2018 Herbicide Guide for Iowa Corn and Soybean Production

Transcription

1 2018 Herbicide Guide for Iowa Corn and Soybean Production Industry Update for 2018 Micheal D. K. Owen, University Professor and extension specialist in agronomy and weed science at Iowa State University. Bob G. Hartzler, professor in agronomy and extension weed specialist at Iowa State University. Introduction There again are no new herbicides with novel mechanisms of action and none are anticipated in the near future. Weed management issues were very evident in 2017 and weed populations with evolved resistance(s) to herbicides continued to escalate statewide. Palmer amaranth populations have been identified in many new Iowa counties and it is likely that populations will eventually be identified in all Iowa counties. Weed management remains a major concern for Iowa agriculture and addressing these burgeoning problems requires greater diversity of tactics beyond herbicides. Most herbicide labels now include sections describing management of herbicide resistant weeds. These sections describe various best management practices (BMPs) which are important for the diversification of weed management programs. Typically, there is also a statement that suspected herbicide resistant weed populations should be reported to the company for investigation. It is hopeful that the BMPs as suggested on most herbicide labels will gain traction and more growers will adopt more diverse tactics to manage weeds. New herbicide resistant (HR) crops represent a continuation of herbicidebased weed management and evolved resistance to the concomitant herbicides may already be evident. With dicamba-tolerant soybeans, the new dicamba formulations represent challenges for managing off-target dicamba movement. Given the problems with dicamba movement attributable to particle drift during application and movement after application from volatilization, it is clear that adoption of the dicambabased technology incurs greater risk than past herbicide technologies. The off-target movement of dicamba is complex and involves a number of factors, some that can be addressed with better application techniques. Factors such as nozzle type, boom height, application speed, wind speed, and direction can be addressed by applicators. Other factors such as the inherent chemical characteristics of dicamba, the high sensitivity of susceptible soybean cultivars and other non-target plants, the effects of rain, temperature, relative humidity, and inversions, not just the day of application but for several days following application, cannot be addressed by applicators and increase the risk of adopting the dicamba-based technology. The need for different technologies to address the burgeoning problem of herbicide resistant weeds must be considered in relation to the risks associated with the technology. Preemergence applications of dicamba with dicamba-resistant soybean represent the least risky use strategy and is recommended. Early postemergence applications in May, when temperatures are typically cooler, have greater risk than the soil applications. The greatest risk from dicamba-based weed management is postemergence applications in June and later. We do not recommend using the dicamba-based weed management at this time due to the greater risk of off-target movement. Contents Industry update for Dicamba Moving Forward...5 Herbicide-resistant Weeds in Iowa...7 Corn herbicide effectiveness ratings...12 Soybean herbicide effectiveness ratings...13 Grazing and haying restrictions...14 Herbicide package mixes...15 Herbicide sites of action...22 Herbicide site of action and typical injury symptoms...25 WC 94 Revised November 2017

2 Regardless of pending changes in herbicides and crop traits, weed management diversification beyond herbicides must be considered in order to support the tools currently available to farmers. Iowa agriculture will not be able to resolve weed management issues by simply spraying herbicides. What follows is a summary of the limited changes in the industry for 2018; the information should not be considered all encompassing. Selected Industry Updates AMVAC Parazone 3SL herbicide (paraquat) is now available from AMVAC. Parazone 3SL is a HG22 and the formulation contains stenching (odor) and emetic materials. The label is similar to other paraquat herbicides. BASF Engenia herbicide was approved for use in Engenia is a 5 lb a.i./gal formulation of N,N-Bis (3- aminopropyl) methylamine [BAPMA] salt of dicamba. The main label specifies uses in asparagus, CRP, corn, cotton, fallow cropland, farmstead turf (non-cropland) and sod farms, grass grown for seed, pasture, hay, rangeland, and farmstead (non-cropland), proso millet, small grain, sorghum, soybean and sugarcane. A supplemental label allows use of Engenia in dicambatolerant soybeans. Application can be made preplant, preemergence or postemergence. Maximum application rate (per application) is 12.8 oz/a. Combined applications per season may not exceed a maximum rate of 51.2 oz/a, e.g. two 12.8 oz/a applications to the soil (preplant and preemergence) and two 12.8 oz/a applications post. Approved nozzles, adjuvants and tank mix partners can be found at campaigns/engenia/tankmixselector. Zidua SC herbicide (HG 15) will be available for the 2018 growing season. The water-based suspension concentrate formulation contains 4.17 lbs. of pyroxasulfone per gallon. The label allows preplant surface, preplant incorporated, preemergence or post emergence applications in both corn and soybeans. Use rates in corn range from 1.75 to 6.50 oz/a and from 1.75 to 5.75 oz/a in soybean. Use rates are based on soil texture and application timing. FIFRA 24(c) Special Local Needs Label allows applications of Zidua herbicide for control of Palmer Amaranth in established federal Conservation Reserve Program (CRP) fields. The label specifies both preemergence and early postemergence applications. Application rate range is from 1.0 to 4.0 oz/a depending on soil texture and application timing. This label is for distribution and use only in the state of Iowa. DOW AGROSCIENCE Corn hybrids with 2,4-D (HG4) resistance (Enlist corn) are globally deregulated for the 2018 crop season. Enlist corn will tolerate applications of conventional corn herbicides and carry herbicide tolerance to glyphosate and Enlist Duo and Enlist One herbicides. Enlist Duo (HG4 and 9) is registered and available for non- Enlist corn and soybean as preplant burndown and preemergence (corn) and preplant burndown (soybeans). Enlist Duo is a premix of 2,4-D choline and glyphosate with Colex-D technology. Enlist Duo is also labeled for use on Enlist corn in 2018, but not available for postemergence use until the Enlist soybeans or E3 soybeans are approved by China. Enlist Duo has less potential for volatilization than other HG4 formulations but care should be taken to avoid conditions that may cause off-target movement. The Enlist Duo label describes appropriate nozzles for application, buffer requirements, and specific application techniques. Enlist Duo can be tank mixed with many surfactants and additives including AMS. Refer to com/en/approved-tank-mixes to confirm a given product or nozzle is approved. Herbicide-resistant weed management requirements are also included in the Enlist Duo label. Enlist One (HG4) is a registered herbicide for the control of annual and perennial weeds for use on non-enlist corn and soybeans as a preplant burndown, preemergence (corn) and preplant burndown (soybeans). Enlist One is also labeled on Enlist corn in 2018, but not available for postemergence use until the Enlist soybeans or E3 Soybeans are approved by China. Enlist One herbicide contains 3.8 lb a.i./gal of 2,4-D choline with Colex-D technology and was developed to give greater flexibility in herbicide tank mix partners to combat hard Herbicide Guide for Iowa Corn and Soybean Production

