Jeremiah Vardiman

Weed management plans need to impact weeds on all aspects of their growth and development for effective control — starting with emergence, to survival of the weed and all the way through seed production.  Long-term reduction of the weed seed bank requires minimizing the seed produced per area by a combination of reducing weed establishment and viable seed produced per plant. 

As herbicide resistance becomes more prevalent in weeds, how should weed management plans be changed to keep good weed control? 

To answer this, the latest research from the University of Wyoming’s plant sciences department explored the impacts from varying crop canopies and acetolactate synthase (ALS)-inhibiting herbicide applications on kochia density and seed production. 

Overall results indicated combining effective crop canopies with effective herbicides provides the best long-term weed management. More specifically, results demonstrated seed production of kochia is influenced more by crop selection than by herbicides. 

Selecting crops with competitive crop canopies reduces weed pressure by out-competing the weeds for resources, specifically light, and can create unfavorable conditions for seed germination.

This study was conducted under field conditions in four locations: Lingle and Powell; Scottsbluff, Nebraska; and Huntley, Montana. The plots were planted to a 5% ALS-resistant kochia seed blend to establish a uniform kochia population in the plots and a known herbicide resistant population. 

This study assessed the effectiveness of four crop canopies — spring wheat, corn, dry beans and sugar beets — and impacts of ALS herbicides versus non-ALS herbicides.

Spring wheat treated with non-ALS herbicide was the best treatment, resulting in no kochia seed production. Spring wheat’s crop canopy and crop management contributes to reduced kochia density and low seed production per plant because of how early in the season the crop is planted, the density of the crop canopy and the harvest of the crop occurring during kochia seed set and maturity. All reduced production of viable seed. 

Adding an effective herbicide treatment further reduced kochia densities within the crop, which completely eliminated seed production in spring wheat plots.

Like spring wheat, corn is also a very competitive crop canopy against weeds. Again, combining the effective herbicide treatment with a competitive crop canopy is the best control option. The results from the corn crop also demonstrated the importance of proper herbicide selection: Non-ALS herbicide treatments resulted in 24% less kochia plants than using an ALS herbicide treatment. Of course, this can be attributed to the non-ALS herbicide’s ability to control both ALS susceptible and resistant kochia.

Unlike spring wheat and corn, dry beans and sugar beets are not very competitive crop canopies because of their short stature. Dry bean and sugar beet plots had the greatest amount of kochia seed production. Sugar beet was the least competitive crop. It allowed the highest kochia densities and seed production of all the plots. 

The management of these crops do not offer great control options for kochia populations, especially because of sugar beet’s slow early-season canopy development and how little impact the harvest has on kochia seed production. 

The sugar beet plots demonstrated that improper herbicide selection can even aggravate weed control. ALS herbicide treatments resulted in seven times more kochia plants producing seed than non-ALS herbicide treatments.

The results for only the herbicide treatments across all of the crops were as expected: Non-ALS herbicide treatments resulted in less kochia plants than with ALS herbicides. In comparison to crop choice, the herbicides’ effect on kochia density was 47 times greater. This indicates herbicides have a greater impact on the control of actively growing kochia plants than crop selection; however, kochia seed production per plant was more influenced by crop choice than herbicide.

This study demonstrates crop selection is a significant contributor to the overall weed management plan for a field. Selecting spring grain, such as wheat or barley, in the crop rotation provides the best competitive crop canopy and crop management for controlling annual weeds similar to kochia. 

This study also demonstrates that relying on effective herbicides in poorly competitive crops, such as sugar beets, does not provide adequate control to reduce seed production. The overall results of this study demonstrate that the best long-term weed management plan combines competitive crop selection and effective herbicides.

(Jeremiah Vardiman is an agriculture and horticulture extension educator for the University of Wyoming Extension. He is based in Powell.)