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2,4-D- and -tolerant — Some Facts to Consider William G. Johnson Introduction publication shares the perspective of some Purdue University scientists on the subject Steven G. Hallett The impending approval and use of Travis R. Legleiter of managing in crops, explains why varieties that are tolerant of the 2,4-D- and dicamba-tolerant crops were Fred Whitford 2,4-D and dicamba has been generating developed, and discusses some of the Purdue Botany and some conversation and debate. concerns surrounding the short- and Plant Pathology Many groups and individuals — including long-term effects of this technology. scientists, agronomic growers, Stephen C. Weller and specialty crop growers — are concerned Background on Weed Bruce P. Bordelon about 2,4-D- and dicamba-resistant crops. Management and B. Rosie Lerner They say such crops are unnecessary, will make farmers more dependent on Current Problems Purdue the intellectual property held by large and Landscape corporations, will injure nontarget crops Since they were introduced by Architecture sensitive to 2,4-D and dicamba, and will in 1996, genetically engineered accelerate the evolution of Ready® (RR) crops revolutionized weed resistance in weeds. management and no-till practices in agronomic cropping systems. RR crops Others argue that 2,4-D and dicamba have were resistant to the herbicide glyphosate, been used on millions of acres since the which meant that producers could apply this Purdue Botany and 1960s and has not resulted in widespread one herbicide postemergence during the damage, so using them on tolerant crops Plant Pathology crop season and achieve excellent, broad- should not concern growers of high-value spectrum . RR soybean was www.ag.purdue.edu/BTNY horticulture crops. introduced in the in 1996 Purdue Horticulture It is important to note that there is no followed shortly thereafter by RR and RR corn. Additional crops (including canola and Landscape unified opinion on this topic among weed scientists or agronomic crop growers. This and ) have also been released. Architecture www.ag.purdue.edu/HLA LOCAL FACES EXTENSIONAGRICULTURE COUNTLESS CONNECTIONS 1-888-EXT-INFO WWW.EXTENSION.PURDUE.EDU 2 ID-453-W • 2,4-D- and Dicamba-tolerant Crops — Some Facts to Consider

Weed management in RR crops has been excellent Companies are searching for new active ingredients, and has given agricultural producers simpler, less but the cost of developing them and the limited expensive weed control with glyphosate-based potential for economic return have made it difficult herbicides. However, after countless glyphosate to bring new products to market. Part of this is applications over many years and many millions of due to the fact that glyphosate has dominated the acres, there has been the widespread development of herbicide market since the introduction of RR crops weed populations that are resistant to glyphosate. in 1996. Furthermore, with no new herbicide modes of action on the horizon and herbicide-resistant The first problem weed known to develop weed populations on the rise in every state that glyphosate resistance since the introduction of RR grows corn and , there are few tools left to technology was marestail (horseweed). Today, there protect crop yields from weed infestations on large are 22 weeds worldwide reported to be resistant to monocultures. glyphosate. In Indiana, marestail, giant , and waterhemp are three problem weeds that have The same technology that allowed glyphosate to documented glyphosate resistance. Several other be used on crops provides the ability to introduce weeds are suspected to have developed resistance. herbicide tolerance traits into crops, thereby allowing existing herbicides to be used in that The Purdue Extension Glyphosate, Weeds, and Crops new crop. So, companies have taken advantage publication series (available from the Education of this technology to provide a solution to weed Store, www.the-education-store.com) documents control problems. This provides “unique” tools for several glyphosate-resistant weeds and recommends managing our current list of herbicide-resistant management practices. weeds. The existence of glyphosate-resistant weeds Companies decided to introduce 2,4-D and dicamba requires producers to use additional herbicide tools resistance into crops for fairly straightforward and cultural practices to effectively manage the reasons. First, these herbicide chemistries have weeds. These additional practices add cost to the shown excellent resilience and few herbicide- production system. Often, there are no effective resistant weeds have occurred after more than herbicides that are easily available for some crops, or 50 years of use. Second, these herbicides provide it is not desirable or possible to use more intensive excellent control of glyphosate-resistant broadleaf tillage on large acreages. The presence of glyphosate- weeds such as marestail, giant ragweed, common resistant weeds imperils the long-term sustainability waterhemp, and other broadleaf weeds. of the RR system unless producers practice more integrated weed management that relies less on the sole use of glyphosate. Developments in Herbicide-resistant Crops New Weed Management Approaches Two major companies have developed cropping In response to the evolution of glyphosate-resistant systems with 2,4-D and dicamba herbicide weeds, agricultural chemical companies have resistance: Dow AgroSciences and Monsanto been investigating new methods of weed control. Company. Dow AgroSciences and Monsanto have They have been developing new herbicides or taken proactive steps to address concerns of off- (more commonly) using target movement by developing new formulations to obtain crop plants that are resistant to other that, according to product labels and technical herbicides. Developing new herbicides is difficult. use guidelines for seed products, will require new To our knowledge, no university in the country application methods. If used, these methods can is evaluating any new herbicide active ingredient. minimize, if not eliminate, off-target movement. The In fact, the crop protection industry has not following are brief summaries of each system. commercialized an herbicide with a new mode of action in at least 10 years.

