July 25, 2021

Gucker: Tall waterhemp not dying

Here in central Illinois, this spring has delivered its share of surprises. The planting season got off to a good start in April with warm and dry weather with a lot of corn and soybean being planted. The first half of May was cool and wet with very little fieldwork taking place until the second half of the month.

The second half of May was warm with good growing conditions for both crops and weeds. I watched tall waterhemp grow 5.5 inches in just 5 days. It went from 2 to 7.5 inches. Unfortunately, it needed to be sprayed at 6 inches to be effectively controlled by an herbicide.

This is the third year in a row that I have seen tall waterhemp over 6 inches tall sprayed with a post-emergent herbicide combination containing dicamba. Last year, I noticed a few fields where it seemed that tall waterhemp had survived a late application of dicamba and appeared to have small seed heads.

In 2019, University of Tennessee weed scientist Dr. Larry Steckel noted seeing more escape pigweeds in fields where dicamba had been sprayed. Steckel suspected that this was the beginning of dicamba resistance development as he had seen earlier with glyphosate.

I, too, am wondering if I am seeing the beginning of dicamba resistance here in central Illinois. If that happens, how are we going to control the tall waterhemp and other pigweeds in our corn and soybean rotations?

In Illinois, we currently have tall waterhemp resistant to seven different herbicide sites of action groups — 2, 4, 5, 9, 14, 15 and 27. These affected herbicide groups include the following commonly used herbicides: glyphosate, atrazine, cyanazine, Callisto, Dual, Laudis, 2,4-D, Impact, Reflex, Outlook, Classic and the list goes on. There are no new herbicide modes of action being developed.

What is the answer to preventing dicamba herbicide resistance? An integrated weed management program for your fields.

What does that look like? It involves using all the weed control methods available to you — cultural, mechanical and chemical.

Plant in a weed-free field. Use mechanical weed control methods. And, yes, they do make 24-row cultivators.

Rotate your herbicides — do not make more than two consecutive applications of herbicides with the same mode of action. Use herbicide tank mixes with modes of effective action.

Rotate crops — small grains and forage crops are very effective in reducing pigweed populations.

Clean your equipment when moving from one field to another. Tillage tools and combines carry weed seeds from one field to another.

We are running out of herbicides that will control tall waterhemp and the other pigweeds. If you want to keep having dicamba as an effective herbicide to control pigweeds in Illinois, attack tall waterhemp and the other pigweeds with an integrated weed management program for your farming operation.

Doug Gucker is University of Illinois Extension local food systems and small farms educator.