3 to control and herbicide-resistant weeds in Enlist corn and soybean. Enlist One has a use rate of 1.5 to 2.0 pints/a and can be applied to Enlist corn from preplant to V8 growth state or 30 inches tall as an over the top application. Corn taller than 30 inches and less than 48 inches requires the use of drop nozzles. The Enlist One label describes appropriate nozzles for application, buffer requirements, and specific application techniques. Enlist One has the ability to be tank mixed with various surfactants, additives, and herbicides: refer to to confirm a given product or nozzle is approved. Herbicide-resistant weed management requirements are also included in the Enlist One label. Elevore (HG4) herbicide will be launched in 2018 for preplant burndown control of annual broadleaf weeds with an emphasis on winter annuals like horseweed/ marestail, henbit, purple deadnettle and early spring annuals like common ragweed and common lambsquarters. Elevore contains lbs of halauxifen acid per gallon and is applied at 1.0 oz/a 14 days prior to planting for corn and soybeans. Refer to client.dow.com/elevoretankmix to determine the herbicides available for tank mixtures with Elevore. Herbicide-resistant weed management requirements are also included in the Elevore label. DUPONT EverpreX (HG 15) is a 7.62 EC formulation of s-metolachlor that is labeled for corn and soybeans. This product can be applied in the fall, early preplant, preplant incorporated, preemergence and postemergence for residual weed control. Note that the postemergence application does not control emerged weeds so if weeds are present at the time of application, a tank mixture with products that provides control of emerged weeds is needed. EverpreX will provide control of annual grasses and some smallseeded annual broadleaf weeds. Revulin Q (HG 2 and 27) is a prepackage mixture of nicosulfuron (HG 2) and mesotrione (HG 27). Label changes for 2018 include the addition of COC and AMS for applications to popcorn and sweet corn, the addition of topramezone (HG 27) (e.g., Impact and Armezon) as a tank mix partner, and ability to apply Revulin Q aerially. Popcorn and sweet corn may now be replanted immediately after a Revulin Q application (see the Rotational Crops Guideline) and new sections describing cover crops and a field bioassay are included on the label. Cover crops can be planted into Revulin Q-treated fields as long as the cover crops are not grazed by livestock or harvested for forage or food. However, not all cover crops have been evaluated for sensitivity to Revulin Q so there may be a risk of injury to the cover crop. The field bioassay of the Revulin Q label describes how to assess the potential for cover crop injury from the herbicide. MONSANTO Harness MAX herbicide (HG 15 and 27) is a premixture of acetochlor (HG 15) and mesotrione (HG 27) and available for preplant, preemergence and postemergence use in field corn, seed corn and yellow popcorn. Postemergence applications of Harness MAX can be made to corn up to 11 inches in height. Harness MAX will provide excellent control of annual small-seeded broadleaf and grass weeds as well as postemergence control of some large-seeded broadleaf weeds. NUFARM Panther Pro (HG 5, 14 and 2) contains 3 lbs metribuzin, 0.67 lbs flumioxazin, and 0.56 lbs imazethapyr per gallon and the use rate on soybeans is 12 to 15 fluid oz/a. The amount of metribuzin (0.28 to 0.35 lb a.i./a) is a bit higher than many other premixtures with metribuzin and may provide soil residual as well as contact activity. Panther Pro is restricted from use on sand soils (regardless of O.M.) as well as sandy loams or loamy sands containing less than 2 percent organic matter. Panther Pro is labeled for burndown/fallow uses as well as soybean preplant and preemergence uses. Panther Pro is very effective on many annual broadleaf weeds such as common lambsquarters, waterhemp and others as well as foxtail species and other annual grass weeds. Panther Pro provides excellent burndown of many seedling broadleaf weeds less than 4 inches in height but the addition of glufosinate, glyphosate or paraquat will help with the control of emerged grasses. Either a crop oil concentrate or methylated seed oil which contains at least 15 percent emulsifiers and 80 percent oil or a non-ionic surfactant at 0.25 percent v/v, may be used when applying Panther Pro as part of a burndown program Herbicide Guide for Iowa Corn and Soybean Production 3