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Dow AgroSciences Technology Monsanto and BASF are developing application programs, enhancing dicamba formulations, and Dow AgroSciences calls their 2,4-D-resistant creating product stewardship programs that aim to technology the Enlist™ Weed Control System in minimize the risk of off-target movement. Using corn, soybean, and cotton. This technology became nonapproved dicamba formulations with higher possible when the company inserted genes into volatility characteristics will not be allowed or high-acreage agronomic crops that allow the recommended by Monsanto. plants to metabolize 2,4-D. Dow AgroSciences also developed a new Enlist Duo™ herbicide that contains Monsanto has communicated several required glyphosate and a new formulation of 2,4-D: choline. parts of the anticipated stewardship program to The herbicide features what Dow AgroSciences calls be included on the EPA label of new dicamba Colex-D™ Technology. The new choline formulation formulations. Proposed application requirements provides ultra-low volatility, minimized potential for include: drift, lower odor, and better handling characteristics • Using application technologies that produce very than commercially available 2,4-D or ester coarse to ultra coarse droplets (and eliminate fine formulations. spray particles). The drift potential of any 2,4-D formulation • Applying herbicides only when wind speeds are depends on using nozzles that reduce driftable less than 10 MPH. fines (droplets less than 200 microns). Proprietary components in the Colex-DTM formulation will • Warning against applications when conditions are reduce the amount of driftable fines. favorable for temperature inversions. All EnlistTM crops will have traits that make them • Requiring buffers between application sites and tolerant to 2,4-D as well as glyphosate. EnlistTM sensitive species. corn will also have tolerance to the “-fop” grass Final application requirements and buffer zones are herbicides that contain quizalofop (such as Targa® not finalized and will be determined with the aid of TM and Assure®). Enlist soybean, cotton, and corn valid research results and input from the appropriate will also be resistant to . regulatory authorities. Depending on regulatory approval, these crops are being targeted for commercial sale in 2013 for corn, 2014 for soybean, and 2015 for cotton.

Monsanto Technology Monsanto is developing crops after discovering a gene that allows plants to metabolize dicamba. Their Plus Xtend System® will allow applicators to spray glyphosate and dicamba over tolerant crops. Monsanto is collaborating with BASF to address dicamba’s potential to injure off-target vegetation through drift or volatilization. The danger for drift is an ever-present concern for any herbicide application and a major concern for all crops. There have been many cases of dicamba drift over the past 20 years, and many people have worried that the The plants in this soybean field have been injured by a growth regulator widespread use of dicamba-resistant corn, soybean, herbicide. and cotton could increase off-target movement of dicamba.

