This article is the second in a three-part series detailing a long-term herbicide resistance study being undertaken by researchers from the University of Wyoming. The first part, in the February issue of Sugar Producer, discussed how crop selection, canopy and management contribute to a fields overall weed management plan. Be sure to check back in our April issue for further information.
How do 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 4 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 weeds. 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 so as not to 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 that 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. Equally important is selecting a crop proven to be competitive against the desired weed species and the weed’s lifecycle. Small grains (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 — similar 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 that diversity to the crop rotation is not necessarily beneficial to weed management, and that 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 by adding sugarbeets to the rotation (corn-sugarbeet), resulting in the greatest amount of seed production per kochia plant, compared to the continuous corn (no crop rotation diversity) treatment that 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 showed the highest seed production.
Lastly, the results of the kochia seedbank density tests indicated that 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 fewest 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-resistant kochia is advantageous. This study also demonstrates that 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.
Show Me the Money
By this point, we’ve established that this study demonstrates that 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’s not that simple. So far, we’ve only discussed the best weed management control methods and have not included other important elements — specifically, the economic factors that influence management control options.
The University of Wyoming’s 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 resistance weed management.
This makes sense because crop prices influence crop rotations and crop selection within that 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 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 the 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’s 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 that 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 suggest 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.