Jun 16, 2016
Rotational restrictions limit corn and soybean herbicide choices

Growing the same crops on soil year after year results in buildup of insects, diseases and weeds that reduce yields and quality. Vegetable growers are looking for new ground on which to produce their crops. An obvious choice is neighboring land being planted to field crops. Field crops may be used by vegetable growers as a rotation to help clean up difficult weed problems, such as yellow nutsedge, quackgrass, horseweed, nightshades, and smartweeds.

Changing crops also helps with herbicide resistance management. Corn and soybeans are available with traits which make them tolerant of various herbicides, including glyphosate (Roundup), glufosinate (Liberty, Rely), 2,4-D and dicamba (Banvel, Clarity). Use of these herbicides normally does not cause rotational problems with vegetable crops, and they can be used to help manage difficult weed populations.

When growers or cooperating land managers grow field crops, an important decision involves choice of preemergence herbicides. Many of the highly active field crop herbicides have long rotational restrictions for vegetable crops on the label. In some cases, there is serious potential injury to following crops. Growers should take rotational restrictions seriously. Preemergence herbicides that will not cause either technical label violations or serious crop injury the following year should be selected.

For soybeans, growers should use varieties with glyphosate or glufosinate tolerance. This allows for use of additional modes of action for postemergence control of emerged weeds. Growers should determine the primary weed problems in the field and select short residual preemergence herbicides that control them.

For instance, for a field with glyphosate-resistant Palmer amaranth, growers may apply a combination of S-metolachlor (Dual Magnum) plus sulfentrazone (Spartan) (or the premix Authority Elite) preemergence, followed by an application of Liberty plus Warrant (acetachlor) to extend the suppression of the Palmer amaranth. For non-GMO soybeans, the postemergence application may include Flexstar or Reflex (fomesafen) or Cobra (lactofen). Fomesafen has a 12 month or less rotational restriction for beans, peas, cantaloupe, cucumber, pepper, tomato, pumpkins, squash and watermelon. Lactofen has no rotational restrictions. Other soybean herbicides with short rotational restrictions are Prowl H2O (pendimethalin) and Valor (flumioxazin). Basagran (bentazon) also is safe postemergence.

For corn, growers may use up to 1 pound of Aatrex (atrazine) preemergence, in tank mix with Dual II Magnum, Prowl H2O or Outlook. The postemergence application should include glyphosate or glufosinate. If resistant weeds are present, the postemergence treatments may include dicamba.

If vegetable growers and their cooperators plan ahead, both should be able to obtain acceptable weed control and optimum yield with moderate cost and no rotational restrictions. Before using any herbicide, read the rotational restrictions on the label.

Zandstra’s work is funded in part by MSU’s AgBioResearch.

Bernard Zandstra, Michigan State University

Source: Michigan State University Extension

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