Agronomy •  2022-12-19

Controlling giant ragweed with a program approach

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Giant ragweed is a big problem – literally. Left to its own devices, giant ragweed can reach a height of almost 10 feet or three metres. Adding to the problem is that there are populations of Group 2- and Group 9-resistant giant ragweed across southwestern Ontario. Soybeans are particularly vulnerable since the plant densities and row spacing that favour crop production also favour giant ragweed growth.

Farmers growing Enlist E3 soybeans can use a program approach to combat giant ragweed, including resistant biotypes, by applying Enlist herbicides in a one- or two-pass system. Enlist herbicides offer alternate modes-of-action to go after both resistant and non-resistant giant ragweed.

The problem: Giant ragweed is an annual, is highly competitive and can result in significant yield loss if left unchecked. According to researchers in Michigan, only one giant ragweed plant per 10 square feet can drop soybean yield by 52 per cent.1

Giant ragweed was the first glyphosate-resistant weed to be identified in Canada2 back in 2008. Since then, populations of Group 2-resistant giant ragweed have been identified, as well as biotypes resistant to both Group 2 and Group 9 herbicides.

It means herbicide choice and application timing are more important than ever when it comes to managing giant ragweed – not only for effective weed control, but also to protect valuable herbicide tools. But herbicides are only one part of the solution. To keep giant ragweed at bay, farmers should also maintain diverse and proper crop rotations, use cover crops, and practice no-till if possible.

Controlling giant ragweed with the Enlist weed control system: As with all weeds, early weed removal is key when it comes to effective giant ragweed control. No-till systems do not favour giant ragweed so in these situations, a pre-plant burndown may be all that’s necessary to gain control of resistant and non-resistant populations. If a second pass is necessary, be sure to take a multi-mode-of-action (MMOA) approach to ensure resistant populations of giant ragweed are controlled.

The Enlist weed control system, which includes Enlist E3 soybean varieties with tolerance to 2,4-D (Group 4) and glufosinate (Group 10), uses herbicide groups that giant ragweed is susceptible to – particularly 2,4-D, which is very effective against giant ragweed, including resistant populations.

  • Enlist Duo™ herbicide a convenient proprietary blend of 2,4-D choline (Group 4) and glyphosate (Group 9).
  • Enlist™ 1 herbicide a stand-alone 2,4-D choline formulation that can be tank-mixed with Liberty® 200 SN (Group 10) or glyphosate.

Both herbicides come with Colex-D technology for near-zero volatility and low drift so it stays where it’s sprayed.

The program approach to control giant ragweed: Giant ragweed emerges early – often before soybeans are planted – which is why the Enlist program approach starts with a pre-plant burndown. If giant ragweed then emerges after the crop does, a second, application with a different herbicide is warranted.

  1. Apply Enlist Duo as a pre-plant burndown. In no-till situations, this pass may be enough to control giant ragweed for the season. This is a MMOA herbicide (Group 4 and Group 9) capable of controlling Group 9- and Group 2-resistant biotypes.
  2. If necessary, apply Enlist 1 tank mixed with Liberty 200 SN at post-emergence. Including a Group 10 in this tank mix allows farmers to apply a second mode-of-action against glyphosate-resistant weeds.

Key takeaways: While giant ragweed can be a colossus in terms of speed of growth and height, it is not insurmountable. Early removal with a pre-plant burndown is key, as is using a MMOA approach to herbicide choice to effectively manage biotypes resistant to Group 9 and Group 2 herbicides. The Enlist weed control system provides soybean growers with the alternative modes-of-action necessary to control resistant giant ragweed.


1Michigan State University website:

2Controlling giant ragweed, Top Crop Manager, December 2015.