2012 Herbicide Guide for Iowa Corn and Soybean Production

Transcription

1 2012 Herbicide Guide for Iowa Corn and Soybean Production New options for weed management in 2012 Micheal D. K. Owen, professor and Extension weed specialist, Agronomy, Iowa State University While there have not been many new things in weed control/management for 2012, some that have occurred are not necessarily good. Herbicide resistance, particularly in common waterhemp has escalated significantly for populations with evolved resistance to glyphosate and resistance to HPPD herbicides has predictably has been identified in a number of locations across Iowa. Unfortunately, again as predicted, no new silver bullets have surfaced and in fact, it is unlikely that new herbicide mechanisms of action will be introduced in the foreseeable future. Thus it comes that much more important to recognize the tactics that are available and establish a diverse long-term approach to using the tools in a sustainable manner. New products and changes While there have not been any new products introduced for 2012 (at this time), there are several products pending registration, new generic herbicides and changes in herbicide labels. The following is a partial list of these changes; the inclusion of products should not be construed as an endorsement by Iowa State University or exclusion considered a lack of support. Ignite (Bayer Crop Science) The Ignite label now describes a single application dose of up to 36 fluid ounces per acre. This application can be followed by one additional application of a maximum 29 fluid ounces per acre for a seasonal maximum Ignite application of 65 fluid ounces per acre. The Ignite applications to corn have not changed; the maximum amount of Ignite in any single application is 22 fluid ounces per acre with a seasonal total of 44 fluid ounces per acre. Vida (Gowan) Vida (pyraflufen-ethyl) is an inhibitor of the PPO enzyme and a potent contact herbicide that can be applied to soybean and corn as a preplant burndown, at planting burndown and after planting burndown but prior to crop emergence for the control of many broadleaf weeds. Vida is now registered as a postemergence directed application in corn (conventional, glyphosate-tolerant, Liberty Link, popcorn, seed corn, corn silage, and corn stover). Sweet corn is not registered for a postemergence directed application. Refer to the label for specific restrictions and directions. Flexstar GT 3.5 (Syngenta) Flexstar GT 3.5 is a different premixture formulation of fomesafen and glyphosate. This premixture contains 5.88% fomesafen and 22.4% glyphosate for a total of 0.56 pounds of fomesafen and 2.26 pounds (acid equivalent) of glyphosate per gallon product. The use rate in Iowa (Region 4) is 2.8 pints per acre. Medal herbicides (Syngenta) Medal herbicides are a new S-metolachlor series of products with 7.62 pounds of active ingredient per acre (Medal and Medal EC), 7.64 pounds of active ingredient (Medal II and Medal II EC) and a premixture Medal II AT which is atrazine and S-metolachlor at 3.1 and 2.4 pounds active ingredient respectively. Warrant (Monsanto) Warrant is an encapsulated formulation of acetochlor that is now labeled WC-94 Revised Dec 2011 for application to field corn as a postemergence application. Applications can be made until the corn is 30 inches in height either broadcast or as a directed treatment (e.g. drop nozzles) to minimize interference of the crop with spray coverage. Warrant should be applied prior to weed emergence and will provide residual control of annual grasses and some small-seeded annual broadleaf weeds. Roundup Ready Plus weed management solutions (Monsanto) Monsanto has partnered with a number of companies to improve weed management in glyphosate-resistant corn and soybean and has incentivized the addition of products other than their proprietary herbicides to provide stewardship to glyphosate and the trait. Additions to the previously listed products are Cobra/Phoenix in soybean and Impact in corn. Basis Blend (DuPont) Basis Blend is a premixture of rimsulfuron (20%) and thifensulfuron (10%) which is suggested to be a better formulation that is easier to handle, mix and clean out of the sprayers than Basis 75% DF. Basis Blend can be applied any time after harvest but prior to ground freeze-up. It can be applied with other Contents New options for A reintroduction to soil-applied herbicides 6 Corn herbicide effectiveness ratings 11 Soybean herbicide effectiveness ratings 12 Grazing and haying restrictions 13 Herbicide package mixes 14 Herbicide site of action and injury symptoms 20

