U.S.: New Mode of Action to Treat Superweeds
23 July 2020 Its seeds are the size of a pencil point, nearly invisible to the naked eye. Left alone, it can grow taller — and faster — than a Christmas tree. Part of an emerging class of so-called “superweeds,” Palmer amaranth continues to frustrate farmers nationwide with its dogged resistance to conventional herbicides. Ask a group of growers what keeps them awake at night and you’re sure to hear this superweed uttered with a sneer.
It wasn’t always this way. For years, pigweeds like Palmer amaranth and waterhemp were considered mere nuisances to growers. The numbers were daunting — a single stalk of Palmer amaranth can generate nearly 600,000 seeds, while waterhemp can produce up to 1 million seeds under optimal conditions — but they were well-controlled thanks to an arsenal of powerful, easily accessible, and relatively affordable contact herbicides like glyphosate and mesotrione,
Over time, however, the weeds got smarter, and those weeds reproduced. Growers started noticing that their traditional herbicide regimens were not getting the job done. Switching to another herbicide didn’t help much either; certain waterhemp strains now resist six herbicide modes of action, while some strains of Palmer amaranth resist up to five. These superweeds, which also include kochia, ragweed, and lambsquarters, are invading hundreds of millions of field acres across the country, leaving growers with lower yields and limited options. Seeds from these weeds can even spread through livestock feed, second-hand farm equipment, combine harvesting, and roadside mowing. According to University of Wisconsin researcher Vince Davis, herbicide-resistant weeds cost U.S. farmers $2 billion a year.
A Reliable Solution to a New Problem
In 2017, Belchim USA launched a partnership with several leading universities across the U.S. to test the selective contact herbicide pyridate against various strains of herbicide-resistant weeds. While pyridate itself has been around for years, researchers are just now exploring new uses and application options for this underutilized mode of action.
In corn field trials, researchers combined pyridate in a tank mix with widely used herbicides like glyphosate, atrazine, and mesotrione and applied the resulting mixture to test post-emergent weed control. When these pyridate solutions were applied to corn fields, researchers observed significant improvements in broadleaf weed control — up to 29% in some cases — with no injury to the corn. In one field trial conducted by University of Illinois, treating glyphosate-resistant Palmer amaranth with a tank mix solution of glyphosate, mesotrione, atrazine, and pyridate yielded 99% weed control 14 days after application. While in another field trial spearheaded by Iowa State University, a tank mix of pyridate, mesotrione, and glyphosate applied to HPPD-resistant common waterhemp resulted in 95% weed control just nine days after application.
Pyridate in Action
Pyridate works by blocking a weed’s photosystem II process, triggering the release of toxic forms of oxygen and causing rapid cell wall degradation. Because it uses an underutilized mode of action, there are multiple varieties of combinations that produce a synergistic effect, increasing weed control.
One example is the combination with HPPD-inhibitor herbicides, like mesotrione, which reduces a weed’s ability to process the toxic oxygen. When combined with pyridate, which increases the amount of toxic oxygen produced within the weed, the combination of herbicides causes rapid weed death.
Another effective solution combines atrazine, another photosystem II inhibitor, with pyridate. While both are from the same class of chemistry, they work in different ways, with systemic herbicide, atrazine, hitting the weed up through the roots and contact herbicide, pyridate, hitting the weed through its leaves.
More is Not Always Better
As more growers are looking for long-term sustainable options that still provide strong weed control, they realize that using more of the same often doesn’t make a difference. Plus, using more of the current chemistries, is often not allowed due to local or state restrictions. Even if they could use more, it is usually not economical; not to mention the lack of effectiveness on herbicide-resistant weed strains. Over time, growers have realized that there is no single solution, which is why tank mixing is already commonly incorporated into crop protection programs.
With the continued spread of herbicide-resistant weed strains, like Palmer amaranth and common waterhemp, as well as other tough-to-kill weeds like kochia, university researchers have been strongly encouraged by the enhanced performance that pyridate provides to current tank-mix regimens. In particular, field trials in corn demonstrated improved effectiveness against herbicide-resistant strains.
University field trials also showed that the required ratio of pyridate combined with other herbicides in tank mix solutions makes it an economical option for growers. For instance, in a field trial conducted at Purdue University, when a tank-mix containing 32 ounces of glyphosate and just eight ounces of pyridate was applied to glyphosate-resistant Palmer amaranth, results showed 98% weed control 14 days after application.
Fighting these superweeds isn’t easy, but by incorporating a product with an underutilized mode of action such as pyridate, growers can regain control of their fields safely and effectively.
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