Herbicide-resistant waterhemp continues to spread and is problematic for many North Dakota soybean farmers. Waterhemp will germinate throughout the growing season, which means obtaining a crop canopy is an important part of waterhemp management. Iron deficiency chlorosis (IDC) is an agronomic problem that can delay canopy closure in many soybean fields in North Dakota, which can also prolong the window for waterhemp germination in fields that have IDC issues. Barley and oats are two crops that can be inter-seeded into soybeans to help alleviate IDC symptoms. Fall seeded cereal rye can suppress early-season flushes of waterhemp, but spring-seeded barley and oats have not been evaluated for waterhemp suppression. This project will look at the early-season companion crop suppression capabilities of a spring-seeded cereal cover crop, as well as the role the companion crop can play in late season waterhemp management due to quicker canopy closure of soybeans not stressed by IDC.

1) Evaluate barley/oat suppression of waterhemp in soybean.
2) Evaluate the time from planting to canopy closure of IDC-sensitive soybean with and without a barley/oat cover crop, and the effect of earlier canopy closure on late-season waterhemp. management.
3) Evaluate the optimal termination timing of a barley/oat cover crop to alleviate IDC symptoms and reduce yield loss due to direct competition of the cover crop with soybean.

-Extension bulletin addressing the use of barley or oats in a systems-approach to alleviate IDC in soybean with the overall goal of waterhemp management.
-Results will be shared at Extension meetings and Scientific meetings.
-Trials will be on display at Weed Science Field Days that are open to the public.

Updated November 30, 2020:
FY 2021 Mid-Year Report ND Soybean Council
December 1, 2020
Title of Project: Utility of a Barley Crop for Iron Deficiency Chlorosis and Waterhemp Management
Principal Investigator: Dr. Joseph T Ikley, NDSU Extension Weed Scientist
Objectives of the Research:
1) Evaluate barley/oat suppression of waterhemp in soybean.
2) Evaluate the time from planting to canopy closure of IDC-sensitive soybean with and without a barley/oat cover crop, and the effect of earlier canopy closure on late-season waterhemp management.
3) Evaluate the optimal termination timing of a barley/oat cover crop to alleviate IDC symptoms and reduce yield loss due to direct competition of the cover crop with soybean.
Completed Work:
Trials were initiated at Northwest 22 in Fargo on May 19, 2020, and at Prosper on May 24, 2020. The trial was arranged in a split-block with one half planted with oats and the other half containing no oats. Oats were drilled in 7.5-inch rows at a 1-inch depth and soybeans (AG06X8) were planted in 30-inch rows at a 1.5-inch depth. Glyphosate (1.125 lb ae A-1) and dicamba (0.5 lb ae A-1) were applied when oats were 6, 12, 18, and 24 inches tall to terminate the companion crop and provide weed control. Weed densities and biomass were recorded at each termination timing. Visual IDC scores were assigned to each plot every other week and chlorophyll content was measured with a Soil Plant Analysis Development (SPAD) Logger at that time. The trial at Prosper was harvested on October 6, 2020 and the trial at Fargo was harvested on October 22, 2020.
A presentation was recorded and submitted to the Graduate Student Contest for the Virtual North Central Weed Science Society meeting that reported preliminary results from these trials. Unfortunately, field tours were not feasible this summer with the COVID-19 restrictions in place all summer.
Preliminary Results:
It is worth noting that the plots were planted as soon as soils were dry enough to do so this spring. The seedbed quickly dried out and the first significant rainfall did not occur until June 7. This led to early season moisture competition between oats, soybean, and weeds. Fargo was particularly dry with no rainfall until 19 days after planting, and with a flush of waterhemp occurring soon after planting.
There were minimal IDC symptoms this year on the Xtend variety planted in the trials. There were no differences in IDC symptoms or chlorophyll content between treatments at Prosper. At the Fargo location, there was a slight increase in chlorophyll content in plots with the oat crop compared to plots without oats (data pooled across all termination timings).
The presence of an oat companion crop had no effect on waterhemp (Fargo) or Powell amaranth (Prosper) densities at any termination timing compared to plots without oats. The presence of the oat crop also had no effect on pigweed biomass at either location at the 6 or 12 inch termination timings. Pigweed biomass was lower in plots with oats compared to plots without oats at both locations at the 18 and 24 inch termination timings. Results at both locations suggest that oat suppression of pigweeds did not begin until after oats were 12-inches tall when oats and pigweeds germinate at the same time.
There were no differences in Powell amaranth control across any termination timings at Prosper. This is due to glyphosate controlling the glyphosate-susceptible Powell amaranth plants. Waterhemp control was greatest at the 6-inch termination timing, but was not different between plots with or without oats. There was less control at later termination timings, and control would not be deemed commercially acceptable at any application after the 6-inch termination timings on the glyphosate-resistant population tested.
Yield was more affected by treatments at the Fargo location than at Prosper. At Fargo, the plots with oats always yielded less than plots without oats across all termination timings. The two treatments with the highest yield were herbicides applied to plots without oats at the 6 and 12-inch termination timings. These still resulted in less yield than plots that were maintained weed-free throughout the year. At Prosper, there were no differences between plots with oats or without oats at any termination timing. The treatments with highest yield were the weed-free check and plots where herbicides were applied at the 6 and 12 inch termination timings. Delaying the first herbicide application until 18 or 24 inch termination timings resulted in lower yield.
Altogether, the oats did not provide suppression of pigweeds until the oats were 18-inches tall. Delaying first herbicide application until oats were 18-inches tall also resulted in lower soybean yields. IDC symptoms were minimal and transient on the variety used in these trials, and the addition of oats did not add significant value to weed control until oat was too competitive with soybean in 2020.
Work to be Completed:
Additional data analysis for individual treatments will be completed. Orthogonal contrasts for weed suppression within termination timing also needs to be completed. Trials will need to be repeated in 2021 for more confidence in data. An early May planting time, which is often a couple weeks prior to waterhemp emergence, would likely yield different results.