2 herbicides (e.g. 2,4-D) and is registered for application to fields that wil be planted to corn or soybean. Valor (Valent) Valor has a modification of the label that describes planting corn seven days after application in no tillage and minimum tillage production systems. Pyroxasulfone (several) Pyroxasulfone is a new product that has been included in the ISU herbicide research program for many years under the KIH-485 description. Considerable research was conducted on corn and soybean and a variety of application timings (e.g. early preplant) and rates were included in this extensive evaluation series (www. weeds.iastate.edu/research/default.htm). Pyroxasulfone was first included in the ISU research program in 2003 as a 3.57 SC formulation and was a Kumiai experimental product. This herbicide is an inhibitor of very long chain fatty acids similar to the mechanism of action demonstrated by S-metolachlor and acetochlor (Group 15). Agreements have been made with BASF, FMC and Valent to market pyroxasulfone in different proprietary products, either alone or in combination with other herbicides. These registrations are pending. New genetically engineered traits (several) The development of new genetically engineered (GE) crop traits continues with regard to dicamba-tolerant soybean (Monsanto) and the DHT soybean and corn (Dow AgroSciences). According to these companies, these new crop traits are on track for commercialization middecade. There has been considerable discussion about the utility of these traits and labeled herbicides as tools to better manage weeds, particularly those weeds (e.g. common waterhemp) that have evolved resistance to glyphosate. Currently, there are concerns about the movement of the herbicides used in these GE crops to sensitive crops (e.g. grapes) and also whether or not the use of the systems will result in new resistant weed biotypes. The companies are expending considerable time and money developing robust stewardship programs and use guidelines in an attempt to proactively mitigate these concerns. However, it is critically important for growers and applicators to recognize that the adoption of crop systems based on these technologies have concomitant risks and limitations; they do not represent the new silver bullet as some uninformed people have been suggesting. The development of Optimum GAT crops has been delayed indefinitely according to DuPont. New herbicide resistant weed concerns New herbicide resistant weed biotypes have been identified in Iowa and the Midwest and weed biotypes with multiple resistances are increasing. HPPD-resistant waterhemp was identified in 2010 in Southeast Iowa in a seed corn production field. Since then, numerous seed corn production fields with putative HPPD resistant common waterhemp have come to the attention of ISU. Extensive infield research was established in 2011 and research efforts are escalating for The evolution of HPPD resistance in seed corn production fields can be attributable to the intensity of HPPD use in these fields and the identified problems ascribed to the level of observation and management in the seed production fields. Importantly, given the strategies used in seed corn production, multiple resistances to glyphosate have also been identified in the weed populations under investigation. It is assumed that the extent of HPPD resistance in Iowa, given the likely movement of the resistance trait via pollen, is greater than in just seed corn production fields but masked in commercial production fields. The occurrence of HPPD resistance in seed corn fields is not unlike the canaries in the mines which were used to detect problems for the miners. ISU will continue to monitor HPPD resistance in Iowa common waterhemp populations is conduct research to describe solutions to the problem. Glyphosate resistance in Iowa common waterhemp, as predicted, has increased dramatically in 2011 and is widely distributed across the state. Through collaboration and support from the Iowa Soybean Association, an extensive collection of field weed populations has been cataloged and these populations will be evaluated for evolved resistance to glyphosate and the other herbicide mechanisms of action commonly used in Iowa. A previous collection of approximately 200 common waterhemp populations selected arbitrarily three years ago was evaluated in the greenhouse for response to glyphosate; approximately 1/3 of those populations were not effectively controlled by glyphosate. It is anticipated that the percentage of Iowa common waterhemp populations with evolved resistance to glyphosate has increased considerably. Furthermore, populations with resistance to PPO inhibitors are also becoming more common. Note that Nebraska recently announced the identification of a population of common waterhemp with resistance to 2,4-D. While common waterhemp is the weed about which most Iowa growers and applicators are concerned, issues with herbicide resistant giant ragweed and horseweed/marestail are also escalating. It is clear that the systems currently used for the production of corn and soybean in Iowa, specifically for weeds, is problematic and inevitably will fail unless changes (other than different herbicides) are included soon. Weed management tactics Knowledge and diversity The need for better information is paramount for effective weed management; simplicity and convenience as experienced during the last 16 years of glyphosate-resistant crop systems has run the course and integrated weed management is necessary for the protection of crop yields, the mitigation of existing herbicide resistant weed issues and the proactive tactics needed to keep additional herbicide resistant weed populations from evolving. Iowa State University Extension and Outreach Weed Science 2

