Weed pressure continues to be a big challenge for growers. There are, however, opportunities for improvement this growing season.
One common misconception is that tillage eliminates the need for a pre-emerge herbicide.
“While tillage can kill some early weeds, it also stimulates weed seed germination by disturbing the soil, especially if there's been time between burndown and tillage,” said Troy Deutmeyer, Pioneer field agronomist in eastern Iowa. “We saw several cases in 2024 where deep incorporation of herbicides (more than 2 inches) diluted product concentration and reduced efficacy. For best results, apply a full rate pre-emerge herbicide to the soil surface and limit incorporation depth to preserve effectiveness.”
In addition, partial rates of pre-emerge and post-emerge herbicide applications have been common in recent years. Partial rates often mean partial control and can accelerate weed resistance, according to Deutmeyer.
“The University of Illinois has documented resistance to waterhemp by several Group 15 herbicides,” he said. “The solution is to apply full labeled rates and include multiple effective modes of action in both pre and post passes.”
Timing Your Post Pass: Earlier Is Better
Another area of improvement is spray timing. Post-emerge applications are often made too late. Targeting early growth stages — V2 corn or V2-V3 soybeans — gives herbicides the best chance of success. Small weeds are easier to kill and the crop canopy allows for better spray penetration and less herbicide uptake by the crop.
Contact herbicides require adequate coverage. Ten gallons per acre is often not enough, especially in dense weed canopies. Increasing spray volume to 15 to 20 gallons per acre improves droplet coverage and control.* Higher volume also tends to produce finer droplets, enhancing uptake. Finer droplets are often preferred, but depending on product type and weather, too fine could cause drift issues.
Many tank mixes claim to offer multiple modes of action, but only one or two components may be truly effective against key weeds like resistant waterhemp. Tools like the Iowa State Weed Guide or other Extension guides can help verify the number and effectiveness of active ingredients. Check product labels and know your targets.
Water Quality: A Hidden Factor in Weed Control
Water quality is often overlooked but can make or break herbicide applications. High pH and water hardness can bind up active ingredients before they reach the plant. Testing the pH and hardness of your water is a must. Based on the results, you may choose to add a water conditioner. Conditioning water with ammonium sulfate before adding a herbicide can tie up calcium and magnesium ions, preserving herbicide availability.
