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Effect of Herbicide Resistance in Weeds on Tillage Intensity

Herbicides used for weed control in soybean have resulted and continue to result in reduced tillage activity that was previously conducted for controlling weeds–i.e. tillage that was used for pre- and post-plant weed control was supplanted by herbicides.  This was especially the case when glyphosate became available and was used for preplant burndown of weeds, and when herbicide-resistant (HR) soybean varieties that allowed POST applications of myriad classes of herbicides onto them became dominant on U.S. soybean acres.

This new paradigm for soybean weed control is now being threatened by the evolvement of weeds that are resistant to multiple classes of herbicides.  This means that reverting to tillage is being viewed as a viable weed control option by producers.  But what does this mean for environmental quality factors that are affected by production agriculture activities that involve tillage?

A meta-analysis conducted by scientists at Iowa State Univ. provides a partial answer to the preceding question.  A summary of the group’s findings is published in a university news release titled “Study finds relationships among herbicide-resistant weeds, tillage practices and agricultural greenhouse gas emissions”.  This summary is based on the results that are published in a report titled  “Emerging weed resistance increases tillage intensity and greenhouse gas emissions in the US corn–soybean cropping system” in Vol. 3 (Apr. 2021) of the journal NATURE FOOD  (https://doi.org/10.1038/s43016-022-00488-w).  Pertinent points from the study follow.

It is generally accepted that a growing number of weed species have and are developing resistance to commonly used herbicides, the reduced effectiveness of herbicides as a weed control tool has and is making tillage an attractive weed control option, and tillage results in the release of soil-stored carbon and nitrogen into the atmosphere.

The authors used a process-based land ecosystem model, a long-term farmers’ survey, and time-series gridded data maps to examine the relationships among genetically engineered herbicide-tolerant (HT) crop adoption, the emergence of weed resistance to herbicides, farmers’ decisions about tillage practices, and how historical tillage practices altered net GHG fluxes in agricultural land.

The main benefit resulting from using HT crops–i.e. reducing tillage operations–might not be sustainable in the long run because of the evolving resistance of weeds to herbicides.  In fact, current evidence suggests that partial reversion to conventional tillage has resulted from this development.

The results of the analysis indicate that tillage intensity in US corn–soybean cropping systems declined substantially during 1998–2008, but shifted to an increasing trend after 2008.  The analysis demonstrated that the early-stage (1998–2008) reduction in tillage intensity was strongly correlated with the increases in national adoption of HT corn hybrids and soybean varieties.

Assuming that all corn and soybean fields under tillage were tilled twice a year (in both autumn and spring), the authors estimated from their analysis that the increased tillage intensity in the period 2009–2016 increased GHG emissions by 20.5 ± 7.2 TgCO2e per year, while the reduced tillage intensity before 2008 caused a reduction in GHG emissions of 7.0 ± 6.6 TgCO2e per year (Tg = teragram = 1,102,311 US tons; CO2e = carbon dioxide equivalent).  

Results from the analysis estimated that the tillage intensity increase during 2009–2016 resulted in a net GHG source of 13.8 ± 5.6 TgCO2e per year, more than double the GHG mitigation rate due to reduced tillage intensity in the preceding decade.

The corresponding changes in accumulated GHG flux induced by changes in tillage intensity in the 1998–2016 period were found to be approximately 78% higher than the estimates derived for spring tillage only (87.5 TgCO2e under tillage twice a year vs. 49.1 TgCO2e under spring tillage only). Despite the uncertainty caused by the lack of detailed tillage frequency data used in the analysis, these estimates do provide a lower and an upper boundary on tillage practice impacts, which strengthened the conclusion that there is substantial GHG mitigation potential through managing tillage practices.

Nearly half of the tillage impact was attributed to tillage-induced CO2 emission from soils, and the rest from direct soil N2O emissions.

Without an effective strategy to control weeds, tillage intensity is expected to increase and subsequently undermine the GHG mitigation achievements from other agricultural activities or sectors. On the other hand, this study implies that farmers’ choices in managing herbicide resistance may help mitigate agricultural GHG emissions.

The authors’ analysis found that the increasing number of herbicide-resistant weed species was closely correlated to the increase in tillage intensity after 2008, with a correlation coefficient of 0.81 in corn and 0.87 in soybean (p < 0.01).

Tillage intensity changes can affect GHG emission beyond the soil; i.e., CO2 emission coming from agricultural machinery used for tillage operations is a GHG source that should be considered and quantified if tillage makes a comeback for weed control.

The results from this analysis imply that the benefit of HT crop adoption in reducing tillage has reached its peak, while continually emerging weed resistance is found to contribute to an increase in tillage intensity.  This will likely lead to further increases in GHG emissions.

Finally, results from this study imply that farmers’ choices in managing HR weeds may contribute to the mitigation of agricultural GHG emissions.  These choices should include rotating herbicide chemistries when possible to delay or prevent onset of HR weeds, incorporating HWSC tactics into cropping systems, and adopting measures that will reduce or deplete weed seed in the soil weed seedbank.  A relatively unexplored area is the need to search for and/or develop biocontrol agents that can reduce populations of problematic weeds in major U.S. crops.

Composed by Larry G. Heatherly, May 2022, larryh91746@gmail.com

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