Summer annual weeds in many soybean fields are almost 3 ft. tall, according to University of Illinois associate professor of weed science Aaron Hager.
 
1) The most common weeds include horseweed/marestail, common lambsquarters, and waterhemp. Hager said they may have escaped preplant tillage operations,  preplant burndown herbicide applications, or 2) emerged following the last preplant tillage operation and before planting.
 
“Plants that have escaped preplant tillage operations often have contorted, ‘C-shaped’ stems as a result of being damaged by the field cultivator,” Hager said. “These plants can be very difficult to control with postemergence soybean herbicides.”
 
Hager offered several possible explanations for why the large weeds escaped a preplant burndown application. They may have developed glyphosate resistance; glyphosate-resistant waterhemp and marestail are found across much of Illinois. Environmental conditions before and during the application were not always conducive to good herbicide performance and may have reduced the activity of the glyphosate tankmix partners against glyphosate-resistant plants.
 
Glyphosate-resistant waterhemp can be controlled by foliar-applied protoporphyrinogen oxidase (PPO) inhibitors (such as lactofen (Cobra), fomesafen (Flexstar) or acifluorfen (Ultra Blazer)) in conventional or glyphosate-resistant soybean varieties, or by glufosinate (Liberty) in glufosinate-resistant (Liberty Link) varieties.
 
“However, it is very important to remember that these herbicides do not extensively translocate within the weed following their absorption through the leaf surface,” said Hager. They are less effective on large weeds than on small weeds (5 in. or less).
 
Few herbicide options are available to control emerged glyphosate-resistant marestail in soybean. Cloransulam (FirstRate) or chlorimuron (Classic) can be applied to conventional soybean varieties or tankmixed with glyphosate for glyphosate-resistant soybean varieties, but control of marestail plants larger than 6 in. is often inconsistent.
 
Another possible reason for the weeds surviving the burndown application is that insect tunneling/feeding damaged the internal stem tissue. Weed species known to harbor stem-boring insects include waterhemp, giant ragweed, horseweed/marestail, annual smartweed species, common ragweed, and common lambsquarters. Insects from several orders including Lepidoptera  and Coleoptera present in these weed species as either larva or adults.
 
This is not the first season where instances of insect tunneling seem to have reduced the effectiveness of translocated herbicides. U of I researchers have investigated the relationship between insects and weeds. Research conducted in 1976 found that most stem-boring insects preferred large-diameter stems. “In other words, it was difficult to find an insect tunneling within the stem of a 4-inch-tall giant ragweed, but much easier to find one tunneling in the stem of a 12-in.-tall giant ragweed,” said Hager.
 
If damaged stem tissue caused by insect tunneling was partly or entirely responsible for the plant surviving a herbicide application, the plant would probably survive a supplemental herbicide application.
 
In all of the situations described above, Hager says that these large weeds might require the implementation of a “non-traditional weed management tactic to achieve complete control.” Specifically, it may be necessary to dig them out or cut them down.