Weed control is a major concern for growers, especially considering the growing trend of herbicide resistance.
“The problem with weeds is the yield loss due to nutrient competition,” said Lynn Sosnoskie, associate professor at Cornell Agritech, at the recent virtual Soybean and Small Grains Congress.
Sosnoskie, who specializes in sustainable weed management in vegetable and fruit crops, said the potential annual yield losses from unmanaged weeds using best management practices but no herbicides could be as high as 50% in corn ($26.7 billion), 52% in soybeans ($17.2 billion), and 71.4% in dry beans ($722 million).
“Herbicides have been the dominant pesticide applied in many crops as a percent of planted acres, and they’re used primarily because they work well, until they don’t,” she said.
That does not mean that herbicides are always totally effective. Using the wrong herbicide, applying at the incorrect rate or spray volume, improper use of spray additives, and several other factors can contribute to herbicides being less effective.
“This is the evolution of a trait,” Sosnoskie said. “We used to be able to kill this weed with this herbicide with this rate, and now we can’t.”
Worldwide, 509 unique cases of herbicide resistance exist.
“The U.S. leads the way with 123 cases,” Sosnoskie said. “This is going to increase by many cases.”
Resistance in New York
Examples of confirmed herbicide-resistant weeds in New York include common lambsquarter, smooth pigweed, common groundsel and common ragweed.
Other species that have gained resistance by herbicide combinations include horseweed, waterhemp and Palmer amaranth.
Last year, Cornell crop specialists around the state obtained marestail seed from the four regions of New York and grew them in a greenhouse for resistance testing. Most of the seeds came from farms, vineyards and roadsides.
The researchers then tested Durango at 22 ounces per acre, First Rate at 0.6 ounces per acre and Classic at 0.75 ounces per acre. In some cases, none of the horseweed plants were killed by any of these herbicides.
Paraquat resistance has been reported in Canada and California. When looking at Paraquat resistance in New York horseweed, researchers found that a variety of factors affected results, including plant size and the timing of treatments; the rate and coverage; and the temperature, humidity and light environment. The research suggests resistance in New York populations up to 58 ounces per acre.
“We didn’t kill them or see reduction in plant growth,” Sosnoskie said. “We were able to control sensitive populations at one-tenth the rate listed on the label.”
Horseweed resistance to glyphosate and ALS-inhibiting herbicides used in soybeans have been observed in New York.
“Two horseweed populations in tree and vine systems appear to be resistant to Paraquat,” Sosnoskie said.
Horseweed spreads on wind currents, comparable to dandelion seeds. Because it can spread so easily, “this is very likely to move away from the site of origin,” Sosnoskie said. “If you get resistance, you will see resistance move around from the site of origin.”
Sosnoskie views herbicide use as a numbers game since herbicides are not completely effective. But at 80% to 90% efficacy, a farmer will have fewer weeds the next year. As research continues to emerge on herbicide recommendations and doses, she recommends using pre-herbicides to start clean and stay clean.
“It reduces early-season competition and associated yield loss, buys you time for post treatments, and reduces densities for post control, which improves success,” she said.
Tackling waterhemp, Palmer amaranth
Waterhemp and Palmer amaranth have become troublesome weeds in corn, sorghum and alfalfa.
“They are very quickly becoming the pigweed varieties we’re becoming more worried about in the Northeast,” Sosnoskie said. “These plants, these populations of species develop resistance, and they develop resistance quickly to the tools we’re using to manage them.”
Some populations are resistant to four or even five herbicides.
Last year, large populations of Palmer amaranth were spotted in Wayne, Stueben and Orange counties, with Stueben and Orange being the worst. For waterhemp, Wayne County and its surrounding counties were the hot spots.
Applying Rely 280 at the standard rate, and up to four times the standard rate, resulted in almost 90% dead plants at two weeks after treatment. The results were similar for Xtendimax, but much lower for Enlist, with only 70% to 80% efficacy.
“These plants are bigger than we wanted them to be when we sprayed, so that’s maybe why we’re not in the 90% control range,” Sosnoskie said of Enlist.
In a study on waterhemp, Classic applied at even four times the standard provided only 50% control. First Rate got even worse results. The best results were from Roundup Powermax, but even at four times the standard rate, it got only 60% control.
“With Roundup Powermax, they look sickly at first but grow out of it,” Sosnoskie said.
Researchers observed similar results in other fields where they studied Palmer amaranth. Applying Roundup, First Rate and Classic caused a little more injury, “but we didn’t see complete control,” Sosnoskie said. Instead, “we saw regrowth and rebound.”
This preliminary research indicates that central New York waterhemp and Palmer amaranth are becoming resistant to glyphosate and ALS-inhibiting herbicides used in soybeans. Sosnoskie reiterated that starting clean and staying clean is vital, along with using pre-herbicide and post-control sprays.
“These species have separate male and female plants,” she said. “It can fertilize the female plants, get seed development, and you can get that resistant trait.”
Keeping equipment clean can help ensure that weed seeds are not brought onto a field or brought from another farm. Managing field edges using multiple herbicide modes of action at the labeled rates and recommended weed sizes is also a good practice, along with using diverse chemistries in season and between seasons. Scouting is also a good way to track weed size to get them when they are small.
Sosnoskie likes the use of electric weeders that work by super-heating the weed with an electric current.
“We can get some fairly good success using the Weed Zapper,” she said.
When studying the Weed Zapper Annihilator in soybeans, researchers got 61% to 84% reduced leaf and stem tissue biomass per plant, meaning 67% to 88% reduction in weed reproductive output. Although it proved less effective in grasses, they found good successes in reducing broadleaves.
Some farmers are even using impact mills that mechanically destroy weed seeds while harvesting. Impact mills destroy weed seeds picked up by a combine.
“It is actually effective for the weeds we predominantly see in the U.S.,” Sosnoskie said.
Sergeant writes from central New York.