Part 3: Latest Research on Herbicide Resistance

By Jeremiah Vardiman

My last article discussed how crop selection, canopy and management contributes to a field’s overall weed management plan. Competitive crop selection and effective herbicides combined showed the best long-term weed management plan. 

So, how does tillage, crop rotation and herbicides affect long-term, herbicide resistant kochia management? 

The latest research from the University of Wyoming’s Plant Sciences Department explored the impacts of tillage, crop rotation and herbicide applications on kochia density (surviving and living plants), seed production (seeds per plant) and seedbank density (seeds in soil). 

Research explored tillage versus minimum tillage, four different crop rotations and three in-crop herbicide regimes within each tillage and crop rotation. Intensive tillage in the spring with a rototiller to invert the soil, comparable to plowing and disking operations, mixed the top four inches of the soil. Soil in minimum tillage plots was only disturbed when necessary because of harvest activities, such as with sugarbeets. 

The four crop rotations were continuous corn, corn-sugarbeet, corn-dry bean-corn-sugarbeet and corn-dry bean-small grain-sugarbeet. The crops are common in Wyoming and also have diversity in canopy, planting and harvest dates.

Three in-crop herbicide regimes were applied within each tillage and crop rotation treatment. The first was entirely ALS-inhibiting herbicides each year. The second was a tank mixture of ALS-inhibiting herbicides and a second mode of action to control ALS-resistant. The last was an annual rotation of non-ALS-inhibiting herbicide one year, then ALS-inhibiting herbicides the next year. 

Herbicides were chosen for the highest labeled rate within the crop and would not cause a carryover problem for subsequent crops.

An annual spring tillage reduced kochia density better when compared to minimum tillage. Weed seeds were buried deep enough to block germination stimuli needed to trigger germination. This forces the seed to deplete energy reserves waiting for germination conditions and can kill the seed if the period is prolonged. 

This management is ideal for weed species with a short-lived seed life, such as kochia, whose seed life is approximately two years.

Kochia plant densities were also the lowest in the most diverse crop rotation – corn-dry bean-small grain-sugarbeet. The two crop rotation, corn-sugarbeet, showed the greatest kochia density followed by continuous corn.

These results prove the importance of diverse crops within a crop rotation, and equally important is selecting a crop proven competitive against the desired weed species and the weed’s lifecycle. Small grains, such as wheat or barley, were the most competitive crop, whereas sugarbeets are a poor competitor against weed species like kochia.

As for the herbicide regimes overlaid on the tillage and crop rotation treatments, ALS-inhibiting herbicide tank mixtures consistently managed the lowest kochia densities. Herbicide mixtures are more effective at controlling resistant weed populations compared to yearly herbicide rotations. 

Weed management programs should diversify herbicide controls every season instead of every other year.

Intensive tillage did not directly impact kochia seed production similarly to minimally tilled plots. Tillage was early in the spring and would not have affected any surviving kochia plant’s seed production during the growing season.

The continuous corn and the four-crop rotation caused the least amount of kochia seed per plant. This indicates diversity to the crop rotation is not necessarily beneficial to weed management, and proper selection of crops within a given rotation is just as important as diversifying a crop rotation. 

This is seen when the crop rotation was diversified with adding sugarbeets to the corn-sugarbeet rotation, resulting in the greatest amount of seed production per kochia plant, compared to the continuous corn – no crop rotation diversity – treatment, which had lower seed production.

Similar to the kochia density, the ALS-inhibiting herbicide tank mixtures consistently managed the lowest kochia densities, while the ALS-inhibitor herbicides-only treatments were the highest seed production. 

Lastly, the results of the kochia seedbank density tests indicated minimally tilled plots were three to five times greater than intensively tilled plots. The four-crop rotation had the least amount of kochia seed per unit area throughout the duration of the study, whereas there was no difference between the other three crop rotation treatments.

 Again, the ALS-inhibitor herbicide tank mixture resulted in the least kochia seeds within the soil seedbank. Interestingly, the annual rotation of ALS-inhibitor with non-ALS-inhibitor herbicide rotation not only was less effective at reducing the weed seedbank, but there was no significant difference in the weed seedbank when compared to the ALS-inhibitor herbicide-only treatment for the four years of the study.

So what does this mean? 

Integrating a diverse weed management program for herbicide resistance kochia is advantageous. This study also demonstrates implementing diverse cultural and mechanical weed management programs effectively outweigh the benefits of only relying on a diverse chemical weed management program. 

An effective long-term proactive weed management program, especially for herbicide resistant weeds, must not only control weeds above the soil surface but also reduce weed seed production and the soil weed seedbank.

Jeremiah Vardiman is a University of Wyoming agriculture and horticulture Extension Educator. He can be reached at jvardima@ uwyo.edu.