The pigweeds, including the waterhemps, present many weed management challenges for crop production. Pigweeds are a complex of important field weeds that can vary in their responses to herbicides and weed management practices. Identification is often difficult and producers are left to guess which species are growing in their fields. However, pigweeds can be identified and management practices chosen to control them effectively. The aim of this brochure is to provide the producer with the information necessary to identify pigweeds common to Iowa so that they can be managed more efficiently.
Pigweeds and waterhemps belong to a group of plants known as the amaranths (genus Amaranthus). There are from 65 to 75 species of amaranths, many of which are important field weeds. They include such well known Iowa pests as redroot pigweed, smooth pigweed, Powell amaranth, and common waterhemp. Other less troublesome weedy pigweeds found in Iowa are spiny amaranth, prostrate pigweed, and tumble pigweed. The term pigweed will be used throughout to refer to any of the above species, including common waterhemp.
Pigweeds grow naturally in open areas with full sun and disturbed soils. They are annual plants, growing rapidly in disturbed areas and producing from tens of thousands to hundreds of thousands of seeds per plant. These habitat preferences enable them to grow well in agricultural fields that mimic their natural environments.
Pigweeds can adapt quickly to new environments and some populations have acquired resistance to many common herbicides, most notably the ALS inhibiting herbicides. Pigweeds are wind pollinated and hybridization between species does occur.
The key to pigweed control is integrated management programs. Pigweeds, like other small-seeded annuals, thrive in no-till systems because they need to germinate close to the soil surface. In addition, pigweeds adapt very quickly to chemical forms of control. The following guidelines should form the basis of a management system.
1) Proper identification is needed for an adequate control to be chosen. This is crucial, as different species of pigweed may respond differently to a particular herbicide.
2) Rotate herbicides annually. Avoid using a single herbicide or class of herbicide for more than two consecutive seasons.
3) Narrow row soybean culture helps manage redroot pigweed due to the weed response to a dense crop canopy but is not as effective on common waterhemp. Late germinating waterhemp can emerge through the crop canopy if the canopy is reduced by other factors such as disease or herbicide injury.
4) Tilling reduces pigweed populations. Plowing or heavy soil disturbance may be recommended in cases of major infestation. Rotary hoeing and cultivation are efficient alternative strategies for control of redroot pigweed, but are less effective on common waterhemp.
5) Crop rotation is important in diversifying the number of available herbicides, the cropping environment, and other weed controls.
An excellent management guide is "Waterhemp Management in Agronomic Crops," University of Illinois Publication X855. Available from Vocation Agriculture Service, Information Services, College of Agricultural, Consumer and Environmental Sciences, University of Illinois, 1401 S. Maryland Dr., Urbana IL 61801 or FAX (217)333-3871.