Apr 23, 2015
Control herbicide-resistant pigweeds in asparagus with alternative modes of action

The Photosystem II (PS II) inhibiting herbicides have been used successfully in asparagus production for over 50 years. Current PS II inhibitors labeled for use in asparagus include Karmex (diuron), Lorox (linuron), Sinbar (terbacil) and Tricor or Sencor (metribuzin). Princep (simazine) is no longer labeled for asparagus but was used for many years. These herbicides inhibit photosynthesis. They tend to have long residual lives in the soil and are ideal for perennial crops such as asparagus.Prolonged use of a herbicide or herbicides with the same mode of action (MOA) leads to development of weed resistance. In general, the more effective a herbicide is, the more rapidly resistance develops. In asparagus, several members of the Amaranthaceae family have been verified as resistant to the PSII inhibitors. Resistant Powell amaranth (Amaranthus powellii), and redroot pigweed (Amaranthus retroflexus) are common in many asparagus fields in western Michigan. Smooth pigweed (Amaranthus hybridus) and prostrate pigweed (Amaranthus blitoides) are closely related species that have been verified resistant to PS II inhibitors in other crops. Several other pigweeds and related species in Michigan (Palmer amaranth, common waterhemp, tall waterhemp, tumble pigweed) are known to be resistant to PS II inhibitors.

Fortunately for asparagus growers, we now have several herbicides with different MOA labeled for use in asparagus. By applying at least two residual herbicides with different MOA preemergence in spring and after final harvest, it should be possible to control resistant pigweeds as well as most other annual broadleaves and grasses. The PSII inhibitors should continue to be part of an asparagus weed control plan because of their broad weed control spectrum, low mammalian toxicity, long residual period and moderate cost.

Several residual herbicides have good pigweed activity. Table 1 lists preemergence herbicides labeled for asparagus, their MOA and their respective Resistance Management Group classification. A weed management plan that includes control of most weeds, including resistant pigweeds, should include both PS II inhibitors and other herbicides with different MOA. All applications should include two MOA. Weed control in spring the following year should include another PSII inhibitor and a herbicide with a different MOA. With this approach, resistant pigweeds will be controlled most of the season.

In addition to preemergence residual herbicides, several postemergence herbicides may be used in asparagus. Table 2 lists postemergence foliar-active herbicides labeled for asparagus. Most of them may be applied in the crop during harvest with a short pre-harvest interval (PHI) of 1-3 days. In general, at least one postemergence application during harvest is needed to control germinating and emerged pigweeds. The cotyledon through the 2 true-leaf stage is the most effective time of control. Lorox should not be used postemergence on resistant pigweeds.

Michigan State University Extension advises that a resistant pigweed control plan should include accurate records of weeds present in a field, herbicide and rate applied, date of application and effectiveness. Visual ratings can be based on a 1-10 scale (10=dead), or a percentage control (100=dead). If certain areas of a field are infested with specific pigweeds or perennials that have escaped herbicides, note the species and location for later application. A good weed identification book helps in determining which weed species are present.

With a resistance management plan in place and careful adherence and observation, it should be possible to control PSII resistant pigweeds.

Bernard Zandstra, Michigan State University Extension and Department of Horticulture

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