Options exist if dicamba for dicamba-tolerant soybeans exits the 2025 soybean herbicide scene.

The 2,4-D choline based Enlist system is one, says Meaghan Anderson, Iowa State University Extension field agronomist. Ditto for the glufosinate-based LibertyLink system. Residual PPO inhibitor herbicides (Group 14) such as fomesafen (Flexstar) and lactofen (Cobra) can also be applied over-the-top, although those containing flumioxazin (Valor) and sulfentrazone (Spartan, Authority) may be applied only three days after planting.

“Since all soybeans are tolerant [to Group 14 herbicides], these don’t require a special trait,” Anderson says.

One drawback to foliar-applied PPO inhibitor herbicides is they have minimal soil activity, says Aaron Hager, University of Illinois Extension weeds specialist.

Using Group 14 and Group 15 herbicides as preemergence residual compounds (such as S-metolachlor and pyroxasulfone) can remove some of the pressure on postemergence herbicides. Still, nixing dicamba would place more pressure on all remaining herbicide sites of action. Repeated use of the same herbicide ultimately selects for herbicide-resistant biotypes in a weed population

“We’re asking them [remaining herbicides] to do a lot in controlling weeds that are now developing resistance to herbicides quite quickly,” Anderson sats.

This includes herbicides within a site of action group. Bayer reported the discovery of two dicamba-resistant waterhemp populations in Iowa — Scott County in 2021 and Marshall County in 2022. Corteva also reported the discovery of a suspected 2,4-D-resistant waterhemp population in Iowa.

“When one herbicide in a group falls, it likely isn’t long until another one falls,” Anderson says.

Preemergence herbicide resistance

Resistance can even build in preemergence chemistry. U of I weed scientists confirmed two waterhemp populations in central Illinois in 2019 that resisted S-metolachlor and other Group 15 herbicides. U of I weed scientists are currently sampling Illinois waterhemp populations for more Group 15 resistance.

Resistance to Group 14 preemergence herbicides is also widespread, Hager says. “The way you experience resistance with the soil-applied PPOs is that the length of residual gets less and less,” he says.

Drought can mask weeds that resist preemergence chemistries, Anderson adds. “It can be explained by the pre not getting activated, or weeds germinating below where the pre was because it didn’t get incorporated by rainfall,” she says.

In years of excessive rainfall, it’s possible preemergence herbicides move out of the weed germination zone or quickly degrade. However, they also could be resistant, she adds.

Don’t forget corn

Decreased postemergence herbicide control in corn is also to blame for waterhemp buildup in rotated soybeans.

“In some cases, this could be due to resistance, but it also could be due to the weather conditions that we’ve had the past several years,” Anderson says. “It’s been generally warm and dry, and in these cases, it can be harder to kill weeds. You may get a good burn and they look like they will die, but they can grow back before corn can shade the row.

“If you’re in a corn-soybean rotation, make sure you can do everything to control waterhemp in both crops,” she adds. “Hundreds of thousands of seeds can be produced by every female waterhemp plant that escapes.”