Modes of Action & Resistance Management

Rotate is good; overlap is better

Old advice was "rotate chemistry each year." Current weed-science consensus: use multiple effective sites of action in the same pass. A weed surviving Group 15 must also survive Group 5 in the tank — the odds of both mutations collapse. Rotation across seasons still matters, but the tank mix does the heavy lifting.

"Effective" is the operative word: adding a group your weeds are already resistant to isn't a second site of action, it's expensive water.

The five non-chemical multipliers

• Start clean — burndown or tillage; never spray a weed taller than the label's max height.
• Full labeled rate — cutting rate is the fastest way to select resistance.
• Residuals — layering pre-emerge chemistry reduces the number of weeds that ever see the post pass.
• Narrow rows / cover crops — canopy closure is free weed control.
• Stop seed set — one escaped waterhemp can produce hundreds of thousands of seeds; clean escapes before harvest.

Fungicides and insecticides deserve the same discipline

FRAC 11 (strobilurin) resistance in frogeye leaf spot and IRAC 3 (pyrethroid) failures are real in row crops. For fungicides, prefer pre-mixes with two effective FRAC groups and hold single-site products for the pass that needs them. For insects, scout to threshold rather than spraying insurance passes — every unnecessary application is selection pressure.

A one-page plan you can actually follow

1. List your top 3 problem pests/weeds per field.
2. Write the group numbers you used last season (from records).
3. Plan this season with ≥2 effective groups per target, changing at least one from last year.
4. Record actual groups applied in your application log — that's the data next year's plan needs.

Group classifications per HRAC/WSSA, FRAC, IRAC. Mixing mechanics: adjuvants & tank mixing.