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Updated June 30, 2021:
Introduction
Iron Deficiency Chlorosis (IDC) is a major problem to soybean production in the upper Midwest due to the alkaline soils in these areas (Naeve 2006). IDC can be characterized by the stunting of the soybean plant, delayed canopy closure, along with yellowing and interveinal chlorosis within the leaves, especially of early trifoliolate leaves (Bai et al. 2018). Hansen et al. (2004) indicated that IDC costs soybean growers 120 million U.S. dollars in North Dakota, South Dakota, and areas in western Minnesota annually. The blending of a companion crop to be terminated in-season has the potential to increase overall health of the soybean within the field early in the season (Naeve 2006). This could also be beneficial through an economic standpoint, as purchasing a companion crop would cost less when compared to purchasing herbicide resistant soybean and increasing the overall rate of soybean within the field (Naeve 2006). Some crops, such as oat, can also be conveniently terminated by using one of many different herbicide systems, such as glyphosate, if combined with glyphosate-resistant soybeans (Naeve 2006) that are already regularly planted throughout the Midwest. If the companion crop is only growing during the vegetative stages of soybean to mitigate IDC symptoms, this correlates well with the timing at which the companion crop would need to be terminated to avoid yield loss due to competition. Companion crops compete with both the cash crop and weeds for light and nutrients, reducing the biomass and density of weeds, and reducing yield of the cash crop (Verret et al. 2017; Naeve 2006).
Research supports that a companion crop planted alongside soybean will reduce symptoms of IDC. The mechanism behind IDC symptom mitigation is largely unknown, although the presence of greater total plant biomass has been shown to mitigate the symptoms of IDC (Naeve 2006). It has been theorized that companion crops are able to absorb excess nitrates. Kaiser et al. (2014) observed that soybean planted with oat as a companion crop had reduced trifoliate nitrate N and Fe within the soybean plants. The increased plant biomass can also come from increasing the planting rate of the soybean within a field, which reduced IDC symptoms as well as increased final soybean yield (Naeve 2006). Previous research about utilizing an oat companion crop in soybean was conducted in fields without glyphosate resistant weeds. Glyphosate-resistant waterhemp (Amaranthus tuberculatus) has become a prevalent weed in many fields where IDC is also problematic. Waterhemp is becoming more difficult to manage in soybean fields in the Northern Great Plains, and utilizing a companion crop to manage waterhemp may prove beneficial, particularly in fields susceptible to IDC.

