Globally, 250 weed species are resistant to 160 herbicides and 23 (out of 26) herbicide site-of-action groups. In Nebraska, eight weed species (common ragweed, common waterhemp, giant ragweed, kochia, marestail, Palmer amaranth, redroot pigweed, and shattercane) have been confirmed resistant to at least one herbicide.  Six of them — common ragweed, common waterhemp, giant ragweed, kochia, marestail, and Palmer amaranth — have been confirmed resistant to glyphosate in Nebraska.

Additionally, some weeds are resistant to several herbicides from different site-of-action groups. These are known as multiple herbicide-resistant weeds; for example, several populations of common waterhemp in eastern Nebraska are resistant to glyphosate and acetolactate synthase (ALS) inhibiting herbicides (Classic®, Pursuit®, Raptor®, etc.) (Read more about multiple herbicide-resistant weeds.)

Earlier this summer at the Weed Management Field Day at UNL’s South Central Agricultural Laboratory at Clay Center, strategies to manage herbicide-resistant weeds were described as proactive and reactive. In case you missed the field day, the following discussion reviews these topics and demonstrations.

Pre-Mix Herbicides

For the last three decades, no new herbicide site-of-action was introduced in the market, though a small number of new active ingredients were commercialized. Most of the new herbicides introduced in the marketplace in recent years contain new mixtures of existing active ingredients.

An effective pre-mix herbicide may comprise two or more active ingredients with two or more sites-of-action. These mixes can provide broad-spectrum weed control in corn and soybean, resulting in reduced selection pressure to a certain herbicide chemistry.  Some pre-mixes provide good contact and residual control for most weed species.

For example, Acuron®, a new broad-spectrum corn herbicide, is a premix of four active ingredients (atrazine, bicyclopyrone, mesotrione, and S-metolachlor) which belong to three site-of-action groups (5, 27, and 15). Bicyclopyrone is a new active ingredient belonging to the HPPD inhibitor group (27), the same group with mesotrione (Callisto®). Acuron can be applied as pre-emergence or post-emergenece in corn up to 12 inches high and offers both residual and contact activity. (Read about Acuron.) Also, several herbicide pre-mixes contain crop safener (e.g., benoxacor in Acuron and CSI technology in DiFlexx® and DiFlexx® Duo), which may provide some extra time for late post-emergence application of herbicides to control later emerging weeds. DiFlexx Duo can be applied broadcast up to V7 stage or to 36-inch tall corn. Directed application can be made from V7 to V10 growth stages.

See these CropWatch articles for more information about new corn and soybean herbicides.

Key Take-Home Points

To combat the further development of herbicide-resistant weeds and the resulting loss of effective weed management technologies:

1. Apply herbicides at labeled rates and at recommended growth stages and timing.
2. Scout/monitor the field after herbicide application and check for weedy patches that survived the application.
3. Use multiple effective sites-of-action in the form of sequential applications and/or prepackaged mixtures.
4. Use soil residual herbicides with effective multiple sites-of-action.
5. Clean farm machinery to prevent dispersal of weed seeds.
6. Rotation! Rotation! Rotation! Rotate crops and rotate different herbicide-tolerant crop technologies. Change your herbicide program every year to use herbicides from different sites-of-action. (While it may work well one year, to protect long-term use, don’t repeat it the next year.