With the Weed Science Society of America (WSSA) meeting just finished up in New Orleans, Jason Norsworthy says the society’s website “posts all herbicide-resistant weeds, and we hope to have the metolachlor-resistant pigweed from Arkansas added to this site in the coming days. This is actually the first ever Palmer amaranth with resistance to metolachlor; albeit, a few weeks ago, colleagues at the University of Illinois posted confirmation of a waterhemp population with resistance to metolachlor — the first time resistance to this chemistry has been found in a broadleaf weed.”
Norsworthy, a University of Arkansas weed scientist, and Tom Barber, Arkansas Extension weed specialist, “have been working on two different farms in northeast Arkansas, looking at control options for PPO-resistant pigweeds since 2016. Barber is working near Marion and I’m working near Crawfordsville, with about 10 miles between sites.
“The first year we were in these fields we tested a lot of pre-emergence herbicides. One we thought would work fairly well would be Dual Magnum (S-metolachlor). Unfortunately, it didn’t perform up the standard we were expecting at either location even though we had good activation.”
Returning in 2017, “I had a bit better control at my site but Barber did not at his. Then (in 2018), we did some trials looking at comparing rates of metolachlor (Dual Magnum) to some other Group 15 herbicides like Zidua and Outlook. At both sites we again had good activation and the metolachlor showed less activity than the other two herbicides.”
Last fall, the two researchers “took samples and went into the greenhouse and checked the response of these two populations to metolachlor and other Group 15 herbicides relative to two known susceptible populations of pigweed collected in 2001. What we found was an eight- to nine-fold level of resistance to metolachlor relative to the susceptible populations.”
For the other Group 15 herbicides tested (Zidua, Harness and Outlook) “we saw a two- to three-fold reduction in activity for those relative to the susceptible pigweed. While we still had effective control with these herbicides at a field use rate, it does show that we are seeing slight reduction in sensitivity and continuing to rely heavily on these herbicides could soon result in failures similar to that observed for metolachlor.”
Asked about the situation, Keith Driggs, Syngenta agronomic service representative for the Mid-South sales district, says, “There are two populations in Arkansas — both in Crittenden County — where University of Arkansas weed scientists have documented performance issues with S-metolachlor against palmer amaranth. We’ve seen the issues at these sites and are working closely with (them) to further investigate the extent and underlying causes of the Palmer amaranth control issues.”
Further, says Driggs, “There are a lot questions to be answered. One thing that’s clear is this a Group 15 herbicide issue, although S-metolachlor activity has been the most impacted so far at these sites. We believe this situation further reinforces the need to adopt management practices that can help prevent or delay this problem from spreading. S-metolachlor remains highly effective against Palmer amaranth in most areas and remains the standard for residual control of annual grasses and yellow nutsedge.”
Currently, Norsworthy does not think the problem “is extremely widespread. … We conducted a survey in 2017 of pigweed in Arkansas and collected approximately 200 populations across the state. We sprayed them with a labeled rate of metolachlor in the greenhouse and only seven or eight populations had a few survivors. Even when we did have survivors, the overall control was relatively high under the greenhouse conditions.”
Even so, Norsworthy does “think there are more resistant populations out there. It is not just a coincidence that Barber and I happened to start working on the only two metolachlor-resistant fields in Arkansas. “That said, we are not ready to give up on metolachlor — it still provides tremendous benefit for us on many other weeds, especially barnyardgrass, which is a major issue in soybean/rice rotations. I’m also of the opinion that it is an effective option for pigweed in 99 percent of the state’s fields today.”
At the two northeast Arkansas sites, “we can put down metolachlor with metribuzin in soybeans and that combination still looks good. Boundary, a premix of S-metolachlor and metribuzin, is effective at 1 quart per acre when choosing a metribuzin-tolerant soybean.
“I think the big issue up until few years ago — say, 2008/2009 when glyphosate-resistant pigweed blew wide open for us — is in soybeans most of our acres have gotten metolachlor at planting. We came back over the top, and we’ve talked about overlaying residual herbicides with something like Prefix (S-metolachlor plus fomesafen). In soybeans, a lot of Arkansas fields have gotten two applications of metolachlor every year. I believe most cotton fields have also gotten at least one metolachlor application. The same is true for corn acreage.”
With all that going on, after eight or nine years “it’s pretty easy to see how resistance to a chemistry starts to build, especially when the PPO herbicides were no longer helping pigweed because of resistance.”
Driggs adds that “weed resistance to every mode of action out there can develop over time. Palmer amaranth resistance to glyphosate and PPO herbicides is a good example. I can only speak to what’s happening in Arkansas, but there are a couple of populations of waterhemp in Illinois where Group 15 herbicide performance issues have been documented.
“Our position is we should never use a Group 15 herbicide as a residual behind the planter by itself. Start clean and use either a premix herbicide or herbicide tank mix containing multiple, effective modes of action.
“Also, we need to stay clean and that requires overlapping residuals. The reinforcing post-applied herbicide should be applied before the pre-emerge residual breaks down. At these same problem sites in Crittenden County, performance of herbicide programs using these resistance management strategies has been excellent.
“When you think about it, (the two university research locations) were in the area that was basically Ground Zero for development of both glyphosate- and PPO-resistant pigweed. That area has had multiple residual applications, both pre- and post- in soybeans, for as long or longer than anyplace else.”
As farmers have long been warned, an immediate solution to resistance in pigweed is unlikely. “All of the (herbicide) companies are constantly screening for new modes of action,” says Driggs. “None are imminent (coming to market) … that I’m aware of. The development of HPPD-tolerant soybeans holds a lot of promise and in the future will allow us to use additional modes of action as residuals against these weeds. We must steward the modes of action we have now. The University of Arkansas has done a very good job in trying to get people to understand the value of sounds resistance management strategies.”
Source: David Bennett, Delta Farm Press
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