4 SYNGENTA There are numerous modifications on the labels of existing Syngenta proprietary products (i.e., typos for the emergency telephone number on the Acuron label). These changes pertinent for Iowa can be found on the Acuron Flexi, Acuron, Dual Magnum, Evik DF, Flexstar, Fusilade DX, Gramoxone SL, Halex GT, Reflex, and Sequence herbicide labels. VALENT Valor EZ is a 4 lb a.i./gal liquid flumioxazin formulation (HG14) registered for use in corn and soybean. Valor EZ can be applied 7 to 30 days prior to corn planting and can be applied preplant and preemergence to soybeans. The preemergence application can be made up to 3 days after soybean planting. Valor EZ may also be used as part of a fall burndown program, however it is recommended that this product be tank-mixed with 2,4-D or dicamba (HG4) or glyphosate (HG9) herbicides if weeds are present at the time of application. Refer to the Valor EZ label for all use rates and restrictions. There do not appear to be significant changes in the proprietary products from Bayer Crop Science, FMC and Winfield. The path forward The need for better weed management continues to be a critical concern for Iowa agriculture. However, despite the widespread occurrence of herbicide resistance in waterhemp, giant ragweed, and horseweed/ marestail, most fields in Iowa are in a position to allow continued effective weed control if farmers will diversify their tactics. The thoughtful choice of herbicides with still-effective mechanisms of action is critically important, however, given that most waterhemp populations have evolved multiple resistances, knowing which herbicide mechanisms of action are still effective is a major challenge. The drought of new herbicides in the developmental pipeline continues to be a major problem and the long-term forecast for new herbicides with novel mechanisms of action is not bright. While new herbicide-resistant crop cultivars are available, the herbicides for those cultivars already have resistant weed populations. Clearly, issues in weed management continue to be increasingly complex, and there are no simple and convenient answers despite what product marketing might suggest. The problems of offtarget dicamba represents a major problem for agriculture and there does not appear to be a clear answer to these issues Herbicide Guide for Iowa Corn and Soybean Production

5 Dicamba Moving Forward Bob G. Hartzler, professor in agronomy and extension weed specialist at Iowa State University. Dicamba has been an important component of Iowa weed management systems for more than 50 years. The history of its use is somewhat unique in that its popularity has ebbed and flowed over time. The increase in herbicide resistant weeds along with the introduction of dicamba resistant soybean (Xtend) promises a significant increase in dicamba use in both corn and soybean. This article will review the characteristics of dicamba that differentiate it from other products, provide an overview of problems observed in 2017, and how risks can be minimized in The discovery of 2,4-D and other phenoxy herbicides (e.g., 2,4,5-T, MCPA, Silvex, etc.) in the 1940s initiated the era of chemical weed management. These herbicides mimic the action of auxin (IAA), and are frequently referred to as growth regulator, or Group 4, herbicides. Dicamba is in a different class of chemistry than the phenoxy herbicides, but has the same site of action. A common characteristic of the Group 4 herbicides is the ability to induce plant responses at lower fractions of use rates than most other herbicides. For example, it takes one percent of the glyphosate use rate to injure corn, whereas percent of the dicamba use rate can injure soybean. Due to this high level of activity, injury to sensitive plants outside of treated areas has been a problem since the introduction of Group 4 herbicides. Improvements in application technology have reduced, but not eliminated, problems with off-target movement of the Group 4 herbicides. Another distinguishing characteristic of certain Group 4 products is a relatively high vapor pressure that allows the herbicide to evaporate from the target site following application under certain conditions. The combination of off-target movement via volatility and sensitivity of plants to lose doses can lead to off-target injury even when the herbicide is applied in an appropriate manner. Many factors influence the potential for dicamba movement following application, including: 1. Temperature the rate of evaporation of any chemical is directly related to temperature. The potential for dicamba to volatilize increases as temperature increases. A threshold of 85 F has frequently been cited as the temperature where caution should be used when applying dicamba or other volatile growth regulator herbicides in the vicinity of sensitive vegetation. 2. Application surface the amount of dicamba that may evaporate varies depending on the characteristic of the surface to which it is applied. Research has shown that more dicamba evaporates from corn and soybean leaves than soil. Thus, there is greater risk of volatilization with postemergence applications when more of the herbicide is intercepted by the crop rather than reaching the soil surface. 3. Formulation the vapor pressure of herbicides may be affected by their formulation. Amine salts of 2,4-D have a sufficiently low vapor pressure that volatility is not an issue under typical application conditions. Numerous formulations of dicamba have been introduced with the intention of reducing the risk of volatilization. Xtendimax with Vapor Grip Technology, FeXapan plus Vapor Grip Technology and Engenia were developed to reduce vapor losses compared to older formulations. Independent research has verified these formulations reduce volatilization compared to older dicamba formulations (Banvel), but they do not eliminate these losses. The 2017 Iowa experience The Iowa Department of Agriculture and Land Stewardship (IDALS) received a record number of pesticide misuse complaints in 2017 (271), the increase was largely due to offtarget injury complaints associated with dicamba applications (107). The number of formal complaints to IDALS is a small fraction of the total problems associated with pesticide application. At the time of this article, IDALS has not released the breakdown on the percentage of complaints associated with contaminated spray equipment, particle drift, and volatilization. However, most people involved in investigating dicamba complaints acknowledge that all three avenues of exposure were involved with dicamba injury. Problems associated 2018 Herbicide Guide for Iowa Corn and Soybean Production 5

6 with contaminated spray equipment and particle drift can be minimized through better training and decisionmaking; however, risks associated with volatilization are not easily managed since vapor movement is controlled by the environment following application rather than actions of the applicator. Moving forward in 2018 There has been considerable debate on how to reduce the magnitude of off-target injuries observed in The United States Environmental Protection Agency recently made several important changes to the new dicamba formulations labeled for use on dicamba-resistant soybean. The new formulations are now classified as a Restricted Use Product. This classification will require all users of the products to maintain detailed records of all applications. In addition, certified applicators are required to receive dicamba specific training prior to using the products. The maximum wind speed allowed for applications was reduced from 15 mph to 10 mph, and applications are limited to between sunrise and sunset. Label language regarding sprayer cleanout and susceptible crops have been expanded. We feel these label changes are appropriate, but are concerned that they do not address the issue of off-target movement due to volatilization. Based on independent research and field observations, we have concluded that the new formulations have not reduced dicamba volatility sufficiently to allow safe use of postemergence applications in soybean. Dicamba has long been used postemergence in corn with what is considered an acceptable level of risk. However, there are several factors that differentiate postemergence use in soybean from that in corn: 1. The peak postemergence application timeframe in soybean is mid-june, whereas in corn is mid-may. This results in higher temperatures at application, and therefore a greater potential for volatilization. 2. While many plants in the Iowa landscape are sensitive to dicamba, soybean without the dicamba-resistant trait probably are most sensitive. Soybean without the dicamba-resistant trait will be at developmental stages more prone to yield impacts from dicamba injury. Farmers should not have to select dicamba-resistant varieties to protect themselves from off-target injury. 3. Finally, soybean typically are sprayed at stages with more canopy development than corn, resulting in a greater percentage of the herbicide being intercepted by foliage. Since greater losses occur from leaves than soil, there is greater risk of volatility with postemergence applications in soybean than in corn. Due to these risks associated with postemergence applications of dicamba in soybean, it is recommended that the product only be used preemergence in soybean. We recognize this greatly reduces the value of dicamba in managing herbicide-resistant waterhemp; however, the risks associated with postemergence applications exceed the weed management benefits provided by the new technology. Early postemergence applications (V2-V3 soybean) have lower risks than applications made later in the season. While early postemergence applications could avoid many of the problems associated with dicamba volatilization, planning on this strategy would place farmers and applicators in difficult positions when weather conditions or other factors prevent application during the preferred time period. This is why we have chosen to recommend only preemergence applications Herbicide Guide for Iowa Corn and Soybean Production

