Pest Control · May 2025 · 5 min read
Herbicide-resistant weeds now affect over 150 million acres in the U.S., costing growers billions in extra management. Rotating sites of action and diversifying your weed control program are the most effective defenses.
Every herbicide is classified by its site of action (SOA) group number, printed on the label. Group 9 (glyphosate) and Group 2 (ALS inhibitors) are the most overused, with widespread resistance in waterhemp, Palmer amaranth, and marestail. Alternating between different group numbers each year and within a season prevents any single weed population from adapting.
Apply multiple effective SOA groups in a single pass to hit resistant biotypes from different angles. A pre-emergence application combining Group 15 (metolachlor) with Group 5 (atrazine) followed by a post-emergence pass using Group 4 (dicamba or 2,4-D) covers several resistance mechanisms. Always confirm that each tank-mix partner is individually effective on your target weed species.
Herbicides alone cannot solve resistance—integrate narrow row spacing, competitive crop varieties, and cover crops to shade weeds out. Harvest weed seed management, including chaff lining or impact mills, prevents resistant seed from returning to the soil bank. Hand-rogueing small patches of escapes before they set seed can stop a resistant population from spreading across the field.
🗒 Track your applications with our free tool:
Try the Spray Log