The effects of 2019’s extreme weather are long-lasting. In several ways, the heavy rain and flooding seen in the spring and summer pose new challenges for weed control in 2020.
Syngenta agronomist Bob Kacvinsky acknowledges that even before 2019, weed populations had shifted farther north in Nebraska.
“This is not just a 2019 issue, but it’s been several years in the making,” Kacvinsky says. “Our weed populations have been shifting. Palmer amaranth has become a driver weed in almost everyone’s production in Nebraska. I spoke with a farmer this fall who told me 10 years ago, his No. 1 issue was trying to figure out how to control kochia. Since Palmer has moved in, he wishes kochia was his problem weed again, because Palmer is so difficult to control.”
And 2019’s flooding might have resulted in weed seed introduced where populations weren’t present before. Weed seeds can be transported by water and also can be brought in by outside hay sources.
“Think of 2012 in Nebraska,” Kacvinsky says. “We had a big drought year, and people were buying a lot of hay brought in from places like Arkansas, Louisiana and Kentucky, and it carried Palmer and waterhemp seed. Cattle producers were running into problems with Palmer amaranth. You could almost follow that pattern with the cattle in those feedlots. That’s one of the possible sources of Palmer amaranth. We were buying hay from areas that had it.”
But 2019 posed other problems as well. With the onset of herbicide resistance among populations of Palmer amaranth, waterhemp, marestail and other problem weeds, Kacvinsky notes a two-pass system was critical in 2019. For many growers, however, the timing of application didn’t work out this year thanks to the weather.
“We were planting late, or had planting scattered over eight weeks instead of two,” Kacvinsky says. “We had crops at different stages, and horrible weather in June trying to get herbicide applied. A lot of products were sprayed late on weeds that were too tall. For Palmer amaranth and waterhemp, most labels call for you to spray when they’re 2 to 4 inches tall. That’s not very tall, and it’s very early in the season. Because of the weather, many farmers were spraying 8- to 12-inch weeds. When you spray an 8-inch-tall weed with a chemistry designed for 4-inch-tall weeds, it’s the same as cutting your rate in half or more.”
“Timing is critical, especially on Palmer and waterhemp,” he adds. “Farmers had a difficult time trying to get timing right. There were escapes, and now we’re seeing it in the combines. The more we’re letting weeds get 10 to 12 inches tall before we spray, the more tolerance we build into the population. It only takes a couple of years for these weeds to get to a point we can’t control them with the program we had two to three years ago.”
Kacvinsky notes it typically takes five to six years maximum for a weed species to develop tolerance to a given chemistry.
“If a Palmer plant comes up in the middle of May and is 6 to 8 feet tall at the end of the season, it can produce half a million to 1.5 million seeds,” he says. “The first year, 20% will germinate, so 200,000 plants from the one you let go. Even if I can control 99%, I’ve got a mess. That’s the challenge — it only takes one or two years of letting weeds go to seed that you’ve got a mess that’s difficult to control.”
“We’ve got farmers with good successes, they’re using multiple modes of action, three or four, and using full use rates, which is essential,” Kacvinsky adds. “Cutting use rate is an automatic way to destroy a chemistry’s use in your farm. That’s occurred with virtually every chemistry in the marketplace.”
Source: Tyler Harris, Nebraska Farmer
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