3 Unfortunately, while many (and possibly a majority) growers understand that herbicide resistant weeds are an increasing problem, they seem to still be in denial; they fail to recognize that the problem likely exists close to home and that action to manage the problem is needed immediately. Recall that typically a weed population must have about 30% of the individuals with evolved resistance to a herbicide before a grower recognizes that the issue exists. The information that must be acquired includes a cursory understanding about weed biology and ecology, the herbicide resistance(s) that are likely to evolve or have evolved, and what tactics are effective to manage these weeds. Part of the problem is the marketing of herbicides; many companies are now describing premixtures of products that include more than one herbicide mechanism of action. The concept of multiple mechanisms of herbicide action effectively helping control herbicide resistant weeds and delay the evolution of future herbicide resistances has gained some traction with growers. However, without better knowledge of the mechanisms of action that are in the premixtures, the marketing of these products is misleading at best. Consider that most of the premixtures available for soybeans includes a PPO herbicide and the other product is an ALS inhibitor herbicide; given that common waterhemp in Iowa already evolved ALS resistance, these products have only one effective mechanism of action and thus do not represent an effective resistance management program. While redundancy of tactics (multiple herbicide mechanisms of action in each application) is an important strategy, the herbicides included must have activity on the target weeds. Another strategy that has gained acceptance for herbicide resistant weed management is the rotation of herbicide mechanism of action. Indeed, this can be the start of a herbicide resistant weed management program, but if that is the only tactic used, herbicide resistance will be delayed one year for every year of herbicide mechanism of action rotation but resistance will inevitably evolve. Another common strategy that is marketed is the need for multiple herbicide mechanisms of action in a weed management program. This typically is established by using different herbicides for sequential applications. Unfortunately, the use of this strategy is diminished as it is the last herbicide applied that imparts the selection for resistance. Again, the most effective way to resolve this problem is to use multiple mechanisms for every herbicide application timing. The most important consideration for weed management in crop production, whether herbicide resistant weed populations exist or not, is the need for diversity in weed management strategies. If diverse strategies beyond simply adding different herbicides are not included, the crop system may not be sustainable. History has proven time and again that herbicide-based weed management will inevitably fail. Mechanical and cultural strategies need to be included in a crop production system. The greater the diversity, the more ecologically sound and economically profitable the crop production system will be. Herbicides will continue to be a key feature of Iowa corn and soybean production, but without other integrated weed management (IWM) strategies, weed management will soon become increasing difficult and crop yields will dramatically decline. Conclusions The future of weed management in the relatively near future is better utilization of existing technologies and the inclusion of older herbicide chemistries (when and where appropriate) and mechanical and cultural tactics. The key to profitable and sustainable weed management is diversity. If a diverse suite of weed management tactics is not used, economic losses attributable to weeds will escalate and herbicide resistant weed populations will become more widely distributed. No new herbicide mechanisms of action have been identified for the short and longer term future. While new herbicideresistant crop traits may possibly become available in the three to five year future, these traits are not the answers to existing weed management concerns; they are good tools for weed control but must be used in an appropriate fashion to maximize the benefits and minimize the risks. Iowa State University Extension and Outreach Weed Science 3

4 Table 1. Herbicides used in combination with glyphosate for control of giant ragweed, common lambsquarters and common waterhemp in corn (adapted from NDSU/UMN Extension publication). (P, F, G, E are poor, fair, good and excellent, respectively) Common waterhemp 1,2,3,4,5 Common lambsquarters Giant ragweed 1,2 PRE in sequence with glyphosate Atrazine (0.5 to 1.0 lb ai/a) F/G G/E G/E Balance Flex F G/E G Banvel/Clarity F G G Callisto F G/E E Camix G G/E E Harness/Surpass/Dual/Outlook P G/E F/G Hornet F/G P/F G Integrity G G/E G/E Linex/Lorox F/G G G Lumax G E G/E Prequel F/G E G Prowl P G G/E SureStart F/G G/E E POST as part of a tank mixture with glyphosate Aim F F/G G Atrazine (0.38 to 1.0 lb ai/a) G E E Banvel/Clarity E G G/E Basis P P G/E Buctril G G/E G Cadet P F F/G Callisto G E G/E Capreno G G/E G/E Hornet G/E P/F P/F Impact G G/E G/E Laudis G G/E G/E Option P P P Permit P/F P P Realm Q G E E Resolve Q P P F Resource P F F Status/Distinct G/E G/E G/E Alternative Technology Ignite in Liberty Link corn hybrids G/E G F 1 ALS inhibitor herbicide resistant biotypes have been confirmed in Iowa 2 Glyphosate resistant biotypes have been confirmed in Iowa 3 PPO inhibitor herbicide resistant biotypes have been confirmed in Iowa 4 HPPD inhibitor herbicide resistant biotypes have been confirmed in Iowa 5 Triazine herbicide resistant biotypes have been confirmed in Iowa Iowa State University Extension and Outreach Weed Science 4

5 Table 2. Herbicides used in combination with glyphosate for control of giant ragweed, common lambsquarters and common waterhemp in soybean (adapted from NDSU/UMN Extension publication). (P, F, G, E are poor, fair, good and excellent, respectively) Common waterhemp 1,2,3,4,5 Common lambsquarters Giant ragweed 1,2 PRE in sequence with glyphosate IntRRo (alachlor) P F/G P/F Dual/Outlook P F/G P/F Authority Assist P G/E E Authority First/Sonic G G/E G/E Authority MTZ P/F G/E G/E Boundary P/F G/E G Enlite F FG/E F Envive G G/E G/E FirstRate G/E P G Gangster F/G G G/E Linex/Lorox F/G G G Optill F/G G G/E Prefix F G G Prowl P G G Sencor P E E Sharpen (1 oz/a) F G G/E Spartan F E G/E Treflan P G G Valor F G/E E POST as part of a tank mixture with glyphosate Cadet P F F/G Classic F P P Cobra/Phoenix F/G E F FirstRate E P P Flexstar G E F Harmony GT P P G/E Pursuit F P P/F Raptor G P G Resource P G F Storm G E F Synchrony F/G P G/E Ultra Blazer F E F Alternative Technology Ignite in Liberty Link soybean hybrids G/E G G 1 ALS inhibitor herbicide resistant biotypes have been confirmed in Iowa 2 Glyphosate resistant biotypes have been confirmed in Iowa 3 PPO inhibitor herbicide resistant biotypes have been confirmed in Iowa 4 HPPD inhibitor herbicide resistant biotypes have been confirmed in Iowa 5 Triazine herbicide resistant biotypes have been confirmed in Iowa Iowa State University Extension and Outreach Weed Science 5

