If you have soybeans in your rotation — and 55% of Dryland No-Tiller readers indicate they do — you’ll want to pay increasing attention to any Palmer amaranth or waterhemp showing up in crops.
Dallas Peterson, a weed management specialist for Kansas State University Extension, recently penned an article about the increased incidence of these weeds in Kansas soybeans this year.
Why is this? Pigweed pressure has actually been increasing gradually the last 10 years. “But this year pigweed pressure has taken a big step up,” he says.
He cites two main reasons for this: growing resistance to glyphosate, and wet weather in May and early June, which may have caused early pre-plant (EPP) herbicides to wear off by the time soybeans were planted.
In some cases, pigweed began emerging before soybeans could be planted, and there may not have been time to apply burndown or pre-emergence residual herbicide during soybean planting, Peterson says.
Meanwhile, farmers still relying on post-emergence herbicides, particularly glyphosate, to control pigweeds are having an increasingly hard time getting good control, he says.
There are options other than glyphosate for post-emergence control, but most of those options require weeds to be less than 3-4 inches tall, Peterson says. That means producers have to watch their fields closely early in the season, and spray the weeds when they first see them emerging.
A good residual herbicide program in the spring will likely be important for pigweed management in the future, Peterson says, regardless of the post-emergence program. Where glyphosate-resistant pigweeds have become a problem, growers may want to consider LibertyLink or conventional soybeans.
However, even these soybean varieties will need to be part of a planned program that utilizes residual herbicides and timely applications, Peterson says.
There may be new varieties of soybeans coming in the future with resistance to 2,4-D (Enlist) or dicamba (Xtend) if key export markets get approved. However, these options also work best in a program approach using residual herbicides and timely post-emergence applications, Peterson explains, citing tests that showed glyphosate with 2,4-D or dicamba had problems controlling 6-inch-tall Palmer amaranth this summer.
For no-tillers, he says, the best approach is to start with a two-pass program early. Apply EPP residual herbicides at a two-thirds rate in mid- to late-April, then follow up with the rest of the residual herbicide at planting, Peterson suggests.
If pigweeds are emerged at planting time, it will be important to include a burndown herbicide to control those weeds as well, he says. If farmers want to rely strictly on a single EPP treatment, they should include an adequate rate of a residual herbicide product in the mix.
Then they should be ready to apply any needed post-emergence herbicides early, before weeds get to be 3-4 inches tall. On fields with heavy pigweed pressure, growers may want to add additional residual herbicides to the post-emergence treatment.
Some research work is being done in Illinois using no-till and cover crops to keep Palmer under control, noting that the weed’s seeds are generally short lived. Leaving seeds on the surface allows more seeds to be consumed by insects, birds and mice, and seeds aren’t brought to the surface by tillage to germinate.
The concern about Palmer amaranth isn’t anything to ignore — just ask your colleagues in the southeastern U.S. who’ve had to resort to hand pulling, hiring hoe gangs or tilling up entire fields to get this voracious weed under control.
Palmer is now spreading north into the Midwestern Corn Belt, where growers are learning how to scout and develop herbicide rotations that will control it. Some studies have shown the weed can erase up to 79% of soybean yields.
Also consider that Palmer can grow 2½ inches per day in ideal conditions. A single female plant can produce up to 600,000 tiny seeds, which can be windblown, washed away and spread through drainage ditches, hitch a ride on used farm equipment, survive in lime piles and be spread, or be spread in manure from livestock where cotton seed was used as feed.
And if you’re concerned about moisture in your fields, how much do you think will be left after these fast-growing weeds shoot up to a foot tall or more?
I think this warrants no-tillers taking a second look at their scouting and herbicide programs to ensure problems don’t develop with pigweeds that will last for years in their fields.