weed resistance

When fighting weed resistance, experts agree that corn producers should make herbicide applications on the heels of harvest.

Post-harvest: Be proactive with control of resistant weeds

Considering that most corn in the southern states is harvested in August and September, Palmer amaranth has plenty of time to propagate before the first frost. In fact, all resistant weeds can flourish in fallow cornfields for up to three months while producers are busy picking cotton or cutting soybeans.

Everyone looks forward to a bit of after-harvest “kicking back,” but when it comes to dealing with resistant weeds, experts say farmers should plan to attack before year’s end.

Considering that most corn in the southern states is harvested in August and September, Palmer amaranth has plenty of time to propagate before the first frost. In fact, all resistant weeds can flourish in fallow cornfields for up to three months while producers are busy picking cotton or cutting soybeans.

“If you do nothing, weeds will continue to grow and replenish the seed bank of what you’ll have to deal with in the spring,” says Daniel Stephenson, weed scientist at the Lousiana State University AgCenter.

Stephenson points out that in Louisiana crops compete with two to four species of grasses and anywhere from two to 20 species of broadleaf weeds. Thus, not applying a post-harvest, residual herbicide spells trouble. He suggests corn producers take action at least once a month after harvest to manage yield-robbing weeds.

It’s recommended that producers mow stover roughly two weeks after harvest. Two to four weeks after that, they should rework field beds, then begin to consider which non-selective, residual herbicides to apply by mid-to-late October.

A fall herbicide application, or “the Halloween application,” as Stephenson has coined it, is particularly effective on henbit that emerges in September and October. By January, henbit is so big it’s hard to burn down, causing major problems for corn typically planted early — from mid-February to about March 10.

“Make plans to go in clean for spring planting,” Stephenson says. “Don’t wait until January for cleanup, when environmental conditions like rain and wind can keep you out of the field.

“Fall application of residual herbicides is the best practice to control henbit and other broadleaf weeds; it’s really a valuable, economical decision.

“Producers in Louisiana are not spending money to control just one weed, as in the case of Palmer amaranth,” he says. “They are managing for all of them.”

In the Southeast, herbicide-resistant Palmer amaranth continues to be the biggest resistance challenge, says Eric Prostko, University of Georgia Extension weed specialist. In 2013, producers also faced challenges with rainfall that hindered spring herbicide applications.

Although they needed the moisture, the timing of the rains prevented some early season herbicide applications, allowing more weeds to become problems.

“We saw later season problems with what are typically secondary weeds, such as sicklepod and morningglory, because producers couldn’t get into fields to apply postemergence herbicides,” he says, “But every year, we’re getting a little better at controlling glyphosate-resistant Palmer amaranth.”

Prostko explains that four key practices continue to help producers manage the prolific pigweed:

  1. Tillage remains an important management tool. Turning ground with a moldboard plow enables producers to bury the seed at a depth that prevents approximately half from germinating the next growing season.
  2. Start the growing season with clean fields. This can be accomplished using tillage, cover crops, especially cereal rye, and/or herbicides.
  3. Continue to use residual herbicides and be timely with post-emergence herbicide treatments.
  4. Removing escapes continues to be crucial in managing resistance.

“Everyone knows how many hundreds of thousands of seeds a single Palmer amaranth plant can produce,” Prostko says, “It’s no longer uncommon to see producers hand-pulling escapes in their fields.”

Atrazine, he points out, also continues to be an important herbicide for the majority of field corn producers, although some areas of Georgia have experienced resistance. “It’s tough to beat this chemical in price and effectiveness,” he says.

New weed-resistance tools, including new herbicides and genetic traits, are coming in the next few years, but it’s difficult to determine when and where they will fit into current practices, Prostko says. He also points out that most are not stand-alone products.

“In the meantime, producers still need to do what they’re doing to manage the pigweed problem,” he says.

Similarly, Bob Scott, University of Arkansas Extension weed scientist, says that given the year they had in his state in 2013, producers need to repeat the mantra they are accustomed to hearing: “Be proactive, don’t wait for glyphosate resistance to blow up on you.

“Palmer amaranth has plenty of time to germinate after corn harvest and before frost, he says.” In such cases, he recommends that producers utilize Boundary, Dual, Valor and a few others to prevent pigweed from going to seed. On farms where they aren’t rotating crops, he recommends considering residuals.

Proactive and quick action by producers remains the front line of defense against herbicide resistance. In Louisiana, for example, glyphosate-resistant johnsongrass hasn’t spread like resistant Palmer amaranth due to producers spot spraying it. However, producers in the central part of the state who are faced with herbicide-resistant johnsongrass still have three or four herbicide options. Stephenson recommends Corvus, Capreno, Accent or generic nicosulfuron, and Liberty over LibertyLink corn hybrids. All have proven to be very successful controlling johnsongrass in corn.

In Tennessee, the biggest problem for corn producers is highly competitive Italian ryegrass.

According to Larry Steckel, Extension weed scientist with the University of Tennessee, producers were going into fields in late October and early November with Gramoxone or Dual to burn down weeds. Some fields were requiring two applications, timed approximately 10 days apart.

“Manage weeds after harvest with tillage or Gramoxone or a residual herbicide,” Steckel says.

“Producers are running Bush Hogs and then spraying, hand weeding — whatever it takes to keep resistant weeds from going to seed. It’s a battle to have time for these measures and still plant wheat, but it has to be done.”

He is also very concerned about loosing atrazine as a weapon against Palmer amaranth, as has happened in some areas of Texas and Georgia. He recommends dicamba or herbicides with more than one mode of action, such as Capreno, Callisto, Halex GT, or premixes such as Status.

“Resistant grass species are becoming an issue for us,” Steckel says. “Roundup has traditionally been a grass herbicide, but even that is failing now. Producers need to be on the lookout for that. Know what’s in your fields.”

See more articles from the Southern Corn and Soybean Production Guide!

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.