7 Herbicide-resistant Weeds in Iowa Micheal D. K. Owen, University Professor and extension specialist in agronomy and weed science at Iowa State University. Introduction Weeds with evolved resistance to herbicides are widely distributed in Iowa. Currently there are 10 weed species identified with evolved resistance to herbicides (Table 1). It is important to recognize that reporting herbicide resistance is voluntary and may not reflect the actual situation, particularly for weed populations with multiple resistances. Waterhemp populations have been reported in Nebraska with evolved resistance to HG 4 herbicides and common sunflower populations have evolved resistance to HG 9. Iowa horseweed/marestail populations have evolved resistance to HG 2 and 5, but this has not been reported to the International Survey of Herbicide Resistant Weeds ( Similarly, giant ragweed populations in Iowa have evolved resistance to HG 27. Importantly, the evolution of herbicide resistance continues to increase in Iowa and herbicide resistant weed population densities in specific fields are increasing, thus becoming an economic concern. In 2010, the Iowa Soybean Association requested that the Iowa State University weed science program survey soybean fields in Iowa to gain a better understanding of the herbicide resistance problem. The task became much greater than originally proposed and has just recently been completed. Approximately 900 waterhemp populations were sampled in Iowa (Figure 1). While the information is dated, and likely under-reports resistance to HG 14 and HG 27, the information provides a comprehensive picture of herbicide resistance in Iowa for waterhemp. Giant ragweed and horseweed/ marestail populations sampled Figure 1. Sample collection locations in Iowa, will be completed next but the number of fields with these weeds Table 1. Weed species with evolved herbicide resistance in Iowa. Adapted from The international survey of herbicide resistant weeds (Heap 2017). Available at Accessed October 4, Date reported Weed species (Latin binomial) Kochia (Kochia scoparia) Common lambsquarters (Chenopodium album) Pennsylvania smartweed (Polygonum pensylvanicum) Giant foxtail (Sertaria faberi) Tall waterhemp (waterhemp) (Amaranthus tuberculatus) Common cocklebur (Xanthium strumarium) Common sunflower (Helianthus annuus) Shattercane (Sorghum bicolor) Giant ragweed (Ambrosia trifia) Horseweed/marestail (Conyza canadensis) is considerably less than those with waterhemp. Herbicide site of action reported in Iowa Multiple herbicide resistances reported in Iowa 5 No 5 No 5 No 5 2 Yes (HG 1 in 1994) Yes (HG 5 in 1996, HG 14, 9, and 27 in 2009) 2 No 2 No 2 No 2 Yes (HG 9 in 2009) and HG 27 in 2016) 9 No 2018 Herbicide Guide for Iowa Corn and Soybean Production 7

8 Methods used The original idea was to arbitrarily sample soybean fields that had weeds visible above the canopy in August and September. Approximately 300 soybean fields were sampled in 2011 and This approach increased the likelihood that the escaped weeds were resistant to herbicides but no information about the herbicide use history was collected. The GPS coordinates of the fields were recorded so return visits were possible. It was decided that a random selection of fields would provide more useful and predictive information about evolved herbicide resistance. Thus in 2013, a procedure was developed that provided the prediction of herbicide resistance at the 95 percent confidence interval in any Iowa soybean field. This procedure used the number of soybean acres reported in each county to determine how many fields should be sampled in order to provide an estimate of herbicide resistance. Collaboration with the Iowa State University Department of Statistics and the Geographic Information Systems laboratory resulted in identifying the GPS coordinates of 400 fields that would be visited. Using the same two inclusionary principles as the 2011 and 2012 collections (e.g., soybean field and weeds visible above the canopy), weeds were collected and screened to allow predictions of herbicide resistance at the 95 percent confidence interval. Approximately 900 waterhemp populations were sampled for the project (Figure 1). Several female waterhemp plants were collected from each field, GPS coordinates of the fields were recorded and samples were dried for a number of days. Seeds were threshed by hand, cleaned, and samples were wet-chilled for several weeks to break dormancy. Seed samples were then air-dried and seeds planted and grown in the greenhouse until plants were approximately 3-4 inches tall, at which time they were treated with a herbicide. Herbicide treatments included an HG 2 herbicide (Pursuit), an HG 5 herbicide (atrazine), an HG 9 herbicide (Roundup), an HG14 herbicide (Cobra), and an HG 27 herbicide (Callisto). All herbicides were applied at label rates with adjuvants included as suggested in the herbicide labels. Nine waterhemp plants from each population were evaluated for control for each herbicide group and populations were evaluated for all five herbicide groups. Herbicide response was visually assessed on a scale of where 0 response indicated no herbicide affect and 100 indicated plant death. Populations were determined to be resistant if the averaged herbicide response was 80 percent or less when compared to a waterhemp population known to be sensitive to all herbicide groups. Results The herbicide groups included in the survey were chosen as they represent the most commonly used herbicides in Iowa. The levels of herbicide resistance found were surprisingly high for the five herbicide groups evaluated. However, given the years these herbicides have been used in Iowa, often in both corn and soybean, and the inevitability of evolved herbicide resistance, perhaps it is not that surprising. It should be recognized that the waterhemp populations in most fields that fulfilled the inclusionary principles were relatively low in population density and often represented scattered patches and individual plants. Given the ability of waterhemp to produce high seed numbers, it is possible that the population density may increase quickly in these fields unless appropriate management tactics are adopted. Resistance to ALS inhibitor herbicides (HG 2) in waterhemp is widely distributed and represents virtually all fields in Iowa based on the 2013 evaluation (Figure 2). While one ALS herbicide was used in the screen (Pursuit), waterhemp demonstrates crossresistance to all HG 2 herbicides regardless of application technique (Tranel and Wright 2002). Thus, HG 2 herbicides are not effective in Figure 2. Evolved resistance in waterhemp to ALS herbicides (HG 2) in Iowa Herbicide Guide for Iowa Corn and Soybean Production

