Nebraska: Gaining The Upper Hand Against Resistant Common Ragweed

Over-reliance on glyphosate as the primary method of weed control has clearly pushed the selection of glyphosate-resistant weeds. In Nebraska, we have confirmed glyphosate-resistant populations of common ragweed, common waterhemp, giant ragweed, marestail, kochia, and Palmer amaranth.

With glyphosate no longer an option for the control of these glyphosate-resistant populations, growers are looking for herbicides with different modes of action. A diversified herbicide program for weed management needs to be used to avoid evolving resistance to multiple modes of action.

A diversified herbicide program should include at least two effective modes of action in each application and should rotate modes of action between years to keep selection pressure low.

Two plots in a study of control of glyphosate-resistant ragweed
Figure 1 (left). Non-treated control. Figure 2 (right). Optill (2 oz/acre) tank-mixed with Outlook (21 fl oz/acre) applied preplant. Photo was taken seven days after herbicide application. Click Image to Enlarge

A field study was conducted in 2015 and 2016 near Adams to evaluate herbicide programs for the control of glyphosate-resistant common ragweed in LibertyLink (glufosinate-resistant) soybean. Adams was where glyphosate-resistant common ragweed was first confirmed in the state.

Glyphosate-resistant common ragweed can be controlled effectively by using a diversified preplant and POST herbicide program in LibertyLink soybean.

Remember This: Common Ragweed Gets An Early Start

Common ragweed has an early spring emergence period in Nebraska with 90% total emergence reached before mid-May. This necessitates using a preplant burndown herbicide to control emerged common ragweed before planting (Figure 1).

  • Herbicide options such as 2,4-D, Authority First, Optill plus Outlook, paraquat, Sharpen plus 2,4-D, or Valor XLT effectively controlled common ragweed before planting and up to three weeks after application (Figure 2).
  • Liberty applied post-emergence alone or in a tank-mixture with Prefix, Pursuit, or Warrant controlled 84%-98% glyphosate-resistant common ragweed (Figure 3).
  • For best control and to reduce the evolution of herbicide resistance no single mode of action should be used preplant or post-emergence.
  • Single applications (only preplant or only POST) resulted in less weed control, greater weed biomass, and less soybean yield.
  • Three applications (preplant, pre-emergence, late post-emergence or preplant, early post-emergence, late post-emergence) did not result in greater common ragweed control, weed biomass reduction, or soybean yield (Figure 4 and 5).

Figure 4. Authority MTZ (20 oz/acre) applied pre-emergence followed by Liberty (36 fl oz/acre) applied post-emergence. Photo was taken 7 days after post-emergence herbicide application.
Figure 4. Authority MTZ (20 oz/acre) applied pre-emergence followed by Liberty (36 fl oz/acre) applied post-emergence. Photo was taken seven days after post-emergence herbicide application.
Figure 5. 2,4-D (2.1 pt/acre) applied preplant followed by Authority MTZ (20 oz/acre) applied pre-emergence followed by Liberty (36 fl oz/acre) applied post-emergence. Photo was taken 7 days after post-emergence herbicide application.
Figure 5. 2,4-D (2.1 pt/acre) applied preplant followed by Authority MTZ (20 oz/acre) applied pre-emergence followed by Liberty (36 fl oz/acre) applied post-emergence. Photo was taken seven days after post-emergence herbicide application.

The cost of preplant followed by post-emergence herbicide programs ranged from $59.17 to $93.09 per acre with application and provided the highest gross profit margins compared to no weed control and single- or triple-application herbicide programs (Table 1).

  • Single application herbicide programs were cheapest but resulted in much less profit due to low soybean yield.

The pre-plant/pre-emergence/post-emergence program provided equal control of glyphosate-resistant common ragweed as the pre-plant/early-post/late-post-emergence program.

Table 1. Cost of herbicide programs for controlling glyphosate-resistant common ragweed in dryland LibertyLink soybean, income from soybean yield, and gross profit margin in a field experiment conducted in Gage County in 2015 and 2016.
HERBICIDE PROGRAM PROGRAM COST A GROSS REVENUE B,C GROSS PROFIT MARGIN D
  ————– $/acre —————
Preplant: Optill [2 oz/acre] + Outlook [21 fl oz/acre ] 79.88 284.64 204.76
Post-emergence: Liberty [36 fl oz/acre]
Preplant: Authority First [6.4 oz/acre] 72.93 253.51 180.58
Post-emergence: Liberty [36 fl oz/acre]
Preplant: Valor XLT [5 oz/acre] 68.12 250.21 182.09
Post-emergence: Liberty [36 fl oz/acre]
Preplant: 2,4-D [2.1 pt/acre] 59.17 250.48 191.31
Post-emergence: Liberty [36 fl oz/acre] + [Pursuit 4 oz/acre]
Preplant: Gramoxone Inteon [4 pt/acre] 71.35 245.2 173.85
Post-emergence: Liberty [36 fl oz/acre] + Classic [0.75 oz/acre]
+ Warrant [2 qt/acre]
Preplant: Sharpen + 2,4-D (preplant) 93.09 278.96 185.87
Post-emergence: Liberty [36 fl oz/acre] + Warrant [2 qt/acre]
Preplant: Valor XLT (preplant) 104.36 264.19 159.83
Early post-emergence: Liberty [29 fl oz/acre] + Prefix [2 pt/acre]
Late post-emergence: Liberty [29 fl oz/acre] + Warrant [1.5 qt/acre]
Preplant: 2,4-D [2.1 pt/acre] 81.04 271.71 190.67
Pre-emergence: Authority MTZ [18 oz/acre]
Post-emergence: Liberty [36 fl oz/acre]

A Required herbicide adjuvants and application cost ($7.33 per application) were included in program cost.
B The average dryland soybean yield was 30 bu per acre for above herbicide programs in 2015 and 2016.
C An average soybean price of $8.87 per bushel was used to calculate gross revenue.
D Gross profit margin was calculated as gross revenue-program cost.

Source URL: https://cropwatch.unl.edu/2018/herbicide-options-managing-glyphosate-resistant-common-ragweed-libertylink-soybean