Marestail

The latest wave of herbicide-resistant soybean varieties will play a crucial role this year in managing the Mid-Atlantic’s toughest weeds.

As marestail and Palmer amaranth have developed defenses against glyphosate and other common herbicides, seed and chemical companies have rolled out bean varieties that tolerate the sprays — like 2,4-D, dicamba and glufosinate —that still work.

Indeed, those three herbicides are about the only ones left that will kill post-emergence marestail.

“Our sort of conventional herbicides that are labeled across all types of soybeans just won’t control them anymore,” said Mark VanGessel, a University of Delaware weed scientist.

VanGessel spoke Jan. 22 during the Pennsylvania Agronomic Education Conference.

As they select their seeds this winter, farmers will have a half-dozen herbicide resistance systems to choose from beyond the familiar Roundup Ready products.

There’s Xtend, which pairs with glyphosate and dicamba; LibertyLink, which goes with glufosinate (Liberty); and XtendFlex, which handles all three herbicides.

Enlist E3 goes with glyphosate, glufosinate and a 2,4-D choline. LLGT27, the latest LibertyLink trait, pairs with glyphosate, glufosinate and a new HPPD inhibitor herbicide that has not yet received federal approval.

It’s a head-spinning list, and one farmers must keep straight.

Many of the new traits have a short, specific list of herbicide formulations that can be used on them.

Even if a crop is tolerant of several different herbicides, those chemicals can’t necessarily be tank-mixed with each other.

And the various products are far from interchangeable. “You lose track of which field is planted with which trait and it gets sprayed with the wrong one, you’ll know it in a hurry,” VanGessel said.

What’s more, the new herbicide formulations come with complex rules about the adjuvants and nozzles that must be used, the downwind buffers from sensitive plants, and the weather conditions in which the products can be deployed. Labels for similar products have different rules, so applicators need to read the fine print.

“These labels are not easy to understand, and they’re written in such a way that they put the onus on the person that’s applying it” if anything goes wrong, VanGessel said.

The proliferation of guardrails is a response to major damage to nontarget crops caused early in the rollout of the new dicamba-tolerant programs. Even with updated, reduced-drift formulations, dicamba and 2,4-D still have some risk of wafting away.

And because they are being used in growing crops, not just at burndown, plenty of vulnerable crops are growing when the new sprays are used.

A key way to reduce drift is to apply large droplets. The heavier droplets fall faster and are less likely to be blown off target than a fine mist.

But large droplets may not be the best choice for the other chemicals in the tank mix. Contact herbicides like Cobra, Reflex and Liberty work best with good coverage, which comes from small droplets.

Within the bounds of the label, farmers have to figure out which nozzles and herbicides will be best for their situation, VanGessel said.

For many grain growers, following Leviticus-grade regulations is a price worth paying to keep marestail and Palmer amaranth at bay.

The yield-robbing weeds grow fast, produce seeds like crazy, and in many regions have built up resistance to glyphosate, ALS inhibitors and other herbicides.

“It’s hard for a lot of folks to hit that ideal window for controlling Palmer amaranth. Sometimes it just gets away from folks,” VanGessel said.

Palmer is hard to kill with any herbicide after it gets bigger than 4 inches tall. The plants may get injured, but they’ll still issue a flood of seeds, he said.

Given the paucity of in-season controls, marestail is best controlled at planting.

Dicamba and 2,4-D are options. So is Sharpen, though depending on the soil type in the field, it must be applied up to 30 days before planting. Elevore, a relatively new plant growth regulator, is effective but very slow acting, VanGessel said.

Burndown before double-crop soybeans can be a particular challenge. Because of the weeds’ resistance to ALS inhibitors, control coming out of small grains is harder to achieve than it once was.

Liberty is effective in this situation as long as there’s good coverage. The new over-the-top dicamba products may be off the table because they can’t be used after June 30, VanGessel said.

No matter the season, farmers would be wise to control weeds in part through non-chemical methods. That means rotating crops, cleaning combines between fields, and planting cover crops. These practices won’t be a cure-all on their own, but they can certainly help, VanGessel said.

At one point, the key to weed suppression with cover crops seemed to be growing as much biomass as possible.

But it now appears that even modest cover crop growth is beneficial. In one trial, growing just 500 pounds of cereal rye per acre cut Palmer amaranth emergence by half, VanGessel said.

A cover crop produces more biomass the closer its termination is to planting for the cash crop. Along with using a roller-crimper, the obvious way to delay cover crop termination is to use an herbicide-tolerant soybean variety.

But for all their promise, the new crop and herbicide programs have their own vulnerabilities.

Dicamba and 2,4-D offer limited residual control, so they may need to be tank-mixed with other herbicides. They can’t work magic on large weeds, and some weeds have even been reported with resistance to these products.

“We need to steward these products (so) that we don’t start to select for additional resistance,” VanGessel said.

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