9 managing waterhemp in Iowa. While waterhemp populations may not be homozygous for the resistance trait, the sensitive waterhemp in these populations is likely a minor component, given the historic use of HG 2 herbicides. Atrazine was used as the representative HG 5 herbicide. There is some difference in opinion whether waterhemp with evolved resistance to atrazine behaves similarly to other HG 5 herbicides such at an asymmetric triazine such as metribuzin. Preliminary research suggests these HG 5 herbicides may still be effective to control waterhemp with evolved resistance to the symmetric triazine herbicides (e.g., atrazine) (Owen, data unreported). Given the continued use of atrazine for many decades, it is not surprising that evolved resistance in waterhemp is so widely distributed in Iowa (Figure 3). Historically, HG 5 herbicide was target site-based and incurred a fitness penalty to the resistant populations (LeBaron 1991). However, with the changes in atrazine usage, application rates have declined considerably which, considering the fitness penalty, should result in the decline of HG 5 biotypes. However, reduced herbicide rates facilitate the evolution of herbicide resistance and metabolic HG 5 resistance has been reported in waterhemp (Gressel 2011; Huffman et al. 2015). Based on the 2013 waterhemp collection, at the 95 percent confidence interval 97 percent of the fields in Iowa have detectable resistance to HG 5. The adoption of crop cultivars with genetically-engineered tolerance to glyphosate in the mid-1990s was arguably the most important change in agriculture since the introduction of the moldboard plow. Despite the public concerns about technology, the benefits outweigh the risks (Duke Figure 3. Evolved resistance in waterhemp to PSII herbicides (HG 5) in Iowa Figure 4. Evolved resistance in waterhemp to glyphosate (HG 9) in Iowa and Powles 2008). However, the evolution of resistance to glyphosate has also changed agriculture and has benefited from the resurgence of the importance of weed management (Chatham et al. 2015). Glyphosate has been used on most of the Iowa corn and soybean acres for more than a decade and the inevitable evolution of glyphosate resistance is widespread and is predicted to be in 98 percent of the fields in Iowa (Figure 4). The occurrence of glyphosate resistance increased from and it is unlikely that glyphosate resistance in waterhemp will decline even if other technologies are adopted. The evolution of HG 14 resistance did not change during the course of this survey (Figure 5). However, given the increased importance of HG 14 herbicides to control glyphosate-resistant waterhemp, it is likely that these data greatly underestimate the occurrence of HG 14 resistance in waterhemp (Thinglum et al. 2011). This herbicide group is seen as the only selective postemergence option in soybean weed control despite the phytotoxicity these herbicides cause. The use of HG 14 herbicides as soil-applied treatments has also increased. The increased use of 2018 Herbicide Guide for Iowa Corn and Soybean Production 9

10 of HG 27 resistance may make management in the field more difficult. Further, the frequency of HG 27 resistance brings into question how effective the anticipated HG 27 resistance in soybean cultivars will be at supporting weed management. Figure 5. Evolved resistance in waterhemp to PPO inhibitor herbicides (HG 14) in Iowa Figure 6. Evolved resistance in waterhemp to HPPD inhibitor herbicides (HG 27) in Iowa HG 14 herbicides will result in more waterhemp populations with resistance (Wuerffel et al. 2015). The last new herbicide mechanism of action commercially introduced was the HG 27 herbicides almost 30 years ago. These herbicides have been widely used in corn, and as a result evolved resistance to HG 27 herbicides is widely distributed in Iowa (Figure 6). It is suggested that these data underestimate the occurrence of HG 27 resistance given the increased use of these products since the survey ended. Resistance to the HG 27 herbicides is reported to be due to metabolism (Huffman et al. 2015). Recent research conducted at Iowa State has verified that HG 27 is dominant or semi-dominant and polygenic with the number of genes involved with the metabolic resistance increasing with the herbicide rate (Kohlhase, unreported). The polygenic nature A problem with herbicide resistance in waterhemp is there does not appear to be a fitness penalty associated with the resistance (Wu et al. 2017). As a result, the resistance trait is likely to be conserved even if the herbicide is not used. New evolved resistances will be added to previously evolved resistance (Patzoldt et al. 2005). Given the dearth of new herbicide mechanisms of action, multiple resistances in waterhemp dramatically increased the difficulty of management. Multiple herbicide resistances in Iowa waterhemp populations is the norm (Figure 7). Waterhemp populations with resistance to three herbicide groups increased over the course of this study, and in 2013, 69 percent of the waterhemp populations demonstrated three-way resistance. This estimate is correct at the 95 percent confidence interval. Not surprising is the observation that the most common three-way resistance are for HG 2, HG 5, and HG 9, the most commonly used herbicide groups. Resistance to four herbicide groups and five herbicide groups (all the herbicide groups used in the screen) did not change over the course of the study with four-way resistance more commonly detected than five-way resistance. Management of multiple herbicide resistant waterhemp is a significant challenge for farmers. Conclusion It is important to understand that most of the fields from which waterhemp populations were collected were transitioning from Herbicide Guide for Iowa Corn and Soybean Production

