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Managing waterhemp with layered residual herbicides

Waterhemp is increasingly difficult to manage, as it’s becoming more widespread and herbicide-resistant populations are expanding, including multiple-resistant populations.

In addition, it has a long emergence pattern and frequently outlasts control of an early preemergence herbicide application.

One strategy is to layer residual herbicides, which helps control glyphosate-resistant waterhemp by extending the duration of seedling control. In fact, research has shown this strategy can provide season-long control for Minnesota soybean.

Research: Layering herbicides for waterhemp control in soybean

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Study findings

In July, weed control evaluations clearly showed an advantage of the two-pass system over single applications of residual herbicides (Figure 2). Figure 2 below shows a comparison of weed control in soybean with a single preemergence application of Outlook (left) and layered applications of Outlook on May 5 and June 8 (right). Photos taken July 14.

weed control in soybean
Single preemergence application of Outlook
weed control in soybean
Layered applications of Outlook on May 5 and June 8

Extended control

The season-long control of the layered or preemergence application followed by a postemergence application (PRE/POST) persisted through the end of the season, while control in the PRE-only treatments continued to diminish (Table 1). By the end of September, waterhemp control was 90 to 95 percent in the two-pass system, compared to 62 to 81 percent in the single PRE application system.

Increased yield

Yields reflected the weed control results with a 10 bushel average difference between the layered and single PRE applications. The yield range of the layered herbicide treatments was 46 to 51 bushels per acre, while the PRE-only treatments ranged from 32 to 43 bushels per acre.

While waterhemp is a challenging weed to manage, this trial demonstrates that layering effective residual herbicides is a strategy that could provide season-long control in Minnesota.

View the full report

Table 1 shows yields after a residual herbicide was applied either PRE only (after planting, on May 5, 2015) or layered PRE/POST (May 15 and June 8, 2015) in Rochester in 2015. Notes:

  • All PRE treatments included 4 fluid ounces of Pursuit per acre.

  • Waterhemp density on June 8 was 51 per square feet in Pursuit check.

  • The Pursuit weedy check yield was 14 bushels per acre.

Table 1: Common waterhemp control and soybean yield with residual herbicides

Herbicide Rate Application time 5/27/15 6/10/15 6/26/15 7/8/15 9/29/15 Yield
Dual II Magnum 1.5 pints per acre PRE 99% control 96% control 91% control 85% control 81% control 43 bushels per acre
Dual II Magnum 1.5 pints / 1.0 pints per acre PRE / POST 99% control 98% control 96% control 97% control 95% control 49 bushels per acre
Outlook 18 fluid ounces per acre PRE 99% control 96% control 85% control 73% control 71% control 40 bushels per acre
Outlook 14 fluid ounces / 10 fluid ounces per acre PRE / POST 99% control 98% control 97% control 97% control 94% control 51 bushels per acre
Warrant 1.6 quarts per acre PRE 99% control 91% control 82% control 69% control 62% control 32 bushels per acre
Warrant 1.6 quarts / 1.6 quarts per acre PRE / POST 98% control 95% control 95% control 94% control 90% control 46 bushels per acre
Least significant difference (LSD) p=0.10 -- -- 1% control 1% control 5% control 7% control 7% control 4 bushels per acre

Lisa Behnken, Extension educator; Fritz Breitenbach, former Extension integrated pest management specialist; Jeff Gunsolus, Extension weed scientist and Phyllis Bongard, Extension communications specialist

Reviewed in 2018

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