6 Global USA 5 0 Figure 1. Occurrence of weeds with evolved resistance glyphosate. Adapted from the International Survey of Herbicide Resistant Weeds ( (Heap, 2011) A reintroduction to soil-applied herbicides Bob Hartzler, professor and Extension weed specialist, Agronomy, Iowa State University Although the growth regulator herbicides (2,4-D; dicamba, etc.) were responsible for ushering in the chemical era of weed control in the late 1940 s, it was the introduction of the triazine, dinitroaniline and amide herbicides that transformed weed control in corn and soybean. These products were the backbone of weed control systems until the mid-80 s when the introduction of ALS inhibitors and other postemergence products provided more consistent postemergence weed control. The introduction of glyphosate resistant crops in the late 1990 s completed the transition from soilapplied to postemergence programs for the majority of Cornbelt farmers. The heavy reliance on glyphosate for over a decade has created a situation where soil-applied products will once again be an essential component of weed management systems due to herbicide resistance. This paper will discuss factors that influence the performance of soil-applied products for those who have little experience with these products, or simply need a refresher. Preemergence herbicides are most effective when they are absorbed by weed seeds initiating the germination process; however, only a small portion of the applied herbicide actually is taken up by the intended target. The majority of herbicide degrades within the field, but a portion of the herbicide may be lost from the field due to leaching, runoff, or volatilization. The ultimate fate of an herbicide is largely dictated by adsorption of the herbicide molecules to soil colloids. Herbicide adsorption to soil colloids There are two pools of herbicides present in the soil: the larger pool is the herbicide bound to soil colloids, the smaller pool is the herbicide that is dissolved in the soil water. An equilibrium (the percentage of herbicide present in each pool) is maintained between these two pools, thus herbicide molecules are able to move back and forth (sorption:desorption) between the two pools as herbicide is lost from one of the pools. The equilibrium is determined primarily by the adsorptive capacity of the soil and the chemical characteristics of the herbicide. Since only the herbicide in the soil solution is available to plants, a basic knowledge of herbicide adsorption is essential to understand the behavior and performance of preemergence herbicides. Although adsorption places the majority of herbicide into a bank where it cannot be immediately absorbed by weeds, it is critical since it maintains the majority of herbicide near the soil surface where weed seeds germinate, and adsorption protects groundwater from herbicide leaching through the profile. Soil factors influencing adsorption The adsorptive capacity of a soil is determined by its clay and organic matter content. For most Iowa soils, organic matter is responsible for the majority of herbicide binding. It is important to distinguish between the different types of soil organic matter found in soil. Herbicides bind to the highly degraded, stable forms of organic Iowa State University Extension and Outreach Weed Science 6

7 matter referred to as humic matter or humic acids. The humic acid content of a soil is usually closely related to the organic matter content. Crop residue present on the soil surface in conservation tillage systems or mixed within the soil profile by tillage is not involved in the binding of soil-applied herbicides. Herbicide rates found on the product label take into account herbicide adsorption and are designed to insure that sufficient herbicide is present in the soil solution to control susceptible weeds. Rates of soil applied herbicides generally increase as clay and organic matter increase. For example, the recommended rate for Dual II Magnum increases approximately 10% for each 1% increase in soil organic matter. Many herbicide labels prohibit use on high organic matter soils, such as peats, due to inactivation of herbicide by excessive binding of the herbicide to the soil. Herbicides with a low margin of crop safety often prohibit use on soils with low adsorptive capacity due to the potential for crop injury due to high availability of the herbicide within the soil. Soil ph influences binding of herbicides that are classified as basic chemical compounds (versus acidic or non-ionic compounds). These molecules have a neutral or positive charge depending on the soil ph. In neutral or basic soils (ph 7) a basic herbicide will have a neutral charge, whereas under acidic soil conditions (ph < 7) the herbicide takes on a positive charge. Due to the positive charge on the molecule in acid soils, basic herbicides are more tightly bound to soil colloids in soils with a low ph. The triazine herbicides are the primary examples of herbicides with a basic nature. The metribuzin label warns that use of the product on soils with ph of 7.5 or higher may result in crop injury. The increased risk of injury in alkaline soils is due to the greater amount of herbicide in soil solution. Herbicide factors influencing adsorption The degree of soil adsorption of a herbicide is determined by its chemical characteristics. The sorption coefficient (K) is a measure of the tendency for an herbicide to be adsorbed by the soil. It is usually expressed either as K d or K oc. The K value for an herbicide will vary among soils due to the different binding capacity of soils. The K oc is adjusted for the organic matter content of a soil, whereas the K d takes into account binding to clay and organic matter. The K values determined on different soils or under different laboratory conditions will vary somewhat, but they are still very useful in predicting herbicide behavior. A simple description of the K value is that it is a ratio of the herbicide bound to soil colloids to the herbicide present in the soil solution. Herbicide (soil) K = Herbicide (water) Thus an herbicide that has a high K value will have a high percentage of the herbicide bound to soil colloids, and thus less is dissolved in the soil solution where it would be available for absorption by plants. Herbicides with low K values have more of the herbicide in the soil solution, thus have greater availability to plants and are more mobile in the soil profile. A second parameter that can influence herbicide behavior is an herbicide s water solubility; however, water solubility usually is relatively insignificant compared to the sorption coefficient. Initially this may not seem logical since the fraction of herbicide dissolved in soil water is responsible for herbicide activity. Consider that most herbicides are applied at a pound or less per acre and that an acre inch of water weighs approximately 220,000 lbs. For most herbicides, water solubility is not a limiting factor due to the large volume of water present in the soil compared to the low rate that herbicides are applied. The sorption coefficient helps explain the performance of herbicides applied to the soil (Table 3). Both glyphosate and paraquat have very high K values compared to other herbicides. Neither product has significant soil activity since they are bound so tightly to soil colloids that they are unavailable to plants. The labels of both products state that the use of water containing soil sediments as a carrier will reduce performance due to inactivation of the herbicide by binding to the colloids. Pendimethalin has a high K oc compared to other preemergence herbicides, which explains why it requires more rainfall to provide consistent control than herbicides with lower sorption coefficients. Dicamba has one of the lowest sorption coefficients of commonly used herbicides. Although dicamba is registered for preemergence use in corn, applications made prior to germination of the corn seed have a relatively high risk of crop injury due to dicamba s mobility in the soil. Applications made before or shortly after corn planting may allow dicamba to reach the depth of the corn seed and be absorbed as the seed imbibes water and result in damage to the seedling. Table 3. Chemical properties of several herbicides. Common name Tradename K oc H 2 O Solubility (ppm) acetochlor Harness atrazine Aatrex dicamba Banvel/Clarity 2 250,000 glyphosate Roundup 24,000 10,500 mesotrione Callisto metolachlor Dual paraquat Gramoxone 1,000, ,000 pendimethalin Prowl 17, Source: WSSA Herbicide Handbook; IUPAC Pesticide Properties Database Iowa State University Extension and Outreach Weed Science 7

