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Tillage and Weed Dynamics in Soybean Production

Soybean production in the U.S. has gone through myriad shifts in management practices that involve tillage, planting date, mono-vs. rotational cropping, weed control, etc. It is surmised that all of these shifts have affected or will affect yield-impacting factors such as weed populations.

Tillage practices have arguably changed the most of all the management practices used for crop production. It is well known that tillage is an important agronomic practice that has been and still is used for seedbed preparation and weed control. However, it is just as well known and accepted that many tillage operations destroy residues from a previous crop, negatively affect soil physical and microbial properties, and contribute to increased soil erosion. Thus, the percentage of U.S. crops that are grown with little or no tillage has steadily increased to the present time. This increase in minimum and no-till systems has been aided by herbicide-based weed control both before and after planting a crop. The summaries of results from research reported in the following two publications provide insight into how this change from conventional to minimum/no-till systems for producing a crop has or does affect weed population dynamics in crop fields.

A report from Texas A&M Univ. titled “Thirty-six years of no-tillage regime altered weed population dynamics in soybean” by Govindasamy et al. was published in Agronomy Journal (2021, Vol. 113, Issue 2; https://doi:10.1002/agj2.20631). The research reported in this article assesses long-term no-tillage (NT) and conventional tillage (CT) system effects on weed populations in a continuous soybean production system. Details about and results from that research follow.

•    A long-term tillage experiment using soybean was conducted near College Station, Texas starting in 1982. Soybean was the summer crop and the field was left fallow during the off-season in all years of the study.

•    The soil at the site is a Weswood silty clay loam that shrinks/forms cracks when it dries.

•    The experiment was conducted with two levels of tillage–no-till (NT) and conventional tillage (CT). The NT treatment was never disturbed by any tillage operation during the 36-year period of the study. The CT treatment consisted of disk harrowing after soybean harvest, followed by chisel plowing, another disking, and ridging prior to winter. In the spring, the ridges were flattened for soybean planting. Two in-season interrow cultivations were used to control weeds. This CT tillage regime was consistently used throughout the 36 years of the experiment.

•    In 2016 and 2017, seed of RR2X soybean was planted in May of each year. Pendimethalin was applied PRE the day after soybean planting in both the NT and CT systems, and this herbicide program was followed during the 36-year period. The soybean crop was grown rainfed (no irrigation) in all years; thus cracks would have formed in the soil.

•    Since the tillage x year interaction was never significant for any measured weed variable, data from 2016 and 2017 were combined for all results.

•    The germinable seedbank (GSB) count showed that the majority of the weed species were common between NT and CT.

•    The majority–72%– of the weed species in NT were small-seeded annual broadleafs, which was attributed to their abundance in the shallow soil depths of the NT treatment compared to their deeper burial in the CT treatment.

•    Total weed density was 4 and 2 times greater for summer and winter annuals, respectively, in the NT treatment than in the CT treatment.

•    Weed seedling emergence was delayed in the NT vs. the CT treatment. This was attributed to the greater residue cover and lower soil surface temperature in NT.

•    Weed species diversity as measured by the GSB and the extractable seedbank (ESB) was not significantly affected by tillage system in this study.

•    More small-seeded weed seeds were in the shallow depths of the NT treatment. Seeds of small-seeded species had a much lower presence in depths below 2 in. in the NT compared to the CT treatment. This was related to emergence of these weeds–i.e., seed of some small-seeded weed species only emerged in the NT treatment.

•    The authors concluded that: 1) the NT vs. the CT system had greater weed densities with generally delayed weed seedling emergence; 2) the increased density of small-seeded weed seeds near the soil surface in NT influenced a shift in weed dominance, thus indicating that weed management programs should be adapted when a CT system is changed to an NT one; 3) long-term NT leads to a shift towards small-seeded annual weed species; 4) the cracking soil at this site may have influenced the vertical weed seedbank distribution patterns that were observed in the soil; and 5) growers transitioning from CT to NT should be aware of the potential changes in weed population dynamics that likely will dictate changing strategies for effective weed management in soybeans.

In an article titled “Influence of tillage method on management of Amaranthus species in soybean” by Farmer et al. (Weed Technology 2017, 31:10-20), results from field studies that were conducted in 2014 and 2015 are presented. Research was conducted in 7 states to determine the effects of tillage system and herbicide program on season-long emergence of Amaranthus species in glufosinate-resistant soybean. Details about and results from that research follow.

•    Studies were conducted to determine the effects of tillage system and herbicide program on emergence of Amaranthus species in glufosinate-tolerant soybean. Amaranthus species of concern were Palmer amaranth, spiny amaranth, and waterhemp. Palmer amaranth and waterhemp are the two most problematic weeds in the soybean-producing regions of the U.S., especially since they have developed resistance to several herbicide modes of action.

