Weed Management: Water-hemp Popping Up

Water-hemp is becoming a real headache as a weed. Drive around the countryside in August, regardless of what state you’re in, and you can see water-hemp popping out of soybean fields. In the fields that remain clean all the way to harvest, I’m pretty certain that the grower is probably following a 3-prong approach to keep the population under control that includes:
• Residual at planting
• Tank mix at post application
• Using different modes of action in the previous corn crop

So why has water hemp become such an issue lately? I attribute it to three things:

First, waterhemp germinates for several months before emerging. It used to be that if we got weeds controlled at planting or a few weeks after with postapplication, we didn’t have to worry about late germinating weeds because there weren’t many and shading suppressed any growth. That doesn’t seem to be the case today.

Second, water-hemp is resistant to glyphosate and ALS herbicides (Pursuit®, Classic). There are ever increasing populations of water-hemp that are resistant to glyphosate and they are spreading and becoming a larger part of field populations. Over-dependence on glyphosate has certainly contributed to the spread of this weed.

Third, wet weather in May and early June delayed planting and prevented either application of pre-emerge herbicides or timely applications of postemerge herbicides. In addition, the effectiveness of early preplant herbicides application had worn down by the time beans were planted. This may have been the biggest cause of the huge outbreak of water-hemp later this summer.

Going forward, here are the strategies:
• Don’t depend solely on glyphosate (I think everyone knows that by now)
• Consider planting LibertyLink® or 2,4-D or dicamba resistant soybeans in your rotation Spray when weeds are less than 3 to 4 inches in height
• Use a residue herbicide at planting that has efficacy against water-hemp
• If your field has a history of water-hemp and problems with control, use a tank-mix for post- application
• If the weather turns wet (like in 2015), react quickly and adjust your weed control program to keep weeds at bay and prevent a buildup in weed seed

2015 was a tough year for achieving effective weed control, but some growers achieved it. Continue to look at your weed control program through two lenses; what normally works and (if weather throws you a curve again), how you’re going to adjust.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or leave a comment below.