8 Environmental factors influencing adsorption Since adsorption is a physical process, temperatures within the range experienced in the field have little impact on binding of herbicides to soil colloids. Rainfall impacts herbicide performance by facilitating movement in the soil profile (leaching) and by influencing soil moisture content. As soil moisture decreases the film of water surrounding soil particles becomes thinner, resulting in less volume to dissolve herbicide molecules and greater adsorption to soil colloids. Preemergence herbicides are less active during periods when soil moisture is limiting. Absorption by plants To be effective, preemergence herbicides must be present in the soil solution surrounding weed seeds as the seed initiates germination. Thus, an herbicide must be positioned within the soil profile at the depth of weed establishment. Since most weeds have small seeds, the majority of seeds germinate in the upper inch of the soil profile. The term activation is commonly used to describe movement of a soil-applied herbicide from the soil surface into the soil profile. For most situations, a half inch rain is sufficient to move the chemical to the soil depth required for effective weed control. However, the rainfall required for activation will increase slightly with increasing adsorptive capacity of the soil and sorption coefficient of the herbicide. In addition, more rainfall will be needed in situations where an herbicide is applied to a very dry soil. With the exception of the dinitroaniline herbicides (pendimethalin, trifluralin), differences in the sorption coefficient (K) of commonly used preemergence herbicides are not sufficient to result in significant differences in the amount of rain required for activation. Herbicide absorption from the soil is a passive process. The initial step in seed germination is imbibition of water from the soil. Herbicide molecules present in the soil solution are carried into the seed with the water. Since weeds are most vulnerable to preemergence herbicides just as they initiate germination, having the herbicide present in the germination zone at this time is critical. Herbicide applications made at planting generally are dependent upon rainfall within three to five days to ensure effective control of weeds that germinate shortly after planting and herbicide application. A few preemergence herbicides can be absorbed by roots of emerged seedlings and provide control of established plants. This phenomenon typically occurs when weeds are able to establish due to dry conditions that minimize herbicide availability. Rain shortly after the weeds emerge releases herbicide bound to soil colloids into the soil solution, allowing absorption of the herbicide by the established weeds. It takes higher concentrations of herbicides to kill established weeds than a germinating seed, thus chemicals with low K values and the ability to translocate within the plant are more likely to kill established seedlings than herbicides without these characteristics. The bleaching herbicides (HPPD inhibitors) are promoted for their ability to control established plants through recharge, whereas amide type and dinitroaniline herbicides have little effect on emerged weeds. While the ability to control emerged weeds can on occasion improve weed control, this type of activity is much less consistent than an herbicide acting on a germinating seeds. Thus fields with escaped weeds should be monitored closely to determine the need for remedial control measures. Herbicide degradation The persistence of an herbicide is typically described in terms of half-life (t ½ ), the time required for 50% of the herbicide present in the soil to break down. Herbicides begin to degrade as soon as they are introduced in the environment, but the rate of breakdown varies widely among chemicals and environmental conditions (Figure 1). In this example, the half-life for the chemicalwas 5 weeks under favorable conditions, but increased to 9 weeks under unfavorable conditions. The primary factors that influence degradation rate are soil characteristics, temperature and rainfall. Herbicides may be broken down by chemical or biological mechanisms, or both. Biological degradation is more responsive to environmental factors than chemical processes. The range of soil temperatures encountered during the growing season typically do not have a major influence on degradation rates, but extended dry periods can result in prolonged persistence of a herbicide. Ideally a preemergence chemical could be applied in early spring and would control weeds until the crop canopy closes. After that it would dissipate quickly so that it would not interfere Iowa State University Extension and Outreach Weed Science 8

