Herbicide-resistant Crops

Part 2, Section 1: Pest Management

Pest Management

Weed Control

Herbicide-resistant Crops

Herbicide-resistant crops are available for use in crop production systems. Corn and soybean varieties are currently the only agronomic crops with herbicide resistance that can be grown in the northeast. Herbicide resistance in crops results from two different procedures: tolerance selection and genetic engineering techniques.

Tolerance selection involves selecting naturally occurring herbicide-tolerant cells from a particular crop cultivar or cell culture and incorporating them into crop varieties and hybrids using traditional breeding techniques. Genetically engineering herbicide-resistant plants involves transferring a gene with a certain trait from one organism to another (e.g., from bacteria to plant) using complex technology. The transferred genetic trait(s) is then incorporated into crop varieties using breeding techniques. (The same genetic engineering technology is used to create crop resistance to insects—e.g., Bt corn.) Several crop protection chemical companies and seed companies are involved in developing and marketing these crops.

Following is a brief summary of the herbicide-resistant corn and soybean varieties currently available or soon to be released. See the corn and soybeans sections of this guide for detailed information on herbicide use for these crops.

IMI (IR/IT) or Clearfield (CL) corn was developed by tolerance selection to be resistant/tolerant to imidazolinone herbicides (e.g., Pursuit, Scepter). Although these hybrids initially were introduced to help manage herbicide carryover, Pursuit and Pursuit-containing products such as Lightning can be applied directly to the IMI corn hybrids as part of the weed-management program. Some IMI varieties (IR) also are tolerant of some sulfonylurea (e.g., Accent, Spirit) and sulfonamide (Python) herbicides and are used to reduce injury potential from these products when they are applied alone or in combination with organophosphate (OP) insecticides.

LibertyLink/GR corn is genetically engineered to allow over-the-top applications of Liberty (glufosinate) herbicide. This program should provide broad-spectrum control of annual broadleafs and grasses of low to moderate pressure. Sequential applications or tank mixtures may be required for new weed flushes and perennials.

Optimum GAT corn and soybean were developed using genetic engineering techniques. They allow postemergence applications of glyphosate products directly to corn and soybean. Unlike Roundup Ready, the resistance is achieved by increased metabolism of the herbicide. In addition to glyphosate resistance, Optimum GAT crops are resistant to the ALS-inhibitors, also achieved via genetic engineering. Remember, glyphosate and ALS-inhibitor resistant weeds are becoming an increasing issue in Northeast crops.

SR (sethoxydim resistant)/Poast Protected corn was developed using tolerance selection techniques to allow over-the-top applications of Poast (sethoxydim). This can provide control of annual grasses in a planned postemergence program or help manage escaped grasses.

Roundup Ready corn and soybean were developed using genetic engineering techniques. They have an altered target site not sensitive to glyphosate. They allow postemergence applications of Roundup and some other glyphosate products directly to corn and soybean. Glyphosate resistant weeds are becoming an increasing issue in Roundup Ready Crops.

The STS soybean/herbicide system enhances crop safety from certain sulfonylurea herbicides such as Harmony GT and Classic. Varieties were developed using tolerance selection methods to increase herbicide tolerance. The STS seed/herbicide system is designed to provide good weed control without crop injury. Although these varieties were developed to be used in combination with STS-labeled herbicides, they also provide greater safety from many of the ALS-type herbicides.

LibertyLink soybeans are genetically engineered to allow over-the-top applications of Liberty (glufosinate) herbicide. This program should provide broad-spectrum control of annual broadleafs and grasses of low to moderate pressure. Sequential applications or tank mixtures may be required for new weed flushes and for perennials.

Herbicide-resistant Crop Concerns

Several concerns are associated with the use of herbicide-resistant crops. These include: (a) drift to nearby susceptible vegetation; (b) misapplication to normal crops; (c) herbicide-tolerant crops becoming weedy and difficult to control; (d) illegal use of bin run seed; (e) negative public reaction to genetic engineering; (f) commodity marketability issues; and (g) increased selection for resistant weed species or shifts in weed populations. Herbicide-resistant crops require greater management to prevent problems such as misapplication and weed resistance. The following section provides further information on the potential for herbicide-resistant weeds.