Pest Patrol: Plan to manage waterhemp in 2020

#PestPatrol with Mike Cowbrough, OMAFRA

Figure 1: Light green waterhemp plants approaching the top of the soybean canopy after two applications of glyphosate that proved ineffective. At this stage, no herbicide will adequately control waterhemp.

Waterhemp has spread further across Ontario during the 2019 season and can now be found in eight counties (Bruce, Chatham-Kent, Essex, Haldimand, Lambton, Middlesex, Norfolk and Northumberland).

A close relative of pigweed, waterhemp is a competitive annual broadleaf weed that has been found resistant in the province to four different herbicide groups (Group 9: Roundup, Group 2: Classic, Group 5: Atrazine/Sencor and Group 14: Reflex). This makes management a challenge, particularly in soybeans.

Unfortunately, for many farmers who have found waterhemp on their farm, by the time plants are large enough for easier identification, they are past a reasonable stage for control with herbicides. In some instances, the waterhemp population is so abundant, the best option is destroy the crop to prevent the production of more seed (Figure 1 at top).

In corn and soybeans, the use of pre-emergence herbicides is critical for the management of waterhemp. Because the removal of early-season weed competition in corn and soybeans is so important for yield maximization, it will be money well spent even if you don’t have waterhemp on your farm. The pre-emergence herbicides that are effective on waterhemp are also effective on a number of competitive annual weeds.

A closeup of a large waterhemp plant. The key feature that differentiates it from common pigweed plants (e.g. green and redroot pigweed) is its hairless stem with leaves that are much narrower and have wavy margins. Although crude, waterhemp is often cited as a “really weird-looking pigweed.” Once most people see waterhemp for the first time, they can easily identify it after. photo: Supplied

Fortunately, the University of Guelph (Ridgetown Campus) has been evaluating various options in corn, soybean and cereals. Here is a rundown of herbicides that have been found to provide better than 80 per cent control of waterhemp.

Corn (Pre-emergence herbicides)

  • Greater than 90 per cent control: Acuron, Lumax EZ.
  • Between 80 to 85 per cent: Integrity, Converge XT, Callisto + Aatrex 480.

Soybean (Pre-emergence herbicides)

  • Greater than 90 per cent control: Fierce.
  • Between 85 to 89 per cent: Authority Supreme, Boundary LQD, TriActor, Sencor, Valtera.
  • Between 80 to 85 per cent: Dual II Magnum.

Wheat (Post-emergence herbicides)

  • Greater than 90 per cent control: Lontrel, Pixxaro, Infinity, Infinity FX and 2,4-D Ester.
  • Between 80 to 85 per cent control: Trophy, Estaptrop.

Cover crops

  • Fall-seeded cereal rye has been observed to reduce waterhemp biomass.

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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