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Monsanto is performing extensive work to develop The main concern about these technologies is that dicamba-resistant soybean, corn, and cotton. off-site movement via particle drift or volatility Cotton will be tolerant to glyphosate, dicamba, and will damage sensitive crops (including agronomic glufosinate. The target dates for dicamba-resistant crops that are not resistant to 2,4-D or dicamba), crops depends on regulatory approval, but the most specialty crops (such as tomato, grape, and melons), current estimates say they will be available for 2015. and nursery plantings. There is also concern that these technologies will injure home gardens and landscapes. Logic for this Technology Another major concern is that once these crops The logic for these technologies is that traits for become commercialized, the acreage sprayed with crop resistance to 2,4-D or dicamba, when stacked 2,4-D and dicamba would increase to encompass with other herbicide resistance traits will reduce the the majority of agronomic acres in the United States. risk of crop injury often associated with pre-plant Since much of this acreage could be sprayed two or 2,4-D or dicamba applications. They’ll also improve three times each growing season, some argue that it control of difficult and resistant weeds in no-till greatly increases the potential for off-site movement and conventional systems any time from preplant — even if most applications occur under the best of into the growing season. The technologies will offer conditions — and for weeds to develop resistance to growers flexibility to control weeds, allow them to 2,4-D or dicamba. continue using reduced tillage practices, and will help reduce the risk of selecting for glyphosate- The authors definitely appreciate these concerns, resistant weeds. so the rest of this publication addresses issues that affect off-site movement. Both technologies will include the use of preemergence residual herbicides and postemergence herbicides. The goal is to effectively Factors Affecting Off-site Movement limit the potential for weeds to develop herbicide Understanding off-site movement potential involves resistance, which would make the new herbicide- specific knowledge of the herbicide’s chemical tolerant crop technologies more sustainable. and physical properties that can influence off-site movement. It is important to know the differences Concerns About Off-site Movement between drift and volatility. Concerns exist because the most common drift complaint in the spring consists of preplant Drift burndown herbicides used for no-till corn and Drift is the physical movement of spray particles soybeans that have injured ornamental plants. The by wind after the particles leave the sprayer and herbicides involved in these complaints almost before they reach the intended target. Drift occurs always include one or more of the following: 2,4- when spray applications occur in unfavorable D, glyphosate, , , or . weather conditions, but most commonly happens Growers of crops or plants that are sensitive to these when windy conditions are combined with poor herbicides, rural homeowners, and those concerned application techniques. One of these poor application about maintaining wild habitats for pollinators techniques is using spray nozzles that produce are concerned that widespread use of 2,4-D and small droplets (less than 200 microns). These small dicamba will have detrimental effects on sensitive droplets are light and easily carried by wind. plants. These concerns exist despite the fact that both herbicides have been used postemergence for Drift is a major concern with all chemicals but in more than 40 years (dicamba) and 60 years (2,4-D) reality, application technology and restrictions about in many different crops and are still widely used applying herbicides during adverse environmental today. conditions, when properly followed, greatly reduce this problem.

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Volatility Herbicides like Treflan® (which are still popular) have a fairly high vapor pressure so they must be Volatility is much different than drift. Volatility is incorporated in the soil to trap the herbicide in the the movement of the gaseous form of the herbicide soil matrix. Command® was somewhat infamous after it has been deposited on its intended target as a for off-site movement in the late 1980s because liquid. After deposition, the herbicide changes from of its high vapor pressure, but refinements in its a liquid to a gaseous form, and the gaseous form formulation eliminated this concern. It is important moves off the target with wind currents. to note that most herbicides introduced today have Volatilization involves a phase change in which a lower volatility formulations that have reduced off- liquid or solid phase may be transformed into vapor site movement concerns. by rising temperatures or drops in external pressure. The big concern with 2,4-D and dicamba is that The tendency of an organic substance to volatilize volatility and drift problems still exist. The known is expressed by its vapor pressure — more volatile volatility problems of certain 2,4-D and dicamba compounds have higher vapor pressure values. formulations are also well documented and are An herbicide’s volatility is influenced by many being addressed by both companies through the factors, including its vapor pressure, concentration, development of newer, lower volatility formulations. and rate of transport to the surface of the leaf or However, a concern is that low-cost, generic soil. Other factors influence volatility including formulations of both 2,4-D and dicamba are still the temperature of the air, leaf, or soil; the water readily available and growers may turn to these content of leaf or soil surface; and the velocity options if economics favor a lower application cost. of air movement above the surface of the leaf or soil. Reducing the water content on the leaf or soil surface can reduce volatility by increasing sites for adsorption. Conversely, water can assist volatility through “wicking,” which is an upward movement effect in which compounds are transported to the soil surface as water moves upward in the soil profile due to water evaporating at the soil surface. The distribution of a chemical between water and air is an expression of Henry’s Law. Without going into confusing detail, Henry’s Law accounts for the concentration of the in water and air along with the influence of temperature, herbicide solubility, molecular weight, and (most importantly) its vapor pressure. If an herbicide has a vapor pressure that allows a greater proportion of it to be present in air, that chemical more readily goes from a liquid to a gas and can move away from its target to a nontarget plant. This is a situation to avoid, and the herbicide industry has developed newer chemistries that have vapor pressures that are not conducive to volatilization.

