Waterhemp Its relation to pigweed, already found resistant, is of concern to researchers
Recent discovery in the U.S. of a population of waterhemp resistant to 2,4-D grabbed a lot of attention, as the old standby chemical was seen as an option for controlling the weed that’s already become resistant to glyphosate.
But the waterhemp discovered in a Nebraska grass seed field are not the first weeds resistant to 2,4-D. The first case was in 1957, when wild carrot in Ontario was not controlled. At present populations of 30 weed species in 49 places have been confirmed resistant to the old herbicide. Some, including false cleavers found in Alberta in 1996, escape by evading more than one of the herbicide’s modes of action.
“The synthetic auxins are classified as low risk for herbicide resistance,” says Hugh Beckie, an Agriculture Canada weed scientist. “There’s a lot we don’t understand about them, but we know they have multiple mechanisms of action.”
The Group 4 herbicides, 2,4-D and other phenoxy products — dicamba, clo- and amino-pyralid and picloram — disrupt cell growth and protein production in newly forming stems causing misshapen shoots and tumours.
“The waterhemp complex, (there are several species that interbreed) is notorious for its ability to develop widespread resistance because it includes several species that outcross and interbreed, a bit like kochia,” says Beckie.
So far no waterhemp has been found in surveys of weeds in Western Canada. However, the pigweed family is related to the waterhemps and Beckie says some resistance to 2,4-D has been found in green pigweed in Manitoba.
“It’s quite surprising in a way that we’ve seen relatively little resistance to 2,4-D when we’ve been using it for so long. (The first synthetic plant hormone was first released in 1946 in the U.K.) The good news on 2,4-D is that it isn’t the greatest herbicide. It has a limited-action spectrum, but it has been a good standby.”
The problem Beckie sees for both 2,4-D and dicamba is that resistance to these two herbicides is part of the new “stacked-trait” genetically modified crops to be released as soon as next year in the U.S., by Monsanto and Dow. His fear is that the stacked-trait crops will lead to a lot more selection pressure on weeds controlled by 2,4-D and that cross-resistance to other Group 4 chemicals will develop.
“I don’t expect we’ll see a big surge in resistance,” he says. “I think we’ll see a slow increase in resistance.”
The multiple mechanisms of action against weeds of the Group 4 chemicals should keep resistance low. Also, the 2,4-D and dicamba tolerance are stacked with glyphosate tolerance, so stacked traits will likely be sprayed with mixtures of these chemicals so the risk that even plants in a particular field can withstand both chemicals is quite low.
But Beckie worries that pollen or seed could spread resistance across a region, particularly in areas like Manitoba that are prone to flooding. “We just need to be aware of the risk,” he says. Tank mixing herbicides with different modes of action, rotating into a broader range of crops, more competitive crops and crops with different seeding times, higher seeding rates can all help control the development of resistance.
Beckie recognizes that by the time you’re done spraying, you really want a break. But, he says, you can control even Group 1 and 2 resistance if you scout your fields after spraying. “After spraying you can see weed patches that aren’t controlled while the plants are small,” he says. “If you prevent those weeds from setting seed, you can stop that patch from spreading. If you combine those patches they’ll spread right across the field. We’ve seen a big difference in farm fields — where the farmer controlled seed set, over six years, the patches of resistant weeds stayed the same size. But where the surviving weeds were left till after combining, for six years, the resistant patches expanded and expanded.”