9 with future cropping plans or move into water resources or other areas where it is not wanted. Unfortunately, the dynamics of herbicide degradation and environmental variability prevent such a simple solution to weed management. The initial rate of degradation in spring is relatively rapid compared to degradation rates later in the season. This is due to a combination of greater initial herbicide availability and more favorable conditions for biological degradation in the spring (temperature, moisture) than occurs later in the season. Since herbicide degradation rates slow as the season progresses, a product that persists long enough to provide full-season weed control may pose a threat to susceptible rotational crops the following growing season. Soil ph influences degradation rates of several herbicides that are used in Iowa corn and soybean production. The persistance of atrazine and chlorimuron increases with soil ph, and this limits their use in areas of the state with alkaline soils due to carryover risks to rotational crops. Atrazine degrades more rapidly when bound to soil colloids, thus the greater availability of atrazine in high ph soils increases the half-life of the chemical. Chlorimuron and other sulfonylurea herbicides are degraded by both chemical and biological processes in acidic or neutral soils, with chemical hydrolysis being the most important mechanism. In alkaline soils, only biological degradation is involved and the persistence is greatly increased. Like atrazine, mesotrione binds to soil colloids less under alkaline conditions; however, mesotrione breaks down more rapidly when in solution than when bound to colloids. Thus, mesotrione is more persistent under alkaline conditions. The imidazolinone herbicides (Pursuit, Scepter) also have increased persistence in acid soils. Application timing impacts on herbicide performance Preemergence herbicides can be applied over an extended period of time, from fall applications made more than six months prior to crop planting, until after the crop has emerged. The primary influence of application timing is in determining the time period when the herbicide will be present at effective concentrations in the soil. Application timing also influences the probability of the herbicide being activated by rainfall before weeds become established. Several preemergence herbicides are registered for fall applications. Fall applications are most appropriate for controlling winter annual weeds such as marestail/horseweed, field pennycress and henbit in no-till fields. Fall applications can also control earlyemerging summer annuals; however, degradation of the product between application and establishment of the crop significantly reduces the length of in-season weed control. The only advantage of fall applications for inseason weed control is eliminating a field operation in the spring, thus the benefit of this strategy should be carefully evaluated. The potential success of this approach increases as one moves north in the state due to the longer winter which reduces the time the herbicide is vulnerable to degradation. Early preplant applications (EPP) are made several weeks ahead of planting. This strategy increases the probability that the herbicide will be moved into the soil profile by rainfall before annual weeds begin to germinate compared to at planting applications. EPP generally require higher use rates than applications made at planting to provide equivalent periods of weed control. The difference in length of control between EPP and at planting applications is magnified when planting is delayed due to wet springs. A factor to consider with EPP treatments is the impact of final seedbed preparation and planting operations on the distribution of the herbicide within the soil profile. In systems where tillage is used to prepare the seedbed after the EPP has been made, improper tillage can either leave a streaky pattern of herbicide across the field or place the herbicide too deep within the profile, effectively diluting the chemical to non-effective concentrations. Planters that move significant amounts of soil from the row can displace the herbicide, leaving an unprotected strip within the row for weeds to establish. Preemergence applications made within a few days of planting provide the greatest likelihood of full-season weed control since the herbicide is placed in the field when it is needed. It is important to provide the crop with an even start with weeds, so any emerged weeds present at planting should be killed as close to planting as possible. The primary disadvantage of at planting applications is the need for timely rainfall to activate the herbicide. Assuming soils have reached temperatures favorable for germination at planting, failure to receive activating rainfall within three to five days of application may allow early germinating weeds to escape control. In no-till where burndown herbicides are used to kill established weeds rather than tillage, the window for rainfall is narrower due to the lack of soil disturbance to kill weeds that have initiated germination but have yet to emerge. Fields should be monitored closely when limited rainfall following application increases the likelihood of escapes. Rotary hoeing can be effective at reducing control failures due to lack of timely rain, but this tillage operation needs to be completed before weeds have emerged to be most effective. Iowa State University Extension and Outreach Weed Science 9

