To date, Illinois farmers have been able to avoid the yield-sucking super weeds that must be yanked from southern fields by hand.

Unfortunately, mechanical intervention may soon be necessary to control waterhemp as it cruises toward super-weed status. University of Illinois weed scientist Aaron Hager says his laboratory has confirmed waterhemp biotypes with stacked resistance to four different herbicide families.

"The numbers suggest that this is not an infrequent occurrence," Hager notes. "These stacked biotypes will only become more prevalent as time goes on."

Thus far, U of I has confirmed waterhemp populations that are resistant to ALS and PPO inhibitors, triazines and glyphosate herbicide families. These stacked species are still susceptible to HPPD inhibitors. However, Hager expects HPPD resistance to be stacked along with the other four in the near future.

Resistance evolution

U of I crop sciences professor Patrick Tranel says they first saw resistance to ALS chemistry in the mid-1990s. From a scientific standpoint, ALS-resistant waterhemp plants contain a mutation which affects the target site, making ALS herbicides ineffective. Perhaps more importantly, ALS resistance is a dominant trait, inherited by the plant's offspring.

Shortly after ALS resistance was identified, researchers began finding instances of triazine resistance. Tranel says there are two main types, one of which comes with a significant growth penalty for the waterhemp plant.

The other type, which is more common, does not appear to reduce the vigor of the plant. It is transmitted via pollen and seed. In addition, the second type of triazine resistance stacks with other resistances.

In the early 2000s, U of I scientists found evidence of resistance to PPO inhibitors. Another target-site mutation, this resistance creates a moderate level of resistance to PPO inhibitors.

When PPO resistance was discovered, farmers were in the midst of adopting Roundup-Ready technology. Tranel says this kept the problem at bay. Even though PPO use has declined, Tranel says there's enough out there to keep the selection pressure on for PPO resistant biotypes of waterhemp. "It remains a lurking menace," he adds.

Of course, the entire Cornbelt has paid close attention to the evolution of waterhemp's resistance to glyphosate. Tranel says much is still not known about this resistance only that it appears to also affect glyphosate's target site. This type of resistance can be moved in pollen and seed.

At this point, even less is known about HPPD resistance. Unlike other resistances, most scientists theorize this is a metabolic resistance. Hager says researchers are working from two primary confirmed examples of HPPD resistance. One was discovered in Iowa, the other in Illinois. Both were in seed corn fields.

"There is no evidence that it will stay confined to seed corn," Hager adds.

Quantifying resistance

In 2009, U of I began surveys in an attempt to quantify just how prevalent stacked resistance is in waterhemp populations. Tranel says growers were asked to send plants, which were suspected of being herbicide resistant, to the Urbana lab. Plants were tested for ALS, PPO and glyphosate resistance.

The first year, 93 plants were sent in. Of those, 5% were confirmed to be resistant to all three chemistries. A staggering 58% were resistant to glyphosate. In 2010, 122 plants were sent in from 24 different fields. Of those, 7% were resistant to all three herbicide families. Glyphosate resistance was found in 66% of the plants.

Tranel says the test results also proved that glyphosate resistance is not developing in one specific area of Illinois. It's showing up throughout the entire state.

"If a farmer thinks he has glyphosate resistance, he probably does," Tranel notes.

Flint is editor of Prairie Farmer magazine, a sister Farm Progress publication in Illinois.