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Part 4: Latest Research on Herbicide Resistance

by Wyoming Livestock Roundup

By Jeremiah Vardiman

My last article discussed how tillage, crop rotation and herbicide regimens contribute to a field’s overall weed management plan. 

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

Of course, this sounds wonderful and should be the management strategy for every producer, correct? 

Obviously, it is not this simple. The previous articles have only discussed the best weed management control methods and have not included other important elements, specifically, the economic factors influencing management control options.

The latest research from the University of Wyoming’s (UW) Plant Sciences Department explored the economic benefits and risks with implementing a diverse herbicide resistant weed management program. 

Previous research indicated economics are typically the predominant factor influencing adoption of any management strategy. Also, economics is the predominant factor influencing the adoption of herbicide resistance weed management. 

This makes sense because crop prices influence crop rotations and crop selection within the rotation. In addition to crop prices, assumed costs associated with herbicide resistant weed management have also influenced why producers do not adopt these more diverse practices.

In this study, yield and kochia density for all crops were affected by crop rotation and herbicide regimes. This impact can be seen from the overall corn yields, because corn was the only crop in all four rotation treatments, which increased as the diversity of crop rotation increased. 

The lowest corn yield (97 bushels per acre) resulted in the least diverse crop rotation (continuous corn), and the highest corn yield (194 bushels per acre) was in the most diverse crop rotation (corn-dry bean-small grain-sugarbeet). 

The impacts of herbicide treatments were also noticeable in the corn yields, with the lowest yields in plots only treated with acetolactate synthase (ALS) herbicides, and the highest yields were in plots treated with herbicide tank mixtures. 

Small grains were the only crops that did not show varying yield trends when compared to the other crops. This is attributed to this crop selection’s competitive advantage for low kochia densities.

Tillage treatments were not consistently beneficial to yields. However, it did often help with kochia density in the plot. To an extent, tillage did benefit dry bean yields, which were the highest in plots with ALS-inhibitor herbicide tank mixtures and intensive tillage.

Increase in crop yields is just part of an agricultural operation’s economics. Returns over variable costs and an owner’s labor have impacts as well. 

This study demonstrated diverse weed management programs were associated with lower kochia densities and higher yields, in addition to being the most profitable management programs after four years, with crop rotations and herbicide regimes being the predominant drivers of these results. 

This study found the highest returns for each crop rotation, tillage and herbicide regime were found in the four-crop rotation which was treated with a herbicide mixture and intensive tillage and averaged $275.40 per acre, while this same treatments in minimally tilled averaged $162.70 per acre.

 These findings support the concept that diverse, herbicide resistant weed management programs can be more profitable than non-diverse programs. The overall results suggests a reduction in management diversity was nearly always associated with an increase in kochia density and a decrease in sustainable profits.

So what does this mean? 

This study demonstrates diverse weed management programs implemented effectively result in significantly greater overall profits. Profits were three times greater in the four-crop rotation with intensive tillage and herbicide mixture treatments when compared to the same treatments in the three-crop rotation. 

The four-crop rotation with the same treatments was twice as profitable as the two-crop rotation under the same treatment, and nearly nine times more profitable when compared to the single crop rotation. Undoubtedly, the small grains crop drove the increase in profits because of its high competitiveness against kochia plants, not because it is the most economically 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. 

As for kochia, the most effective, diverse and profitable management program incorporated selecting a competitive crop (small grains), having a four-crop rotation, herbicide tank mixtures which were effective against ALS resistant kochia and intensive tillage.Jeremiah Vardiman is a University of Wyoming agriculture and horticulture Extension Educator. He can be reached at jvardima@

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