11/11/2020

Field Facts: Waterhemp

A Difficult, Herbicide-Resistant Weed

Waterhemp is one of the most aggressive, resistant weeds Midwest farmers deal with. It’s a summer annual plant with herbicide resistance recorded in 19 states. Populations in certain states have shown resistance to up to six sites of action.A joint study written by professors from the University of Illinois, the University of Missouri and Iowa State University reports waterhemp can reduce soybean yields by up to 44% and corn yields by up to 15%.

  • Common name: waterhemp
  • Scientific name: Amaranthus tuberculatus
  • Cotyledons: egg-shaped
  • Leaf shape: lance-shaped
  • Reproduction: dioecious, with male and female flowers on separate plants (female plants have shiny, black seeds, while male plants do not)
  • Flowers: male and female waterhemp have long, thin inflorescences


Fast facts

  • Waterhemp is easily confused with other amaranths, particularly Palmer amaranth. Waterhemp leaves tend to be more narrow and lance-shaped than its cousins’ leaves.
  • Waterhemp grows faster than most other weeds or crops. It typically grows about one inch per day.1
  • Waterhemp produces about 250,000 seeds per plant, with some plants producing as many as 1 million seeds.1
  • Waterhemp seeds can remain viable in the soil for several years.1
  • The weed can be poisonous to livestock if ingested.
  • A joint study written by professors from the University of Illinois, the University of Missouri and Iowa State University reports waterhemp can reduce soybean yields by 44% and corn yields by 15%.

  
Herbicide resistance

  • Because waterhemp is dioecious, with separate male and female plants, it has the potential for greater genetic diversity. This gives it a better ability to develop herbicide resistance than non-dioecious weeds.1
  • In 2017, populations of waterhemp were recorded as showing resistance to six commonly used herbicide groups.1 In the summer of 2019, a group of researchers at the University of Illinois reported finding a population of waterhemp with resistance to seven modes of action.
  • According to WeedScience.org, 19 states have confirmed herbicide-resistant waterhemp in various crops.

 

Crops_with_Herbicide_Resistant_Waterhemp_According_to_WeedScience_org.png

Control tips

  • A program approach to weed control with multiple modes of action and residual activity is recommended to control waterhemp. A program approach includes applications of a burndown, preemergence and postemergence.
  • Farmers also can use several cultural practices to control waterhemp. Some of those include:1
    • Planting in narrow rows to help soybeans, for example, outcompete the weed for sunlight and nutrients.
    • Deep tillage to reduce the amount of waterhemp seeds that germinate by burying them at unfavorable depths.
    • Planting fall-seeded cover crops like cereal rye.

1United Soybean Board, ‘Waterhemp,’ I Will Take Action, Oct. 10, 2020, https://iwilltakeaction.com/weed/common-waterhemp.

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