Potential management strategies to control resistant pigweed in muck soils

Adapted from The Grower article “Strategies to control resistant pigweed in muck soils” from April 2012

Ontario carrot growers in particular are struggling with resistant pigweed, a problem that was studied extensively in 2011. As far back as 1997, resistance to group 5 herbicides was noted. Then in 1998, resistance to group 2 herbicides was noted. Resistance to group 7 herbicides appeared in 1999. There are some weed populations with multiple resistance (i.e. resistance to both group 5 and group 7 herbicides or maybe even to three different herbicide groups). Weed specialists also suspect resistance to group 14 and group 9 herbicides.

Pigweed in the field

Different pigweed species are showing resistance to herbicides, including:

  • Redroot pigweed (Amaranthus retroflexus, L.)
  • Green pigweed (Amaranthus powellii, S. Watson)
  • Smooth pigweed (Amaranthus hybridus, L.)
  • Common waterhemp (Amaranthus tuberculatus, syn, rudis)
  • Prostrate pigweed (Amaranthus blitoides, S. Watson) – currently being tested for confirmation of resistance

Without effective herbicides, growers are using alternative methods to manage pigweed, including wick-weeding with glyphosate and hand hoeing with removal of pigweed from the field which are generally not economically feasible for some crops.

Wick-weeding pigweed

Potential Solutions

In 2011, Kristen Callow, OMAFRA’s Weed Management – Horticulture Program Lead established on-farm demo plots to assess alternate methods of weed control for carrots on muck soil through funding from the Agricultural Adaptation Program. In general, there were fewer weeds in the PRE herbicide applications followed by banding of Lorox + inter-row cultivation treatment compared to the standard carrot herbicide program of PRE and POST treatments (Lorox and Gesagard in combination with adjuvants). Producers in Ontario can potentially control herbicide resistant pigweed with herbicide banding and inter-row cultivation.

Another thing to take into account is your rotation. Rotation to crops (i.e. onions, beets) in which herbicides can be used to control pigweed can be beneficial. See table below for herbicides that can be used on common rotational crops of carrots. These are provided as a general outline only. Please refer to complete labels before using these products.

Crop Herbicide Group
Onions Chateau, Goal, Aim 14
Prowl 3
Pardner 6
Frontier 15
Beets Nortron – soil type restrictions 16
Upbeet 2

New solutions coming soon (hopefully)

Prowl H2O looks promising and will hopefully be registered in the near future (not yet registered). Also, a new minor use project evaluating Blazer has been established in the past couple months, which means that this product will likely be registered here in the next few years.

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