10 Many preemergence herbicides allow application after the crop has emerged, but some products prohibit this use due to foliar activity that can result in crop injury. Preemergence herbicides applied after planting extend the period of weed control later into the growing season than applications made earlier in the season. This extended control can be valuable for weeds with prolonged emergence periods such as waterhemp. Since these applications are often made during peak weed emergence periods, lack of activating rain soon after application can result in inconsistent performance. Crop injury In the era of Roundup Ready crops, farmers have become accustomed to herbicides that have a large margin of crop safety. Although most preemergence herbicides used today have less risk of significant injury than some that were used in the past, there is the potential for adverse crop response with many products. The factors that influence this risk are: 1) crop tolerance to the herbicide, 2) soil characteristics, and 3) environmental conditions. Each herbicide has a specific margin of safety on an individual crop, and ratings of crop tolerance to herbicides are provided by most land grant universities. Certain herbicide formulations include a safener that enhances tolerance. Safened products include Dual II Magnum, Harness, Balance Flexx, etc. There can be varietal differences within a crop, but these differences are usually relatively small compared to the other factors that determine crop response. The adsorptive capacity of a soil often influences crop response due to increased availability of the herbicide in soils with a low affinity for herbicides. Some products recommend not using the product on soils with low adsorptive capacity due to injury risk. The rate structure specified on herbicide labels is designed to avoid overwhelming the crops tolerance mechanisms, but variability of soil types within a field often makes it difficult to adjust rates accurately according to soil type. Environmental conditions influence both the availability of the herbicide and a crop s tolerance mechanisms. Excess soil moisture increases the availability of the herbicide by increasing the amount of herbicide in soil solution. Herbicide selectivity normally is achieved by differential metabolism: the crop is able to metabolize the herbicide more rapidly than weeds, thus the weed dies due to toxic concentrations accumulating within the plant, whereas the crop detoxifies the herbicide before it is harmed by the herbicide. When a crop is under stress due to weather, disease, exposure to other chemicals, or other factors its ability to metabolize the herbicide may be compromised. A reduced rate of metabolism can allow the herbicide to reach toxic concentrations within the plant. Determining the impact of injury on yield potential is difficult. Since preemergence herbicides cause injury early in the season, plants often have time to recover from the setback and yields will not be reduced unless significant stand loss occurs. Summary Preemergence herbicides will play an increasingly important role in weed management due to the evolution of glyphosate resistant weeds, either to reduce the risk of these weeds invading fields or to manage resistant populations. The keys to successful preemergence weed control are: 1) select a product that is effective against the weeds present in the field, 2) select a rate that is appropriate for the target weeds and soil properties of the field, and 3) apply the herbicide uniformly across the field and at an appropriate time. The availability of the herbicide within the soil profile determines the effectiveness of weed control and the risk of crop injury. Thus, knowing the soil characteristics of the field and the adsorptive characteristics of the herbicide is critical in diagnosing problems with performance of soilapplied herbicides. Iowa State University Extension and Outreach Weed Science 10

11 Weed response to selected herbicides E = excellent G = good F = fair P = poor Grasses Broadleaves Perennials Crop tolerance Crabgrass Fall panicum Foxtail Woolly cupgrass Amaranthus spp. 2, 4, 5,6 Black nightshade Common ragweed Giant Lambsquarter Smartweed Velvetleaf Canada thistle Quackgrass Yellow nutsedge Preplant/Preemergence Atrazine E F P F P P E G G E F-G E E G G P F F Axiom, Breakfree, Dual II Magnum, Frontier, Outlook, etc E E E E F F F-G G P P P P P P P P P G Balance Fexx 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 Callisto E P P P P P G-E G-E F-G F-G F E F-G G-E E P P P Degree, Harness, Surpass, Topnotch, etc E E E E F-G F-G G G P P P P-F P-F P P P P G Hornet WDG G P P P P P G-E F-G G G G G G-E G-E G P P P Linex/Lorox G P P P P P G-E F F G P G-E G-E F F P P P Pendimax, Prowl, etc F-G G-E G-E G-E G G G P P P P G-E F P P-F P P P Pursuit 3 E 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 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) G P P P P P G-E G-E G G G G-E G G-E G-E P P G Postemergence Accent, Steadfast G-E P G G-E G-E E G P F P P P G P F F G F Aim G P P P P P F-G G P P F G P P E P P P Atrazine G F P F P P E E E E G E E E E F* F G Basagran E P P P P P P P E E F P E G G-E G* P G* Basis, Basis Blend F F F-G G F G G P F F P G-E G-E G-E G P G P Banvel, Clarity, etc F-G P P P P P G-E G E G-E E G E G F-G G* P P Beacon G P F-G P-F P E E G G G E P G G F-G F-G* G F Buctril G P P P P P G G-E E E G G-E G-E E G P P P Callisto G-E P P P P P E E G-E F G G E G-E E P P P Distinct F-G P F F P F G-E G E G-E G G E G G G* P P Equip F-G P G G-E F-G E G E E E G G E E G-E G* G P Glyphosate (Roundup, Touchdown) 3 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 G P P P P P G-E F E E G-E F G-E E G-E G P P Ignite 3 E E G G-E E E G E E E G G E E E F-G G P Impact G-E F-G F G F F G-E G-E G-E G G G G E E P P P Lightning 3 G-E G G E G E F-G E E G F-G G-E E E E G F F NorthStar G P F-G F P E F-G G E E E G E E G F-G G F Option G P G G-E F-G E G E F F P P P G G P G P Permit, Halomax, etc G P P P P P E P G-E G-E G P G-E E E P P G Pursuit 3 G-E G G F-G F E F-G E G-E G F G E G G-E F P P Resolve F F F-G G F G G P F F P G-E G P F-G F G F Resource G-E P P P P P G P F F-G P F P P E P P P Yukon F-G P P P P P G G G-E G-E G G G-E E E P P G 2,4-D F P P P P P G F E G G-E G F G G F* P P 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. Cocklebur 2 ragweed 2, 4 Sunflower 2 Corn Herbicide Effectiveness Ratings 1 Shattercane 2 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 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. *Degree of perennial weed control is often a result of repeated application. Iowa State University Extension Weed Science 11

