Experiment 1: Deep tillage effects on waterhemp populations

Field trials examining the effects of deep tillage on waterhemp populations were established in the falls prior to the 2017 and 2018 growing seasons. These trials consisted of three different tillage treatments: 1) fall moldboard plow followed by shallow spring tillage, 2) shallow spring tillage, and 3) no-tillage. Prior to the fall tillage each year, 10 random soil cores (4-inch diameter) were taken to establish the baseline waterhemp seedbank. Soil cores were separated into two depths: 0-2 and 2-7 inches deep. Additional soil cores (six subsamples per plot) at the same depths were taken following spring tillage. Half of the subsamples were elutriated for waterhemp seed and counted, the other half were grown in the greenhouse and emerged waterhemp plants were counted.

Roundup Ready 2 Xtend soybeans were planted on May 18, 2017 and May 30, 2018 in 30-inch rows. Within each tillage system a subset of herbicide treatments was established. These treatments included: 1) high waterhemp management (PRE fb. POST + residual), 2) standard waterhemp management (PRE fb. POST), and 3) no waterhemp management. Three 0.25 m2 permanent quadrats were established per plot for weekly waterhemp emergence counts and treatments were evaluated for weed control throughout the growing season. Prior to soybean harvest additional soil samples (six subsamples per plot) were taken from standard management herbicide plots from within each tillage treatment. Soybeans were harvested and yield was adjusted to 13% moisture. All treatments were replicated four times each year.

Results and observations:
There was a difference in the baseline seed number and distribution between in the two years. Higher amounts of waterhemp seed were counted in 2017 compared with 2018 and more seed was found in the upper 2-inches in 2017.

Seed counts and distribution were:
2017: 0-2” = 786 seeds; 2-7” = 141 seeds
2018: 0-2” = 20 seeds; 2-7” = 52 seeds

Moldboard plowing altered the vertical distribution of waterhemp seed in both the prior to planting and harvest soil samples.
- Regardless of tillage, fall moldboard or spring soil finish, there was 66% less waterhemp seed in the upper 2 inches of the soil profile compared with the no-tillage treatment when sampled prior to planting.
- However, by the harvest soil sampling period there was no difference in the number of seeds in the upper 2-inches of soil.
- Higher seed numbers were found in the 2-7 inch samples in the moldboard plowed treatments (33-35% more) in the prior to planting soil samples.
- These numbers ended up being 11 and 65% less seed in the spring tillage and no-till treatments that than moldboard plow treatments by the at harvest sampling period.

Tillage affected the total number of waterhemp emergence when the populations was high in 2017. Moldboard plowing and spring shallow tillage reduced the number of emerged seed by 2.5 times compared with the no-tillage treatment. Under a lower population in 2018, tillage did not affect the number of waterhemp plants that emerged.

Even though the total number of waterhemp plants that emerged was not always affected by tillage, the moldboard plowed treatments resulted in an earlier more uniform flush of waterhemp than other tillage treatments.
- It took 13 d to reach 80% waterhemp emergence with the fall moldboard plowed treatments and 29-30 d in the spring soil finished and no-till treatments, respectively.

The soil-applied (PRE) herbicide treatment of Fierce at 3 oz/A in the standard and high management treatments provided good to excellent waterhemp control for six weeks.

Post-emergence follow up applications of XtendiMax (dicamba) and XtendiMax + Warrant (acetochlor) provided excellent control of waterhemp throughout the remainder of the season.

Soybean yield was similar for the standard and high management treatments. Low management herbicide treatments (no waterhemp control) had over a 20% reduction in soybean yield in comparison to standard and high management treatments when combined over tillage systems and years.

In conclusion, moldboard plowing reduced the number of seeds in the upper 2” of the soil at planting and waterhemp emergence was earlier and more uniform.
- Under high populations (927 seeds core-1) total waterhemp emergence was reduced.
- Under low populations (72 seeds core-1) there was no effect on total waterhemp emergence.

If moldboard plowing is used for waterhemp management, seeds need to be left buried for approximately 4 years for complete degradation (Steckel 2007).
- During this time frame waterhemp may be more difficult to manage due to extended emergence patterns in shallow and no-tillage systems.

Both standard and high management herbicide programs effectively managed waterhemp in all tillage systems.

Experiment 2: Effectiveness of Liberty-based weed control programs for management of multiple-resistant waterhemp

A field experiment was established to compare several Liberty-based weed control systems using various soil-applied herbicides for waterhemp control. LibertyLink ‘MCIA2314LL’ soybean were planted in 30-inch rows on May 30. Following planting, 15 different PRE herbicide treatments were applied. Liberty was applied when the majority of the PRE treatments needed a POST herbicide application for waterhemp control. Waterhemp control was monitored throughout the season.

Results and observations:
There was significant variation in the initial control and the length of residual activity for the different soil-applied herbicides.

At the time of the POST Liberty application, 40 DAP, all treatments provided 90% or greater waterhemp control with the exception of: Valor (2 oz), Metribuzin (8 oz), Valor + Metribuzin, and Warrant.

After the POST Liberty application all treatments provided greater than 90% waterhemp control.

By harvest (Sept. 28) all treatments with an effective PRE followed by Liberty provided 95% greater waterhemp control.

Experiment 3: Waterhemp control systems in Roundup Ready 2 Xtend and LibertyLink soybean

A field experiment was established to compare three different soybean trait systems for waterhemp control. Roundup Ready 2 Xtend ‘AG26X8’ and LibertyLink ‘MCIA2314LL’ soybean were planted in 30-inch rows on May 30. Following planting, PRE herbicide treatments of Valor at 2 oz/A were applied to selected treatments. Follow-up POST treatments included: 1) XtendiMax + Roundup PowerMax, 2) XtendiMax + Roundup PowerMax + Warrant, and 3) XtendiMax + Roundup PowerMax + Warrant Ultra. The Roundup Ready follow-up treatments of: 1) Roundup PowerMax, 2) Roundup PowerMax + Flexstar, 3) Roundup PowerMax + Warrant Ultra, and 4) Roundup PowerMax + Cobra (12.5 fl oz) were also applied to Roundup Ready 2 Xtend soybean. The follow-up treatments in the Liberty Link soybean were: 1) Liberty, 2) Liberty + Warrant, and 3) Liberty + Warrant Ultra. Additional treatments included Roundup PowerMax alone and Liberty alone. Weed control was evaluated throughout the season and soybean yields will be taken if possible.

Results and observations:
Soil-applied applications of Valor at 2 oz/A provided 76-85% control of waterhemp until 40 DAP when the POST herbicide applications were made.

Soybean injury from POST applications only occurred when the active ingredient fomesafen (Flexstar or Warrant Ultra) or lactofen (Cobra) was applied. Injury ranged from 14-23%, 7 DAT.

Waterhemp control 30 DAT was greater than 90% with Liberty and XtendiMax treatments, with the exception of Liberty alone. Waterhemp control was between 80 to 88% when Flexstar or Cobra was applied.

Overall from two years of this research the LibertyLink and Roundup Ready 2 Xtend systems were more effective than the Roundup Ready systems.