Nearly 2 million acres in Nebraska are infested with glyphosate-resistant marestail, and 2 million more are infested with glyphosate-resistant waterhemp. Those are the two biggest problem weeds in Nebraska, although Palmer amaranth has been creeping its way farther north in the last few years.
With resistant weeds a growing problem in Nebraska and all over the Midwest, some growers have opted for mechanical control using tillage. Glyphosate-tolerant corn and soybeans were one of the big drivers behind the growth in no-till acres in the 1990s. With six weeds in Nebraska resistant to glyphosate and other chemistries, it’s easy to wonder: Must no-till and integrated resistance management be exclusive from one another?
It’s worth considering that not all weeds are affected equally by tillage. Some, especially pigweeds, can actually be stimulated by tillage and exposure to sunlight.
“The consequence of tilling is you’re reducing soil structure; you’re increasing potential for soil erosion. You’re also controlling weeds with mechanical control, but you’re potentially going to bring weed seed to the surface,” says Keith Glewen, Nebraska Extension educator.
“One weed that’s scaring a lot of people is Palmer amaranth. It’s such a small seed that just several days after row tillage, it will emerge,” says Paul Jasa, Nebraska Extension engineer. “Tillage can be effective for controlling winter annuals or a weed that’s emerged in the early spring such as giant ragweed and marestail. For those weeds coming up later, all tillage does is encourage them to grow. When seeds are buried and come to the surface, and there’s a flash of sunlight, they can actually be stimulated to grow.”
Don’t give up on herbicides
Amit Jhala, Nebraska Extension weed management specialist, notes in 100% no-till systems, growers must rely more heavily on herbicide programs with multiple sites of action. Herbicides aren’t the only tool available, however. Cover crops are another tool, especially for winter annual weed suppression.
“If you have winter annual weeds like marestail, you can plant cover crops like cereal rye in the fall after corn and soybeans,” Jhala says. “Winter annuals will emerge in the fall and very early in the season. So, if you already have cover crops in the field planted in fall and have a good stand, it may help to suppress winter annual weeds.”
However, Jhala notes cover crops won’t be useful as the only method for suppressing summer annual weeds such as common waterhemp and Palmer amaranth. These rotations still rely on an effective pre-emergence herbicide program with multiple effective sites of action. And, the herbicide program that’s right for the field depends on the crop rotation and the weed challenges specific to that field. Therefore, field scouting and keeping notes for an individual field is important.
“For pre-emergence herbicides for corn and soybean, there are a number of products. When it comes to a post-emergence herbicide, you may need to add something else with residual activity,” says Jhala. There are some options that can provide overlapping residual activity if they really have a problem with common waterhemp or Palmer amaranth. "For example, Warrant, Outlook, Prefix, and Zidua can be applied in soybeans depending on soybean growth stage. For example, Warrant can be applied up to R2 soybean growth stage and Zidua can be applied up to V3 soybean (third trifoliate) growth stage."
“If you don’t have that many weeds and are planting corn, applying a preemergence herbicide with two to three sites of action, and later a foliar active post-emergence program, can get the job done. By then, the corn crop will close the canopy, so you really don’t need an overlapping residual in most cases,” Jhala adds. “There is no one single herbicide program that will control weeds in every field. Scouting and knowing what weeds are in the field are crucial.”
Worth the cost
Jasa notes at Rogers Memorial Farm, different preplant herbicides are used for soybeans rotating to corn versus soybeans rotating to wheat.
“We have three early preplant programs we use on our soybeans because of the next crop in the rotation,” he says. “With a grass crop like grain sorghum, it’s easy to go after broadleaves. Next year with soybeans it’s easy to go after grasses like foxtail and shattercane. Do your scouting, and you may your change crop rotation or herbicide program next year.”
“The other problem is most post-emerge herbicide labels give a rate for 2- to 4-inch-tall weeds. I see a lot of people spraying 2- to 4-foot weeds that think the herbicide’s too expensive, so they apply a lower rate. You can’t cut rates, and you have to pay attention to weed size,” Jasa says. “I equate it to giving weed a flu shot. If I take a reduced rate of herbicide on a big weed, that’s just giving it a flu shot and teaching it resistance.”
For fields with heavy waterhemp and Palmer amaranth populations, keeping them under control chemically won’t be cheap — but it’s worth it.
“If you want overlapping residual program where you start with preemergence with multiple effective sites of action followed by glyphosate tank-mixed with Warrant, Outlook, Zidua or Prefix, it might cost $50 to $70 per acre, depending on herbicide program you chose," Jhala says. “It’s worth it, and it’s not only one season. If you let your fields get infested with waterhemp and Palmer amaranth, and then harvest them and spread seeds across the field, don’t clean out your combine and spread them to a nearby field, you have to deal with five times more pressure next year. Weeds are a permanent problem.”