•    The increase in the development of herbicide-resistant (HR) Amaranthus species supports the need for producers to diversify weed management practices, and this can include cultural control practices that will involve tillage. Tillage practices that involve soil inversion to place weed seed at depths that will reduce or prevent their germination and emergence likely should be considered.

•    The use of both PRE and POST residual herbicide applications is known to reduce weed densities and is an integral component of HR Palmer amaranth management (click here for an up-to-date USB publication that verifies this).

•    The objectives of the research were to 1) determine the effect of tillage system with and without a PRE herbicide on season-long emergence of Amaranthus species in glufosinate-tolerant soybean, and 2) determine the effect of tillage system on the vertical distribution of Amaranthus seed in the soil profile.

•    Tillage systems were: 1) fall moldboard plow, spring field cultivator–referred to as deep tillage (DT); 2) fall chisel plow, spring field cultivator–referred to as conventional tillage (CT); 3) spring vertical tillage–referred to as minimum tillage (MT); and 4) no-till with spring weed burndown by herbicide–referred to as no-tillage (NT).

•    Each tillage treatment received either a 1) PRE herbicide followed by POST glufosinate + metolachlor (PRE + POST), or 2) POST-only glufosinate (POST).

•    DT reduced Amaranthus emergence by a significant 62%, 67%, and 73% compared to emergence in the CT, MT, and NT treatments, respectively. This was attributed to the deep burial of seeds of Amaranthus species in the DT treatment.

•    The PRE + POST herbicide program resulted in an 87% reduction in Amaranthus species emergence compared with the POST treatment. Thus, application of a residual herbicide both at and/or after planting will reduce the selection pressure for resistance to POST herbicides.

•    The combination of DT and PRE + POST resulted in a 97% reduction in Amaranthus species emergence compared to the MT and POST program combination. The DT and PRE + POST treatment combination resulted in a lower weed density than the other three tillage treatments combined with PRE + POST. Where only POST was used, DT combined with POST resulted in lower weed emergence than that from the other three tillage treatments combined with POST, which had significantly similar weed emergence. Thus, regardless of herbicide program used in this study, DT burial of seed of Amaranthus species was a major factor in their reduced emergence.

•    The DT treatment resulted in lower Amaranthus species emergence from all soil depths above 8 in. 72% of Amaranthus species seeds in DT were located deeper than 2 in.

•    In this study, only 28% of Amaranthus species seeds were located in the top 2 in. of soil in the DT treatment compared to 79%, 81%, and 77% in the CT, MT, and NT treatments, respectively.

•    Based on these results, 1) one-time deep tillage with an inversion tool such as a moldboard plow will provide the greatest reduction in emergence of Amaranthus species compared to CT, MT, and NT, and 2) the combination of DT and PRE + POST will provide the overall greatest reduction in Amaranthus species emergence throughout the season.

•    The authors concluded that the combination of effective cultural practices such as DT in this study combined with residual herbicides that contain multiple, effective sites of action can provide a substantial reduction in the occurrence of HR Amaranthus species in soybean.

The above summaries of results presented in the two cited articles are not meant to support one tillage system over another, or cause a producer to change a tillage system currently in use. Rather, they are intended to inform growers using any tillage system about the weed dynamics that may result from use of that system. Hopefully, this information will better prepare producers for the weed species that may or may not be promoted by a particular tillage system so that proper variety herbicide technology and weed control measures can be chosen based on the anticipated weed spectrum that is likely to develop where a particular tillage system is used. Also, the above results strongly indicate that a continuous NT system may be a significant contributor to increased presence of small-seeded weeds such as Palmer amaranth, which is known to have developed resistance to herbicides with multiple sites of action.

All of this brings one point to the forefront in the battle against HR weeds–i.e., soybean producers may have to rely on inversion tillage in specific situations where HR weeds have become a problem that current herbicide technology cannot or does not manage. This may mean that specific fields or even portions of a specific field will have to rely on a tillage operation with an appropriate implement to help control problematic HR weeds such as Palmer amaranth. To not consider this option in such a dire situation may mean an irreversible yield decline on a site that such tillage could reverse. Again, an individual producer will have to make that decision based on his/her knowledge of weed spectrum and soil characteristics at the site of concern.

Caveat. The term “deep tillage” used in the above summary does not mean the same as the term often implies in other writings. For example, “subsoiling” is also referred to as deep tillage, but it is not inversion tillage such as that resulting from the use of a moldboard plow. In fact, deep tillage with subsoiling is usually quite deeper than that with a moldboard plow, but often results in very little soil surface disturbance or inversion of soil. So in reality, the inversion component of tillage provided by a moldboard plow is likely more important than the “deep” component when the objective is to move weed seed deeper into the soil profile in order to remove them from the germinable weed seedbank. This is something that should be considered when using weed seed burial by tillage as a weed control measure.

Composed by Larry G. Heatherly, July 2021,