Objectives of the Research:
1) Evaluate barley/oat suppression of waterhemp in soybean.
2) Evaluate the time from planting to canopy closure of IDC-sensitive soybean with and without a barley/oat cover crop, and the effect of earlier canopy closure on late-season waterhemp management.
3) Evaluate the optimal termination timing of a barley/oat cover crop to alleviate IDC symptoms and reduce yield loss due to direct competition of the cover crop with soybean.
Materials and Methods:
Trials were initiated at NDSU’s Northwest 22 research site near Fargo, ND on May 19, 2020, and at a research site near Prosper, ND on May 24, 2020. The soil near Fargo is a Fargo Series, Silty Clay with 5% OM and a pH of 7.4; while the soil near Prosper is a Kindred-Bearden Silty Clay Loam with 4.3% OM and a pH of 7. The experiment was a 2x5x2 factorial treatment structure arranged in a RCBD split-block with one four replications. Treatment factors were A) Presence of oat companion crop, B) termination timing of the companion crop and C) number of POST herbicide applications. Factor A had two levels, oat and no oat. Factor B had 5 levels: 6, 12, 18, and 24 inch oat height used to determine termination timing, as well as a no oat termination. Factor C had two levels, one or two POST treatments of herbicide. A weed-free, oat-free treatment was included for comparison. The main pigweed species at the Fargo location was waterhemp (Amaranthus tuberculatus), while the main pigweed species seen at the Prosper location was Powell amaranth (Amaranthus powelli).
Oats (ND Rockford) were drilled in 7.5-inch rows at a seeding rate of 60 pounds per acre and a 1-inch depth. Soybeans (AG06X8) were planted in 30-inch rows at a seeding rate of 156,000 seed per acre and a 1.5-inch depth. Glyphosate (1.125 lb ae A-1) and dicamba (0.5 lb ae A-1) were applied when oats were 6, 12, 18, and 24 inches tall to terminate the companion crop and provide weed control. Any plots with a secondary POST treatment were applied with the same mixture of herbicide 14 days after the initial termination. Treatments were applied with a CO2-pressurized backpack sprayer calibrated to deliver 15 GPA at a speed of 3 MPH with TTI 11002 (Turbo TeeJet Induction, TeeJet Technologies, Glendale Heights, IL) in 20 inch spacing with 28 PSI of pressure.
Weed densities and biomass were recorded at each termination timing. All plants were collected from a 1-m2 quadrat, and were clipped at the soil surface and dried at 43 degrees C for 14 days. Visual IDC scores were assigned to each plot every other week and chlorophyll content was measured with a Soil Plant Analysis Development (SPAD) Logger at that time. The visual IDC ratings were recorded using the visual system developed by Helms and Kandel (2020), whereby plots were rated on a scale of 1-5, with 1 representing no symptoms and with a 5 being a dead plant (Helms and Kandel, 2020). The trial at Prosper was harvested on October 6, 2020 and the trial at Fargo was harvested on October 22, 2020. All data were analyzed using a three-factorial analyzation to account for the three different factors within the experiment with PROC GLIMMIX (SAS 9.4) using means separation with Tukey’s HSD test (a < 0.05).


Results:
It is worth noting that the plots were planted as soon as soils were dry enough to do so this spring. The seedbed quickly dried out and the first significant rainfall did not occur until June 7. This led to early season moisture competition between oats, soybean, and weeds. Fargo was particularly dry with no rainfall until 19 days after planting, and with a flush of waterhemp occurring soon after planting.
There were minimal IDC symptoms this year on the Xtend variety planted in the trials. There were no differences in IDC symptoms or chlorophyll content between treatments at Prosper. At the Fargo location, there was a slight increase in chlorophyll content in plots with the oat crop compared to plots without oats (data pooled across all termination timings).
The presence of an oat companion crop had no effect on waterhemp (Fargo) or Powell amaranth (Prosper) densities at any termination timing compared to plots without oats. The presence of the oat crop also had no effect on pigweed biomass at either location at the 6 or 12 inch termination timings. Pigweed biomass was lower in plots with oats compared to plots without oats at both locations at the 18 and 24 inch termination timings. Results at both locations suggest that oat suppression of pigweeds did not begin until after oats were 12-inches tall when oats and pigweeds germinate at the same time.

Figure 1. Biomass of waterhemp at Fargo location in grams per square meter at different termination timings triggered by height of oats. Green bar represents biomass in plots withou t oats, while gold bars represent biomass in plots with oats present.


Figure 2. Biomass of Powell amaranth at Prosper location in grams per square meter at different termination timings triggered by height of oats. Green bar represents biomass in plots without oats, while gold bars represent biomass in plots with oats present.
There were no differences in Powell amaranth control across any termination timings at Prosper. This is due to glyphosate controlling the glyphosate-susceptible Powell amaranth plants. Waterhemp control was greatest at the 6-inch termination timing, but was not different between plots with or without oats. There was less control at later termination timings, and control would not be deemed commercially acceptable at any application after the 6-inch termination timings on the glyphosate-resistant population tested.

Figure 3. Percent control of waterhemp 14 days after treatment. Y-axis is percent visible control on a scale of 0 to 100. X-axis represents different application timings based on oat height in plots with oats. Data are pooled across plots with and without oats.


Yield was more affected by treatments at the Fargo location than at Prosper. At Fargo, the plots with oats always yielded less than plots without oats across all termination timings. The two treatments with the highest yield were herbicides applied to plots without oats at the 6 and 12-inch termination timings. These still resulted in less yield than plots that were maintained weed-free throughout the year. At Prosper, there were no differences between plots with oats or without oats at any termination timing. The treatments with highest yield were the weed-free check and plots where herbicides were applied at the 6 and 12 inch termination timings. Delaying the first herbicide application until 18 or 24 inch termination timings resulted in lower yield.


