Roundup-resistant horseweed was confirmed last spring in Nash and Edgecombe counties. So far, the resistance has been confined to the two counties in the upper Coastal Plain of North Carolina and horseweed, says Alan York, North Carolina State University Extension weed specialist.
He attributes a rash of calls this growing season about weed problems to excessive moisture stress on the cotton crop this year, and delayed application due to the rains.
Glyphosate-resistant horseweed was first discovered in the United States in Delaware soybeans in 2000. It has since spread to Tennessee, Arkansas and Kentucky.
If history were any indication, however, some species of pigweed would be a good candidate to develop resistance to glyphosate, York says.
"The bad thing about horseweed is it’s cross-pollinated," York says. The pollen is wind-blown and it will spread quickly.
The cornerstone of resistance management is alternating chemistry, York says. "The horseweed is basically a burn-down problem, primarily with glyphosate and paraquat, which has trouble with horseweed." He encourages farmers to rotate modes of action, not just herbicides, but all crop-protection chemicals. He also advises tank-mixes, including 2,4-D.
In the case of horseweed resistance found in North Carolina, growers couldn’t have switched chemistry this year because the problem just occurred this year. The weeds were already too big by the time they noticed. They would have to rely on a tank-mix to take out the marestail.
The horseweed’s resistance in North Carolina, however, doesn’t concern York near as much as potential resistance in other weeds such as Palmer amaranth.
Resistance in Palmer amaranth, a far more important economic weed to North Carolina cotton growers, would mean reliance on Treflan or Cotoran, which would not be adequate alone for control. Post-emergence, directed materials would do little good with Palmer amaranth because the weed grows so fast. The weed already has resistance to Staple in some areas.
In pre-Roundup Ready days, Palmer amaranth caused major problems in South Carolina and North Carolina.
What concerns York the most about resistance is, "new chemistry is almost a thing of the past.
"With 90-plus percent of the crop in Roundup Ready cotton, the market has shifted away from ‘conventional chemistry,’" York says. "Companies can’t afford to spend $100 million to develop a new material."
Along those lines, York says the big concern is with one chemistry having the whole market and the potential for weed resistance and weed shifts "and not having new tools to deal with it or maybe not having the old products still on the market. Some of this old chemistry may fall by the wayside and we may wish at some point down the road we had it back."
In a nutshell, resistance develops this way: As chemicals kill off susceptible biotype weeds, any resistant biotypes, should they be present, would survive the herbicide application. Normally, one out of 10 million plants may be resistant to a chemical, York says. "Since 1996, we’ve been putting on tremendous selection pressure by applying glyphosate. One might argue that we should have already encountered problems if they were going to happen... but a lot of people are convinced that it’s going to happen with other weeds."
Going into this winter, York plans to discuss resistance at grower meetings. "We’re not going to be saying, ‘the sky is falling,’ but we are going to be asking folks to keep their eyes open and try to determine if there is a problem before it gets out of hand."