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Part 5: Latest research on herbicide resistance

by Wyoming Livestock Roundup

My last article discussed the economic benefits and risks of implementing a diverse, herbicide-resistant weed management program.

As previously mentioned, the study demonstrates a diverse weed management program implemented effectively results in greater overall profits. Undoubtedly, the small grains crop drove the increase in profits because of its high competitiveness against kochia plants, not because it is the most profitable crop. 

Long-term, effective weed management programs, especially for herbicide-resistant weeds, must focus on incorporating as many diverse, effective control methods as possible for the weed in question. 

Study objectives

Diverse weed management programs are more profitable and provide the best control of kochia populations in the field. This is wonderful, however, the deeper question facing the agriculture industry and particularly producers is, “How does the impact of crop rotation, tillage treatments and herbicide regimes impact resistant weed populations?” 

Is this impact negative, resulting in higher resistance in the weed population or positive with lower levels of resistance? The latest research from the University of Wyoming’s Plant Sciences Department addresses these very questions. 

The last objective of this large study was to quantify the impact of crop rotation, tillage and herbicide regimes on the enrichment of resistant to susceptibility ratios of acetolactate synthase (ALS) resistant kochia. 

In simpler terms, the last objective of the study is to determine if these herbicide resistant weed management practices are increasing or decreasing the level of resistant weeds within the population.

The large study was conducted over four years under field conditions in four locations – Lingle, Powell, Scottsbluff, Neb. and Huntley, Mont. The research focused on kochia management since it is a problematic weed in the major crops grown in Wyoming, such as sugarbeets, corn and dry beans. The plots were seeded with kochia at a rate of 1.8 pounds per acre and a known concentration of five percent ALS-resistant kochia seed.


            Results indicated crop rotation was the only treatment which significantly impacted the resistant proportion of kochia populations actively growing in the field and also found herbicide treatments were only marginally impactful. 

In contradiction to the study’s hypothesis, the more diverse crop rotation resulted in the greatest proportion of resistant kochia plants. However, this crop rotation also resulted in the lowest kochia plant density. 

This means a high proportion of resistance does not mean high population densities or resistant seed per area. The one-crop, two-crop and three-crop rotations were found to contain similar proportions of resistant kochia plants and resulted in the lowest proportions of resistant kochia, even though the kochia population densities were higher.

Tillage treatments, including intensive versus minimal tillage, resulted in no difference in the resistant proportions in the population. The three different herbicide treatments, ALS-inhibiting herbicides every year, tank mix of ALS-inhibiting and second mode of action for ALS-resistant every year and annual rotation of non-ALS inhibiting then ALS-inhibiting herbicide the next year, resulted in no difference as well, which is contradictory to other field studies conducted on the evolution of herbicide resistance. 

This could be attributed to gene flow from kochia pollination – kochia pollen is able to travel large distances. There was no difference in control when treatments were combined.

             This study also found tillage, crop rotation and herbicide treatments had the greatest impact to resistant seed density after four years of the study. Intensive tillage had better control with lower resistant seeds than minimum tillage. 

The most diverse crop rotation also contained the lower seed density when compared to the one-crop, two-crop and three-crop rotations. The lower seed densities in the tillage treatments and most diverse crop rotation are attributed to the low kochia plant densities within these treatments, i.e., fewer kochia plants results in fewer seeds in the seed bank.

            As for the herbicide treatments, the lowest seed density was found in the ALS mixture treatment, which contained a mixture of ALS herbicide and a second mode of action to control ALS-resistance, when compared to an ALS herbicide only and to annual rotation of ALS.

Research implications

            Even though this study indicated there was no difference to resistance between diverse and non-diverse weed management practices, still there are key factors to consider with weed management programs, especially for herbicide resistance.

The more diverse weed management practices always resulted in the lower kochia plant densities and seed production, which results in a decreased amount of resistant seed produced. This demonstrates high levels of resistance are not always found or associated in fields or areas with high density of weeds or resistant seeds in the seedbank. 

It also reinforces the notion that long-term weed management needs to focus on the weed seeds within the soil seedbank, not just the weeds above the soil surface. 

            The overall conclusion for this large research study is proactive herbicide-resistant weed management practices can impact the establishment and development of targeted weeds and herbicide resistant weeds, while also being profitable to implement. This also includes the idea that management strategies should not only focus on herbicide regimens. 

Incorporating as many effective control practices as possible results is the best weed control. In this specific example, tillage, crop selection and crop rotation had significant impacts to kochia density in the field and seed density in the soil seedbank. 

            The results of this study, such as small grains being the most competitive crop, only apply to other annual weeds which are similar in growth and development to kochia. More research is needed to find the applicability of cultural and mechanical weed control practices to other resistant weeds, such as biannual, perennial and complex perennial weeds.

Jeremiah Vardiman is a University of Wyoming Agriculture and Horticulture Extension Educator. He can be reached at jvardima@

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