University of Illinois

No. 2/April 4, 1997

Herbicide-Resistant/-Tolerant Crops--Keeping the 'R's Straight!

IR, SR, RR, GR--it's becoming confusing to remember what each of these abbreviations stands for. Even more confusing is keeping straight what herbicide(s) can or cannot be applied to a particular herbicide-resistant/-tolerant crop. A short review of what these abbreviations stand for, and more importantly what can or cannot be applied to a specific resistant/tolerant crop, follows (Table 2).

Table 2. Herbicides for specific resistant/tolerant crops.

HerbicideResistant/tolerant crop
Pursuit, Contour, Resolve, LightningIR or IT corn
Poast, Poast Plus, HeadlineSR corn
LibertyLiberty Link, Glufosinate Resistant
Roundup UltraRoundup Ready
Synchrony, RelianceSTS varieties

Herbicide-resistant/-tolerant crops have been developed for a wide array of reasons. In some instances, they are viable options to alleviate problems such as herbicide carryover. In other cases, they have created a new market for herbicides or herbicide premixes specifically designed to be used in the resistant/tolerant crop.

How these crops are developed has a large impact on how quickly they can be commercialized. Those crops developed through "traditional" techniques generally come to market in less time than those developed through biotechnology. Crops that have been genetically transformed have a foreign gene inserted into their genome, which in some way provides for resistance to a herbicide that would severely injure nontransformed plants. Genetically transformed plants are under intense regulation by several federal agencies prior to receiving clearance.

IT--imidazolinone tolerant What does imidazolinone mean? The name imidazolinone represents a family of chemically related herbicides that have become widely used for weed control in a variety of crops. Members of the imidazolinone family include products such as Pursuit, Scepter, and Raptor (not yet labeled). Corn hybrids with the IT designation have been bred for enhanced tolerance to this family of herbicides. These hybrids have been utilized to reduce adverse consequences from carryover of herbicides in the imidazolinone family, as well as provide an opportunity to utilize herbicides or herbicide premixes "designed" for these hybrids. Contour (Pursuit + atrazine), Resolve (Pursuit + dicamba), and Lightning (Pursuit + Contain) are herbicide premixes that have been developed exclusively for use in IT or IR hybrids. Keep in mind, however, that these three premix products are to be used only on hybrids designated as IT or IR.

IR--imidazolinone resistant. These hybrids also possess enhanced tolerance to imidazolinone herbicides. The main difference between IT and IR hybrids is that both parents of the IR hybrid contained the tolerance trait, while only one parent contained the tolerance trait with IT hybrids. IR hybrids also have been utilized to reduce the adverse effects that can result when certain ALS herbicides are applied to corn previously treated with certain soil insecticides.

SR--sethoxydim resistant. What is sethoxydim? Sethoxydim is the active ingredient in Poast and Poast Plus. Prior to commercialization of SR corn hybrids, Poast and Poast Plus were used for postemergence control of annual and perennial grass species primarily in soybeans. SR corn hybrids have been developed to allow Poast or Poast Plus to be applied directly to the crop for control of grass species. A herbicide marketed for use in SR corn hybrids is Headline, which consists of Laddok S-12 and Poast Plus. Do not apply Headline to corn hybrids lacking the SR designation.

GR--glufosinate resistant. Glufosinate is the active ingredient in Liberty herbicide. Liberty is a nonselective herbicide and can be applied to corn hybrids designated as Liberty Link or Glufosinate Resistant only. Application of Liberty to corn hybrids lacking either of these designations will result in severe crop loss. In 1998, soybean varieties with resistance to Liberty will likely be commercially available.

RR--Roundup Ready. Roundup is a nonselective herbicide that has been extensively used for many years. With no crop species demonstrating natural tolerance to glyphosate (the active ingredient in Roundup), the use of Roundup has been limited to spot treatments or selective applications that minimize the herbicide's coming in contact with the crop. Soybean varieties have been developed that have resistance to Roundup and will be entering their second year of commercial availability in 1997. Roundup should be applied only to soybean varieties designated as Roundup Ready. Corn hybrids that have resistance to Roundup will, within the next few years, also be commercially available.

STS--sulfonylurea tolerant soybeans. What does sulfonylurea indicate? The name sulfonylurea represents a family of chemically related herbicides that are utilized in numerous crops. These soybean varieties have been developed with enhanced tolerance to certain sulfonylurea herbicides. In particular, these varieties help reduce plant stunting that can occur following application of certain postemergence sulfonylurea herbicides. These soybean varieties do not demonstrate enhanced tolerance to all sulfonylurea herbicides however. Synchrony and Reliance are two premix products, each containing Classic and Pinnacle, that have been developed for use in STS soybean varieties. Do not apply Synchrony or Reliance to soybean varieties not designated as STS.

If one of these resistant/tolerant crops is planted, must it be treated with the herbicide "designed" for use in that crop? Of course not; it can be treated with any herbicide (besides those labeled for other resistant/tolerant crops) labeled for use in the crop.

Aaron Hager and Marshal McGlamery, Department of Crop Sciences, (217)333-4424