11 Owen, M.D.K Diverse approaches to herbicide-resistant weed management. Weed Science 64: Owen, M.D.K., Beckie, H.J., Leeson, J.Y., Norsworthy, J.K., Steckel, L.E Integrated pest management and weed management in the United States and Canada. Pest Management Science 71: Figure 7. Evolved multiple resistance in waterhemp to in Iowa sensitive to resistant and the population density of waterhemp found was likely lower than the level that would be recognized by a farmer and cause a major concern. Nevertheless, the levels of herbicide resistance detected suggests that unless remediation is initiated, wide resistance to herbicides in Iowa waterhemp populations will likely increase. Despite farmers desires to have available a new herbicide, it is impossible to spray the problem of herbicide resistance in waterhemp away. The only solution is the judicious use of herbicides and adoption of greater diversity of weed management tactics (Owen 2016; Owen et al. 2015). References Chatham, L.A., Wu, C., Riggins, C.W., Hager, A.G., Young, B.G., Roskamp, G.K., Tranel, P.J EPSPS Gene Amplification is Present in the majority of glyphosate-resistant Illinois waterhemp (Amaranthus tuberculatus) Populations. Weed Technology 29: Duke, S.O., Powles, S.B Glyphosate: a once-in-a-century herbicide. Pest Management Science 64: In press. Gressel, J Low pesticide rates may hasten the evolution of resistance by increasing mutation frequencies. Pest Management Science 67: Heap, I The international survey of herbicide resistant weeds. Available at Accessed October 9, Huffman, J., Hausman, N.E., Hagar, A.G., Riechers, D.E., Tranel, P.J Genetics and Inheritance of Nontarget-Site Resistances to Atrazine and Mesotrione in a Waterhemp (Amaranthus tuberculatus) Population from Illinois. Weed Science 63: LeBaron, H.M Distribution and seriousness of herbicideresistant weed infestations worldwide. Pages in J. C. Caseley, G. W. Cussons, and R. K. Atkin, eds. Herbicide resistance in weeds and crops. Oxford: Butterworth-Heinemann. Patzoldt, W.L., Tranel, P.J., Hagar, A.G A waterhemp (Amaranthus tuberculatus) biotype with multiple resistance across three herbicide sites of action. Weed Science 53: Thinglum, K.A., Riggins, C.W., Davis, A.S., Bradley, K.W., Al- Khatib, K., Tranel, P.J Wide distribution of the waterhemp (Amaranthus tuberculatus) G210 PPX2 mutation which confers resistance to PPO-inhbiting herbicides. Weed Science 59: Tranel, P.J., Wright, T.R Resistance of weeds to ALSinhibiting herbicides: what have we learned? Weed Science 50: Wu, C., Davis, A.S., Tranel, P.J Limited fitness costs of herbicideresistance traits in Amaranthus tuberculatus facilitate resistance evolution. Pest Management Science. In press. Wuerffel, R.J., Young, J.M., Tranel, P.J., Young, B.G Soil- Residual protoporphyrinogen oxidase inhibiting herbicides influence the frequency of associated resistance in waterhemp (Amaranthus tuberculatus). Weed Science 63: Herbicide Guide for Iowa Corn and Soybean Production 11

12 Herbicide Group Number Crop tolerance Crabgrass Fall panicum Foxtail Woolly cupgrass Waterhemp 2, 4, 5, 6, 7, 8 Black nightshade Common ragweed Giant ragweed Lambsquarter Smartweed Velvetleaf Canada thistle Quackgrass Yellow nutsedge 2, 4, 8 Corn Herbicide Effectiveness Ratings 1 Weed response to selected herbicides E = excellent G = good F = fair P = poor Preplant/Preemergence Grasses Broadleaves Perennials Shattercane 2 Atrazine 5 E F P F P P E G G E F-G E E G G P F F Balance Flexx 27 E G F-G G G-E F-G G-E F P-F F-G P G G-E F G-E P P G Breakfree, Harness, Surpass NXT, etc Cocklebur 2 15 E E E E F-G F-G G G P P P P-F P-F P P P P G Callisto 27 E P P P P P G-E G-E F-G F-G F E F-G G-E E P P P Cinch, Dual Magnum, Outlook, Zidua, etc 15 E E E E F F F-G G P P P P P P P P P G Hornet WDG 2, 4 G P P P P P G-E F-G G G G G G-E G-E G P P P Linex/Lorox 7 G P-F P-F P P P G-E F F G P-F G-E G-E F F P P P Pendimax, Prowl, etc 3 F-G G-E G-E G-E G G G P P P P G-E F P P-F P P P Python 2 G P P P P P E F-G F G F F-G G-E F-G G-E P P P Sharpen (Kixor) 14 G P P P P P G-E G-E G G G G-E G G-E G-E P P G Sunflower 2 Postemergence Accent Q, Steadfast Q 2 G-E P G G-E G-E E G P F P P P G P F F G F Aim 14 G P P P P P F-G G P P F G P P E P P P Armezon, Impact 27 G-E F-G F G F F G-E G-E G-E G G G G E E P P P Atrazine 5 G F P F P P E E E E G E E E E F* F G Basagran 6 E P P P P P P P E E F P E G G-E G* P G* Basis, Basis Blend 2 F F F-G G F G G P F F P G-E G-E G-E G P G P Banvel, Clarity, DiFlexx, Xtendimax with Vapor Grip Technology, Engenia, FeXapan, etc. 4 F-G P P P P P G-E G E G-E E G E G F-G G* P P Beacon 2 G P F-G P-F P E E G G G E P G G F-G F-G* G F Buctril 6 G P P P P P G G-E E E G G-E G-E E G P P P Callisto 27 G-E P P P P P E E G-E F G G E G-E E P P P Glyphosate (Roundup, etc.) 3 9 E E E G-E E E G-E F-G E E G-E G E E G G G-E F Hornet WDG 2, 4 G P P P P P G-E F E E G-E F G-E E G-E G P P Liberty 3 10 E E G G-E E E G E E E G G E E E F-G G P Laudis 27 G-E F-G F G-E F-G F-G E G-E G-E G G G G E E P P P Permit, Halomax, etc. 2 G P P P P P E P G-E G-E G P G-E E E P P G Resolve 2 F F F-G G F G G P F F P G-E G P F-G F G F Resource 14 G-E P P P P P G P F F-G P F P P E P P P Status 4, 19 F-G P F F P F G-E G E G-E G G E G G G* P P 1 Ratings are based on full label rates. Premix products containing ingredients marketed as single a.i. products may not be listed in this table. 2 ALS-resistant biotypes of these weeds have been identified in Iowa. These biotypes may not be controlled by all ALS herbicides. 3 Use only on designated resistant hybrids. 4 Glyphosate-resistant biotypes of these weeds have been identified in Iowa. These biotypes may not be controlled by glyphosate. 5 PPO-resistant biotypes of waterhemp have been identified in Iowa. These biotypes may not be controlled by PPO inhibitor herbicides. 6 HPPD-resistant biotypes of waterhemp have been identified in Iowa. These biotypes may not be controlled by HPPD herbicides. 7 PSII-resistant biotypes of waterhemp have been identificed in Iowa. These biotypes may not be controlled by PSII herbicides. 8 Biotypes of this weed with resistance to multiple sites of herbicide action have been identified in Iowa. * Degree of perennial weed control is often a result of repeated application. This chart should be used only as a guide. Ratings of herbicides may be higher or lower than indicated depending on soil characteristics, managerial factors, environmental variables, and rates applied. The evaluations for herbicides applied to the soil reflect appropriate mechanical weed control practices Herbicide Guide for Iowa Corn and Soybean Production

