Source: Russell Hahn, Department of Crop and Soil Sciences, Cornell University
Herbicide resistant weed populations are an ongoing concern for growers. They are also a concern for companies that develop/market herbicides and genetic traits that make crops resistant to certain herbicides. Growers have a responsibility to use practices that delay or prevent development of herbicide-resistant weed populations. While chemical and seed companies develop products that may contribute to this effort, it is the end users or growers who determine how these products/technologies are used. Ultimately, it is these use patterns that determine the number and distribution of herbicide-resistant weeds, and how long the value of new technologies is preserved. A previous article discussed the scope of herbicide-resistant weeds around the World and of glyphosate-resistant weeds in the U.S. To follow up on that discussion, a review of resistance management strategies for growers seems appropriate. In addition, we'll take a look at current and future industry efforts that may facilitate these efforts.
Grower Practices and Responsibilities
Growers must recognize that repeated use of the same cropping practices, like choice of crop(s), tillage systems, etc., will favor certain weeds. Likewise, repeated use of herbicides with the same site of action may result in herbicide-resistant weed populations. Due to genetic variability, there may be a few weeds in a native population that are resistant to a particular type of herbicide. With repeated use of the same herbicide(s), these surviving weeds are the only ones that reproduce. Over time, this results in a shift to a population that is dominated by the resistant weed biotype.
Cultivation can play a role in preventing weed population shifts by controlling the resistant survivors before they reproduce. Crop rotation can also play an important role in delaying development of herbicide-resistant weed populations. Before the introduction of genetically engineered herbicide-resistant crops, crop rotation often forced changes in herbicide use. Now, if growers are using glyphosate-resistant (GR) corn and GR soybeans, and are relying heavily on glyphosate alone for weed control in both, crop rotation doesn’t really contribute to resistance management. It's the change in herbicides that is the key element. The most important resistance management practices for growers are to rotate the types or genetics of their crops, to rotate herbicides with different sites of action, and to use herbicide combinations or sequential applications with herbicides with different sites of action. To work, this means that more than one of the herbicides used in rotation or combination must control a particular weed. Growers must know how different herbicides work to rotate herbicides most effectively. A herbicide site of action classification system has been approved by the Weed Science Society of America (1). In this system, a group number is given to all herbicides with the same site of action. These group numbers are included in the Cornell Guide for Integrated Field Crop Management and are found on many herbicide labels. This site of action information can assist growers in using a variety of different types of herbicides in their resistance management plans.
Industry strategies to facilitate resistance management focus on educational efforts, on the development of herbicide premix products that include herbicides with more than one site of action, and on development of crop varieties that have resistance to multiple types of herbicides that they would not normally tolerate.
While herbicide premixes with more than one site of action have been in the market for many years (Bicep), there are several new products that have been developed specifically for use on GR crops. Perhaps the best known of these is a GR corn herbicide, Halex GT. Table 1 shows it’s a mixture of glyphosate, a Group 9 herbicide that inhibits an enzyme (EPSP synthase) essential for amino acid synthesis; metolachlor (Dual), a Group 15 herbicide that inhibits long-chain fatty acid synthesis; and mesotrione (Callisto), a Group 27 herbicide that inhibits an enzyme (4-HPPD) that is essential for pigment formation. Two other premixes, Extreme and Flexstar GT are for use in GR soybeans. Extreme combines glyphosate with imazethapyr (Pursuit), a Group 2 herbicide that inhibits an enzyme (ALS or acetolactate synthase) which is essential for amino acid synthesis. Flexstar GT teams glyphosate with fomesafen (Reflex), a Group 14 herbicide that acts as a cell membrane disrupter.
Crops with Multiple Resistance
Crops with resistance to more than one herbicide site of action are not new. However, the intentional development and marketing of varieties that are resistant to herbicides they would not normally tolerate is relatively new. Corn hybrids have been available for several years that are resistant to both glyphosate, a Group 9 herbicide, and to glufosinate (Ignite 280), a Group 10 herbicide. Group 10 herbicides cause ammonia accumulation that destroys plant cells and directly inhibits photosynthesis. SmartStax corn hybrids, with these two types of herbicide resistance along with six types of insect resistance, are an example (Table 2). SmartStax hybrids, developed by Monsanto in cooperation with Dow AgroSciences, were introduced in 2010. There are other hybrids that are resistant to both glyphosate and glufosinate herbicides.
Table 1. Herbicide premixes with multiple sites of action for use on glyphosate-resistant crops
Table 2. Current and future crops with multiple types of genetically engineered herbicide resistance are shown in the table.
(Partners – Pioneer and other companies )
*All dates after 2010 are proposed product launch dates.
On the drawing board are DHT (Dow AgroSciences Herbicide Tolerance) traits for both corn and soybeans. As shown in Table 2, DHT crops will have enhanced tolerance to 2, 4-D, a Group 4 growth regulator or synthetic auxin. This trait will be stacked with glyphosate resistance, and no doubt with insect resistance traits in corn. These DHT hybrids/varieties are being developed with Pioneer and could be introduced as soon as 2013 (corn) and 2015 (soybeans). New 2,4-D formulations, with low spray drift potential and little volatility, are being developed and will have to be used with these DHT varieties. In addition, the use of air induction spray nozzles will be recommended to further reduce the risk of off-site movement of spray particles. Monsanto and BASF are collaborating on soybean varieties that will combine resistance to glyphosate and to dicamba (Clarity, Banvel, etc.), another Group 4 herbicide. Like Dow, Monsanto and BASF are working to develop dicamba formulations that are less volatile than those currently available. These dicamba-resistant soybeans could be available in 2014. Finally, Optimum GAT (glyphosate and ALS tolerant) corn and soybeans are being developed by DuPont/Pioneer. These Optimum GAT hybrids/varieties combine glyphosate resistance with enhanced tolerance to Group 2 herbicides that inhibit ALS (acetolactate synthase). Optimum GAT technology is targeted for introduction in 2015 at the earliest.
With both the premix herbicide products and with the multiple resistance hybrids/varieties, the idea is to use more than one herbicide site of action to control an individual weed species. The theory is that if one site of action doesn’t control the weed, the other one will. Although this concept will help manage existing resistant weed populations and delay development of new ones, it is not guaranteed to work. There are weed populations that have developed resistance to more than one site of action. For example, there are isolated populations of five weeds in the U.S. that are resistant to both glyphosate and ALS inhibitor herbicides. This multiple resistance has occurred with two pigweeds, Palmer amaranth and tall waterhemp, both common and giant ragweed, and horseweed.
Growers must recognize that weed resistance to many sites of action is common, that resistance is manageable, and that most herbicides and genetic traits retain their value despite resistant weeds. Growers must also recognize that the battle against weed population shifts and against the development of resistant weed populations is ongoing. This battle requires an integrated approach to weed management that involves vigilant scouting for weeds that are not being controlled with current practices/herbicides. It also requires that growers use different control tactics over time, including the use of rotations with different crop genetics and the use of herbicides with different sites of action.
1. Mallory-Smith, C. A. and E. J. Retzinger. 2003. Revised classification of herbicides by sites of action for weed resistance management strategies. Weed Technol. 17:605-619.