Figure 4. Soybean yield at Fargo location. X-axis represents different application timings based on oat height in plots with oats. Green bar represents biomass in plots without oats, while gold bars represent biomass in plots with oats present. The weed-free, oat-free check had a yield of 2009 kg ha-1.






Figure 5. Soybean yield at Prosper location. X-axis represents different application timings based on oat height in plots with oats. Green bar represents biomass in plots without oats, while gold bars represent biomass in plots with oats present. The weed-free, oat-free check had a yield of 3670 kg ha-1.

Altogether, the oats did not provide suppression of pigweeds until the oats were 18-inches tall. Delaying first herbicide application until oats were 18-inches tall also resulted in lower soybean yields. Control of glyphosate-resistant waterhemp was reduced at later termination timings, and a follow-up treatment was necessary in all treatments in order to obtain complete control. This indicates that in the absence of any preemergence herbicide, a postemergence application would need to be applied prior to 6 inch tall oat in order to achieve complete waterhemp control with one postemergence application. IDC symptoms were minimal and transient on the variety used in these trials, and the addition of oats did not add significant value to weed control until oat was too competitive with soybean in 2020. This research is being repeated in 2021.



Literature Cited
Bai G, Jenkins S, Yuan W, Graef GL, Ge Y (2018) Field-Based Scoring of Soybean Iron Deficiency Chlorosis Using RGB Imaging and Statistical Learning. Frontiers in plant science 9: 1002-1002
Hansen NC, Jolley VD, Naeve SL, Goos RJ (2004) Iron deficiency of soybean in the North Central U.S. and associated soil properties. Soil Science Society of America Journal 50:983-987.
Helms TC, Kandel H (2020) NDSU Soybean IDC Test Scores Available (08/27/20). NDSU Extension. Helms and Kandel https://www.ag.ndsu.edu/cpr/plant-science/ndsu-2020-soybean-idc-test-scores-available-08-27-20 Accessed on March 21,2021.
Naeves SL (2006) Iron Deficiency Chlorosis in Soybean: Soybean Seeding Rate and Companion Crop Effects. Agronomy Journal 98, 6: 1575-1581
Verret V, Gardarin A, Pelzer E, Mediene S, Makowski D, Valantin-Morison M (2017) Can legume companion plants control weeds without decreasing crop yield? A meta-analysis. Field Crops Research 204: 158-168

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Research Conducted
This research evaluated the ability of a companion crop to alleviate symptoms of Iron Deficiency Chlorosis (IDC) and suppress/control Powell amaranth and glyphosate-resistant waterhemp. We also compared soybean canopy development and yield in plots that either did or did not have an oat companion crop. This research was conducted using a Roundup Ready Xtend (AG06X8) soybean variety and companion crop was terminated using glyphosate + dicamba applied at 6, 12, 18, and 24 -inch height for the companion crop. IDC severity, pigweed control, and yield were evaluated throughout the year. A companion crop-free and weed-free plot was included for comparison.
Why the research is important to ND soybean farmers
Iron Deficiency Chlorosis (IDC) and glyphosate-resistant waterhemp are both problematic for ND soybean farmers. Planting a small grain companion crop has proven beneficial in some environments (like wet years) to manage IDC. Small grains can also be a competitive crop for late emerging summer annual weeds like waterhemp. As herbicide resistant waterhemp continues to spread in ND, we need to evaluate programs with integrated weed management tactics.
Final findings of the research
The oat companion crop did not provide suppression of waterhemp until the oats were over 12 inches tall. This also coincided with yield loss due to companion crop and weed competition. So even though a companion crop may provide some relief from IDC symptoms, we did not see a benefit for pigweed management in this system. Delaying postemergence applications until the companion crop was 6 inches tall also resulted in less than 90% control of glyphosate resistant waterhemp with dicamba. We also found that delaying waterhemp control until reproductive phases of soybean resulted in a 43% reduction of soybean yield. If uncontrolled all season long, waterhemp reduced soybean yield by 96% compared to a weed-free check.
Benefits/Recommendations to North Dakota soybean farmers and industry
Based on results from 2020, a small grain companion crop will not suppress pigweed species in soybean until yield loss is occurring. These companion crops can still help with IDC in some environmental situations, but we did not see the benefit for pigweed management. In a companion study, we did find that group 15 herbicides did not cause major stunting of oats, so this system could still work when paired with certain preemergence herbicides for waterhemp management.

-This project will evaluate additional options for waterhemp control beyond sole reliance on herbicides. This will be important moving forward since resistance to most commercially available herbicides has been found in waterhemp across the US, and there are no new mode of actions coming to market in the near future. We currently do not have 2,4-D or dicamba resistance in waterhemp in North Dakota. If successful, the research will also help preserve new soybean technologies by offering additional waterhemp management tools that can also alleviate negative effects of IDC in soybean.