13 Soybean Herbicide Effectiveness Ratings 1 Grasses Broadleaves Perennials Weed response to selected herbicides E = excellent G = good F = fair P = poor Herbicide Group Number Crop tolerance Crabgrass Fall panicum Foxtail Woolly cupgrass Shattercane 2 2, 4, 5, 6, 7, 8 Waterhemp Black nightshade Cocklebur 2 Common ragweed 2, 4, 8 Giant ragweed Lambsquarter Smartweed Sunflower 2 Velvetleaf Canada thistle Quackgrass Yellow nutsedge Preplant/Preemergence Authority/Spartan 14 G P-F P P-F P P E E F F F G-E F P F-G P P F-G Cinch, Dual II Magnum, Frontier, Warrant, Zidua, etc. 15 E E E E F F F-G G P P P P P P P P P P Command 13 E G-E G-E E F F P F F G P G-E G F E P P P FirstRate/Amplify 2 G-E P P P P P F-G P G G-E G-E G G-E G F-G P P F-G Linex/Lorox 7 F P-F P-F P P P G-E F F G P-F G-E G-E F F P P P Pendimax, Prowl, Sonalan, Treflan, etc 3 G-E E E E E G-E G P P P P G F P P P P P Pursuit 2 G F-G F F-G P-F G F-E G-E F G F G G-E F-G G P P P Python 2 E P P P P P E F F F P F-G G-E F E P P P Metribuzin, Sencor, TriCor, etc 5 F-G P P P-F P P E F F E P E E F-G G-E P P P-F Sharpen 14 G P P P P P F F F F F F F F F P P P Valor SX, Rowel 14 F-G P P P P P G-E E P G F G-E F P F P P P Postemergence Assure II, Fusilade DX, Fusion, Poast Plus, Select, etc. 1 E E E E E E P P P P P P P P P P G-E* P Basagran 6 E P P P P P P-F P-F E E F P E G G-E G* P G* Blazer 14 F-G P P F P F E G F G F F E F F F P P Classic 2 G P P P P P E P E G-E F P G-E E G-E F P G-E Cobra/Phoenix 14 F-G F P P P P E G G-E E F-G F G G F F P P Engenia, FeXapan, Xtendimax with VaporGrip Technology 4 E 9 P P P P P G-E G E G-E E G E G F-G G* P P FirstRate/Amplify 2 G P P P P P P P G-E E E P G E G P P P Glyphosate (Roundup, etc.) 3 9 E E G-E E E E G-E F-G E E G-E G E E G G G-E F Harmony 2 F P P P P P E P F F P G-E G-E G-E G P P P Liberty 3 10 E E G G-E E E G E E E G G E E E F-G G F Pursuit 2 G G G F-G F E F-G E G-E G F P-F E G G-E F P P Raptor 2 G G-E G-E G-E G E F-G E G-E G G G E E G-E F F F Reflex, Flexstar, Rumble, Dawn, Rhythm 14 F-G P P P P P E F-G F G G F G-E F F P-F P P Resource 14 G-E P P P P P G P F F-G P F P P E P P P 1 Ratings in this table are based on full label rates. Premix products containing ingredients marketed as single a.i. products may not be included in this table. 2 ALS-resistant biotypes have been identified in Iowa. These biotypes may not be controlled by all ALS products. 3 Use only on appropriate resistant varieties. 4 Glyphosate-resistant biotypes of these weeds have been identified in Iowa. These biotypes may not be controlled by glyphosate. 5 PPO-resistant biotypes of common waterhemp have been identified in Iowa. These biotypes may not be controlled by PPO inhibitor herbicides. 6 HPPD-resistant biotypes of common waterhemp have been identified in Iowa. These biotypes may not be controlled by HPPD herbicides. 7 PSII-resistant biotypes of these weeds have been identifed in Iowa. These biotypes may not be controlled by PSII inhibitor herbicides. 8 Biotypes of this weed with resistance to multiple sites of herbicide action have been identified in Iowa. 9 Diamba-tolerant soybean cultivars only. * Degree of perennial weed control is often a result of repeated application. This chart should be used only as a guide. Ratings of herbicides may be higher or lower than indicated depending on soil characteristics, managerial factors, environmental variables, and rates applied. The evaluations for herbicides applied to the soil reflect appropriate mechanical weed control practices Herbicide Guide for Iowa Corn and Soybean Production 13

