Experiment 1: Integrating fall-planted cereal cover crops with herbicides for control of glyphosate-resistant horseweed was established in the fall of 2017 by planting cereal rye and winter wheat as cover crops. The experiment was set up as a split-plot design with cover crop treatments as the main plots. The main plots include cereal rye and winter wheat, each drilled on September 27 at 60 and 120 lbs/acre (1X and 2X the seeding rates), and a no cover control. Roundup Ready Xtend soybeans were no-till planted on May 21 in 30-inch rows. Within each cover crop treatment four herbicide treatment subplots were established including burndown treatments prior to planting and planting green treatments with burndown a week after planting.
The herbicide treatments within each main plot are:
1) glyphosate only burndown (GR horseweed survival),
2) glyphosate only burndown fb. glyphosate + dicamba POST,
3) glyphosate + 2,4-D ester burndown fb. glyphosate + dicamba POST,
4) glyphosate + 2,4-D ester + flumioxazin + metribuzin burndown fb. glyphosate POST,
5) glyphosate only burndown 1 WAP, and
6) glyphosate only burndown 1 WAP ‘Planting Green’ fb. glyphosate + dicamba POST.
Cover crop, horseweed, and other weeds were counted, collected, and weighed for dry biomass from two 0.25 m2 quadrats per plot prior to initial burndown. Two 0.25 m2 permanent quadrats were established per plot for weekly horseweed emergence counts and treatments were evaluated for weed control throughout the growing season. Additional measurements included horseweed biomass from two 0.25 m2 quadrats towards the end of the season and soybean harvest. All treatments were replicated at least three times.
Results and observations:
• Initial horseweed emergence in this Isabella County field started on May 5. This is later than we normally expect, but may have been due to the cooler conditions experienced this spring. We were primarily working with spring germinated horseweed, since we managed horseweed in the fall prior to seeding the wheat and cereal rye cover crops. From May 5, new horseweed emerged weekly through early August with peak emergence occurring prior to planting (May 5-May 30). Another large emergence time occurred the week of June 21.
• Cereal rye produced more cover biomass than winter wheat at both termination timings. The high seeding rate provided more cover biomass at early termination but was not different for ‘Planting Green’ treatments. Delaying cover crop termination two weeks (Planting Green) provided 3 and 4X greater cover biomass for cereal rye and winter wheat, respectively.
• Cereal cover crops improved early-season control of horseweed in all herbicide programs, but their effectiveness did not last the entire season. Early terminated cereal rye and winter wheat equally suppressed horseweed and reduced density and biomass by 54 and 49% compared with the no cover control, respectively. Cover crop suppression of horseweed was not apparent at the time of ‘Planting Green’ termination compared with the no cereal cover crop control, since dense stands of other weeds (dandelion and common chickweed) suppressed horseweed equally. However, the cereal covers did reduce the other weed populations reducing the total amount of weed biomass in the cereal cover crop treatments.
• Postemergence applications of XtendiMax (dicamba) were applied on June 29, 2018 (5 weeks after planting). At this time, cover crops did not reduce horseweed emergence but did provide added suppression of horseweed growth which resulted in smaller plants. There was an interaction between cover crop and herbicide treatments in reduction of horseweed biomass. All effective burndown plus residual treatments and the ‘Planting Green’ treatments with cover crops provided the greatest horseweed suppression and control. Treatments with an effective burndown also reduced horseweed biomass and the glyphosate only control treatments also reduced horseweed biomass compared with the no cover controls.
• Horseweed control was excellent after the postemergence herbicides applications and there was no difference between the burndown treatments or cover crops at the end of the season. All plots with the residual herbicides of flumioxazin + metribuzin in the burndown treatment provided above 80% control of horseweed until August 2 (11 WAT) and were never treated POST. This resulted in a few plants setting seed at the end of the season. All other emerged horseweed remained as rosettes until the end of the season and did not produce seed.
• Effective burndown plus residual treatments yielded the highest followed by the ‘Planting Green’ treatments and effective burndown which were significantly greater than the control. Differences in cover crops were not significant across the herbicide treatments. However, when examining only the early termination with glyphosate only and the ‘Planting Green’ treatments, cereal rye and winter wheat were equivalent and yielded better than no cover control. ‘Planting Green’ also yielded higher than early termination timing.
• In conclusion, cover crops, delayed termination of cover crops, and residual herbicides provided early season suppression of horseweed which resulted in higher soybean yields. Winter cereal cover crops alone did not provide season-long horseweed control in no-till soybean. However, winter cereal cover crops may aid a good herbicide program for early-season suppression and help reduce the selection pressure for the further development of herbicide resistance.
Experiment 2: Glyphosate-resistant horseweed control in LibertyLink soybean was established on May 14 by applying 12 different 7-day early preplant burndown-residual herbicide combinations. ‘Credenz 2312’ LibetyLink soybean were planted on May 21 and POST applications of Liberty were made on July 10, approximately 56 DAT. Burndown and residual herbicides were evaluated throughout the season for horseweed control. All treatments were replicated four times and plot sizes were 10 feet wide by 25 feet long.
Results and observations:
• By ~50 DAP, all burndown applications with the exception of Roundup, 2,4-D ester + Roundup, 2,4-D ester + Roundup + Valor, and Liberty provided greater than 90% horseweed control.
• Metribuzin alone or combination residuals metribuzin + Valor, metribuzin + Sharpen, or metribuzin + Spartan (Authority MTZ), and Zidua PRO all provided good residual control.
• After the POST application of Liberty all treatments that provided good burndown resulted in greater than 90% horseweed control by harvest.
• In LibertyLink systems, even though there were some positive results with a good burndown herbicide followed by a POST application of Liberty. The most consistent horseweed control occurred when an effective residual herbicide was tank-mixed with a good burndown herbicide treatment and followed by a POST application of Liberty. Residual herbicides that included metribuzin of 8 oz/A alone or in combination were extremely effective.
Experiment 3: Residual control of glyphosate-resistant horseweed in Roundup Ready 2 Xtend soybean was planted and sprayed preemergence residual and burndown applications on May 21. The soybean variety was ‘Asgrow 26X8’ Roundup Ready 2 Xtend soybean. Twelve different treatments evaluating XtendiMax (dicamba) alone and in combination with other herbicides were compared with Sharpen for initial and residual glyphosate-resistant horseweed control.
Results and observations:
• The residual herbicides of Metribuzin (8 oz), Valor + Metribuzin, Spartan + Metribuzin, Authority MTZ, Fierce MTZ, and Zidua PRO included with XtendiMax (dicamba) + Roundup PowerMax provided 90% or greater control of glyphosate-resistant horseweed, ~90 DAP.
• XtendiMax at 22 fl oz/A did provided some residual control, but control became less than 90%, 48 DAP.
• Sharpen alone, and Valor or Spartan combined with + XtendiMax + Roundup did not provided as long of control than the other residual treatments.
• The benefit to a Roundup Xtend system for horseweed control is the application of XtendiMax in the burndown application combine with one of the effective residual herbicide products. In these systems over the past two seasons by using the combination of a good residual with a labeled dicamba product horseweed control lasted throughout most of the season and only a follow-up application of Roundup (glyphosate) was needed to clean up additional weed problems POST.