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Methods of Minimizing pesticide applicators about locations where spray Off-site Movement drift may be a concern. We feel that more programs like Driftwatch will become common as more As mentioned above, volatility and drift are information becomes available about how best to different factors that affect the off-site movement of use these herbicide-resistant crops while avoiding herbicides. Spray operators can control drift, and potential damage from herbicide use. manufacturers have made substantial improvements The next few years will be interesting as herbicide- to spray nozzle technology in recent years. A recent tolerant crop technology is developed, approved for paper presented at the Weed Science Society of release, labeled, and used. Everyone will be watching America (WSSA) meeting showed that driftable how herbicides are used and applied and how the spray droplets can be reduced from 30 percent issues related to off-site movement are addressed. down to 2 percent by simply upgrading the nozzle There is much work that still needs to be done to technology in current spray setups. This type of ensure that when these herbicide-tolerant crop information will be important for all spray operators systems are commercially available they are effective to consider as they spray near sensitive vegetation. and economical, manage weeds, and do not damage However, the most important thing operators can nontarget plants. do to minimize drift is to spray only when winds are not blowing toward sensitive vegetation. Summary On the other hand, operators cannot control volatility except by choosing nonvolatile or low- • We recognize the need to manage glyphosate- volatile formulations. Volatility occurs after spray resistant weeds for the sustainability of Roundup is deposited on the target, when the herbicide Ready® agronomic cropping systems, which turns into a vapor and moves away from the target. provide economical means to protect crop yields. Operators can control volatility only if they do not • The introduction of 2,4-D- and dicamba-resistant spray formulations that have high vapor pressures. crops represents a new technology that can help Both Dow AgroSciences and Monsanto are maintain near-term productivity in our efficient, developing application techniques that aim to simplified monoculture systems of commodity minimize these concerns. They are taking measures crops. However, we must not become overly reliant that include developing herbicide formulations on this technology as the only solution to manage that are less volatile and have improved handling weeds and maintain high crop productivity. A characteristics, creating better spray nozzle diversified approach to weed management must technology, and improving overall application not be lost. The most durable and productive weed requirements. management system must integrate many tools, including genetic, cultural, nonchemical, and When the new cropping technologies become chemical methods. commercially available, it will be important for users to apply the new herbicide formulations • We recognize the potential risks that the increased responsibly and in ways that minimize volatility use of growth regulator herbicides (2,4-D and concerns and physical drift. Purdue weed scientists dicamba) can have near sensitive crops, organic believe operators can minimize drift by using farms, and rural home gardens and landscapes. proper sprayer technology. Purdue weed scientists • We also note that it is not only 2,4-D- and will need to educate applicators about the factors dicamba-sensitive crops (such as vegetables, (both human and environmental) that increase the trees, and ornamentals) that are threatened, but potential for off-site herbicide movement. that 2,4-D-resistant crops may be vulnerable to Another tool is the Purdue-based Driftwatch dicamba, and dicamba-resistant crops may be program (www.driftwatch.org), a voluntary sensitive vulnerable to 2,4-D. crop reporting system that notifies farmers and

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• We recognize the potential risk that growth • We encourage more stringent label restrictions on regulator herbicides (2,4-D and dicamba) may allowable wind application conditions, (currently have if they are applied later in the growing season at 15 mph) and to restrict any application when than they are currently. winds are blowing toward sensitive crops or homes. Wind restrictions should be much • Dow AgroSciences is developing a low-volatile lower (10 mph), and labels should specify that choline formulation of 2,4-D, drift management applications should also be avoided under solutions, and product stewardship programs as temperature inversion conditions. part of their Colex-D™ Technology. Monsanto and BASF are developing application programs, Reference in this publication to any specific enhanced/improved dicamba formulations, commercial product, process, or service, or and product stewardship programs intended the use of any trade, firm, or corporation to minimize the risk of off-target movement. name is for general informational purposes However, concern remains that some older, more only and does not constitute an endorsement, volatile formulations are slated to receive “label recommendation, or certification of any kind support,” not just the newly developed formula by Purdue University. Individuals using such products assume responsibility for their use • We encourage all growers of sensitive crops in accordance with current directions of the and cropping areas to register their fields with manufacturer. Driftwatch (www.driftwatch.org) so that pesticide applicators will be aware of potential problems. However, we do not believe the onus should be solely on these growers. The manufacturers of herbicide-resistant crop technologies must carefully monitor the situation after they release these crops, and they should document any and all drift cases. • We strongly recommend that the U.S. Environmental Protection Agency restrict, by label, the use of higher volatile formulations on any Roundup Ready® crops and that record- keeping requirements (similar to those in place for restricted use (RUP) products) makes sense to help in any investigation of off-target movement. • It is critical that registering companies develop marketing plans that tie technology purchases to the use of the newly developed formulations to reduce the economic risk of generic formulation use that would lead to unacceptable risk of These tomato plants have been injured by a growth regulator herbicide. potential volatility exposures.

Nov. 2012

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