14 Grazing and Haying Restrictions for Herbicides Used in Grass Pastures Herbicide A.I. Rate/A Grazing Beef and Non-Lactating Animals Hay harvest Removal before slaughter Lactating Dairy Animals Grazing Hay harvest 2, 4-D 2, 4-D 1.5 to 2.0 lb ae 7 days 7 days Clarity and many others dicamba Up to 1 pt days 7 days 37 days 1-2 pt days 21 days 51 days 2-4 pt days 40 days 70 days 4-16 pt days 60 days 90 days Chaparral aminopyralid + metsulfuron methyl oz Cimarron Max (co-pack) metsulfuron methyl + dicamba + 2,4-D oz A pt B days 7 days 37 days Cimarron X-Tra metsulfuron methyl + chlorsulfuron oz Crossbow triclopyr + 2,4-D 1-6 qt 0 14 days 3 days Growing season Escort XP metsulfuron methyl Up to 1.7 oz Growing season ForeFront HL aminopyralid + 2,4-D pt 0 7 days days Grazon P&D picloram + 2,4-D 3-4 pt 0 30 days 3 days 7 days 30 days Milestone aminopyralid 3-7 oz Overdrive dicamba + diflufenzopyr 4-8 oz PastureGard HL triclopyr + fluroxypyr pt 0 14 days 3 days 1 year 1 year Rave dicamba + triasulfuron 2-5 oz 0 37 days 30 days 7 days 37 days Redeem R&P triclopyr + clopyralid pt 0 14 days 3 days Remedy Ultra triclopyr 1-2 qt 0 14 days 3 days Growing season Growing season Surmount picloram + fluroxypyr pts Tordon 22K picloram < 2 pts Growing season Growing season > 2 pts Weedmaster dicamba + 2,4-D 1-4 pts 0 7 days 30 days 7 days 7 days Herbicide Guide for Iowa Corn and Soybean Production

15 Herbicide Package Mixes The following table provides information concerning the active ingredients found in prepackage mixes, the amount of active ingredients applied with a typical use rate, and the equivalent rates of the individual products. Corn Herbicide Premixes or Co-packs and Equivalents Herbicide Group Components (a.i./gal or % a.i.) If you apply (per acre) You have applied (a.i.) An equivalent tank mix of (product) Acuron lb S-metolachlor 3.0 qt 1.6 lb S-metolachlor 27.0 oz Dual II Magnum lb atrazine 0.75 lb atrazine 1.5 pt atrazine 4L mesotrione 0.18 lb mesotrione 5.8 oz Callisto lb bicyclopyrone lb bicyclopyrone N/A Acuron Flexi lb bycylopyrone 2.25 qt lb bicyclopyrone N/A lb mesotrione 0.18 lb mesotrione 5.8 oz Callisto lb S-metolachlor 1.61 lb S-metolachlor 27.0 oz Dual II Magnum Alluvex WSG % rimsulfuron 1.5 oz 0.25 oz rimsulfuron 0.5 oz Harmony SG % thifensulfuron 0.25 oz thifensulfuron 1.0 oz Resolve SG Anthem lb pyroxaslufone 10.0 oz 0.16 lb pyroxasulfone 3.0 oz Zidua lb fluthiacet-methyl lb fluthiacet-methyl 0.7 oz Cadet Anthem Maxx lb pyroxasulfone 5.0 oz 0.16 oz pyroxasulfone 3.0 oz Zidua lb fluthiacet-methyl lb fluthiacet 0.7 oz Cadet Anthem ATZ lb atrazine 2.0 pt 1.0 lb atrazine 2.0 pt atrazine 4L lb pyroxasulfone 0.12 lb pyroxasulfone 2.25 oz Zidua lb fluthiacet lb fluthiacet 0.6 oz Cadet Armezon Pro lb dimethenamid-p 20.0 oz 0.82 lb dimethenamid-p 17.5 oz Outlook lb topramezone lb topramezone 0.73 oz Armezon Basis Blend % rimsulfuron oz oz rimsulfuron 0.67 Resolve % thifensulfuron oz thifensulfuron 0.16 oz Harmony Bicep II MAGNUM, Cinch ATZ, Medal II AT, Charger Max ATZ Bicep Lite II MAGNUM, Cinch ATZ Lite, Charger Max ATZ Lite lb S-metolachlor 2.1 qt 1.26 lb S-metolachlor 21.0 oz Dual II MAGNUM lb atrazine 1.63 lb atrazine 52.0 oz Aatrex 4L lb S-metolachlor 1.5 qt 1.25 lb S-metolachlor 21.0 oz Dual II MAGNUM lb atrazine 1.00 lb atrazine 32.0 oz atrazine 4L Breakfree NXT ATZ lb acetochlor 2.7 qt 2.1 lb acetochlor 2.4 pt Breakfree NXT lb atrazine 1.7 lb atrazine 3.4 pt atrazine 4L Breakfree NXT Lite lb acetochlor 2.0 qt 2.2 lb acetochlor 2.5 pt Breakfree NXT lb atrazine 0.85 lb atrazine 1.7 pt atrazine 4L Callisto GT lb glyphosate 2.0 pt 0.95 lb glyphosate 27.0 oz/a of 4.5 lb ae/gal glyphosate lb mesotrione lb mesotrione 3.04 oz Callisto 2018 Herbicide Guide for Iowa Corn and Soybean Production 15