations had a reduced response to mesotrione at 3 oz product/A, but all were controlled at 4 times this, and all of the 2020 populations were controlled by both rates. The response to 2,4-D was more difficult to assess, because it appeared that the 1X rate of 0.75 lb ae/A was overall less effective in the greenhouse than is usually is in the field. Regardless, it appeared that a couple of populations had reduced response to 2,4-D. The same issue with rate occurred for the screening with s-metolachlor. We observed a similar issue with rate for the s-metolachlor screen, where the 1X rate of 1.5 lb ai/A appeared overall less effective than it should have been. But – we still observed variable response among populations to both rates, and 20 to 33% of populations had a reduced response to the 4X rate.
- A summary of the preceding paragraph would be that we have evidence of resistance developing to those 4 herbicides, with the caveat that we did not do the necessary follow up research to confirm this (does response and heritability studies). These are mostly populations collected during our annual preharvest survey, from fields where there appeared to be indicators of resistance. Some populations were poorly controlled at the standard herbicide use rate, and for some of these, using four times that rate still did not result in more than 80% control. If we have metabolic based resistance developing, it is following the predictions of Dr. Tranel. The presence of this type of resistance for one site of action does mean that resistance to the others occurs. However, there were several populations with reduced response to all four sites of action we used, for which metabolic resistance is an issue.
- The regional project screening did not find reduced response to dicamba or glufosinate in the populations we submitted. Our feeling is that it’s probably a matter of time until we have populations in Ohio with resistance to these herbicides, and also 2,4-D, given the intensive use of these in POST herbicide programs for soybeans.
Putting this in perspective, waterhemp continues a trend of developing resistance to any herbicide used against it repeatedly. Metabolic resistance introduces the additional problem of having a broader assortment of herbicides and herbicide mixtures select for a single mechanism that confers resistance to multiple sites of action, reducing the effectiveness of a number of control options at once. It won’t be enough to alternate herbicide sites of action (which doesn’t happen much anyway) and mix multiple sites of action. Herbicide management does not go far enough toward preventing resistance problems in waterhemp. Ultimately, we need to scout fields often enough to discover control problems, which could be due to resistance, and prevent plants from going to seed.
Source : osu.edu