Mounting resistance threatens pesticide power

Waterhemp resistance to her­bicides is old news. Since arriving on the scene, this pesky weed has quickly developed some level of resistance to multiple sites of action. The subject of this article, water­hemp resistance to Group 27 or HPPD-inhibiting herbicides, is also nothing new. What is new—and concerning—is whether we are ap­proaching a tipping point that leads to widespread resistance.

HPPD stands for the plant enzyme, hydroxyphenylpyruvate dioxygenase, thus the need for a much easier acronym. To simplify a complex process, the inhibition of HPPD by this type of herbicide prevents plant growth by destroying chlorophyll and blocking photosynthesis.

HPPD-inhibitor herbicides include products such as Balance (pre-emer­gent only), Callisto/Explorer, Laudis, and Armezon/Impact. They are also part of many widely used premixes such as Acuron, Resicore and Lexar. HPPDs became popular in the late ’90s and early 2000s for their resid­ual activity as well as post-emergent activity, particularly with waterhemp.

Prior to HPPDs, dicamba was often the go-to herbicide for cleaning up waterhemp in corn, but products such as Callisto quickly took over. Crop safety, ease of use, low rate and overall efficacy won loyalty from growers and retailers alike. Glypho­sate also contributed to the transition from dicamba as Roundup Ready corn gained market share, but resis­tance issues made this a short-lived option for managing waterhemp.

The first chinks in the armor oc­curred over a decade ago. Resistance was reported in two seed corn pro­duction fields in Iowa and Illinois in 2009. Seed corn inbreds are not as competitive with weeds as hybrid corn. The problem is compounded by the open canopy that occurs after detasselling, so multiple post-emer­gent applications were used. The field in Illinois had also been planted in continuous corn, which created additional pressure.

Since that first discovery, HPPD resistance in waterhemp has been confirmed in multiple states, in­cluding Missouri. The University of Missouri reported several sites with resistant populations nearly 10 years ago. These trials looked at mesotrione and did not include atrazine in the mix. Atrazine tank mixed with mesotrione results in synergistic activity on waterhemp and is recommended on the label in certain circumstances. Mesotri­one alone can be effective on small plants, but atrazine becomes very important when taller plants also exist. The label cutoff for waterhemp is 3 inches without atrazine and 5 inches with atrazine. Unfortunately, plants much larger than 5 inches are often sprayed, and atrazine is almost always added when corn measures less than 12 inches.

When the MU data was released, most waterhemp in the country was still being controlled with HPPDs. At the time, the issue did not create major alarm like the other wide­spread resistance issues we had seen.

Fast forward to 2022. Multiple fields were reported to have wa­terhemp survive post-emergent applications of HPPD herbicides. Some of these included plants taller than 5 inches. A few mixes did not appear to have the correct adjuvant load. However, there were many fields where the weeds were small, the adjuvants were correct, and the weeds still survived.

In at least one field I observed, a second post-emergent pass using a different HPPD active ingredient failed after the first post-emergent pass of mesotrione failed. We collected seed in a number of these fields for resistance testing. At press time, the seeds have germinated un­der greenhouse conditions, and the plants are slowly growing. It will still be several weeks before we know whether the plants are resistant.

Even without those test results, growers should incorporate pre-emptive practices into corn weed-control programs. It is much easier to kill a weed in the soil when it has one growing point than when it has emerged with multiple grow­ing points. Relying on a post-emer­gent application to clean up emerged waterhemp in corn is fast becom­ing risky business, just as it is in soybeans. Using multiple effective modes of action prior to waterhemp emergence is key.

If you are forced to control emerged weeds, consider using an­other mode of action. In corn, that usually brings us back to dicam­ba. If you do use an HPPD after emergence, it’s important to scout the field to confirm the weed kill. Scouting may give you a chance to come back with another mode and still clean up missed weeds.

Last, but definitely not least, fol­low the label and include the proper adjuvants at the correct rates.

Regardless of whether last year’s issues are confirmed to be herbi­cide resistance or are attributed to weather or other conditions, we know it is now tougher to control weeds that we’ve controlled in the past. Incorporating an aggressive “start clean, stay clean” approach will help preserve one of the few weed control tools we have for waterhemp.