12 Soybean Herbicide Effectiveness Ratings 1 Weed response to selected herbicides E = excellent G = good F = fair P = poor Grasses Broadleaves Perennials Crop tolerance Crabgrass Fall panicum Foxtail Woolly cupgrass Shattercane 2 Amaranthus spp. Black nightshade Cocklebur 2 Common ragweed Giant ragweed 2, 4 Lambsquarter Smartweed Sunflower 2 Velvetleaf Canada thistle Quackgrass Yellow nutsedge 2, 4, 5,6 Preplant/Preemergence Authority/Spartan G P P P P P E E F F F G-E F P F-G P P F-G Command E G-E G-E E F F P F F G P G-E G F E P P P Dual II Magnum, INTO, Frontier, etc E E E E F F F-G G P P P P P P P P P P FirstRate/Amplify 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 F P P P P P G-E F F G P G-E G-E F F P P P Sencor, TriCor, etc F-G P P P-F P P E F F E P E E F-G G-E P P P-F Pendimax, Prowl, Sonalan, Treflan, etc G-E E E E E G-E G P P P P G F P P P P P Pursuit 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 E P P P P P E F F F P F-G G-E F E P P P Valor SX F-G P-F P-F P-F P P G-E E F G F E F P F P P P Postemergence Assure II, Fusilade DX, Fusion, Poast Plus, Select, etc. E E E E E E P P P P P P P P P P G-E* P Basagran E P P P P P P-F P-F E E F P E G G-E G* P G* Blazer F-G P P F P F E G F G F F E F F F P P Classic G P P P P P E P E G-E F P G-E E G-E F P G-E Cobra/Phoenix F-G F P P P P E G G-E E F-G F G G F F P P FirstRate/Amplify G P P P P P P P G-E E E P G E G P P P Glyphosate (Roundup, Touchdown) 3 E E G-E E E E G-E F-G E E G-E G E E G G G-E F Harmony GT F P P P P P E P F F P G-E G-E G-E G P P P Ignite E E G G-E E E G E E E G G E E E F-G G F Pursuit G G G F-G F E F-G E G-E G F P-F E G G-E F P P Raptor 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 F-G P P P P P E F-G F G G F G-E F F P-F P P Resource 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. *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. Iowa State University Extension Weed Science 12

13 Grazing and haying restrictions for herbicides used in grass pastures Herbicide A.I. Rate/A Grazing Beef and Non-Lactating Animals Lactating Dairy Animals Hay harvest Removal before slaughter Grazing Hay harvest Ally oz 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 0 7 days 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 Growing season Escort XP metsulfuron methyl Up to 1.7 oz oz NA 3 days NA NA 3 days ForeFront HL aminopyralid + 2,4-D pt 0 7 days days Grazon P&D picloram + 2,4-D 3-4 pt 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 Growing season Growing season Remedy Ultra triclopyr 1-2 qt 0 14 days 3 days Growing season Growing season Surmount picloram + fluroxypyr pts Tordon 22K picloram < 2 pts > 2 pts Weedmaster dicamba + 2,4-D 1-4 pts 0 37 days 30 days 7 days 37 days 2,4-D (many tradenames) Uses may vary among products 2,4-D 2-4 pt 4 lb/g 0 30 days 3 days 7 days 30 days Iowa State University Extension and Outreach Weed Science 13

14 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 Components (a.i./gal or % a.i.) If you apply (per acre) You have applied (a.i.) An equivalent tank mix of (product) Basis 75DF 50% rimsulfuron 0.33 oz oz rimsulfuron oz rimsulfuron 25% thifensulfuron oz thifensulfuron 0.33 oz Pinnacle 25DF Basis Blend 20% rimsulfuron oz oz rimsulfuron rimsulfuron 10% thifensulfuron oz thifensulfuron 0.33 Pinnacle 25 DF Bicep II MAG. 5.5L, Cinch ATZ Bicep Lite II MAG, Cinch ATZ Lite 2.4 lb S-metolachlor 2.1 qt 1.26 lb S-metolachlor 21 oz Dual II MAGNUM 3.1 lb atrazine 1.63 lb atrazine 52 oz Aatrex 4L 3.33 lb S-metolachlor 1.5 qt 1.24 lb S-metolachlor 21 oz Dual II MAGNUM 2.67 lb atrazine 1.00 lb atrazine 32 oz atrazine 4L Breakfree ATZ 5.25L 3.0 lb acetochlor 2.7 qt 2.0 lb acetochlor 2.5 pt Breakfree 6.4E 2.25 lb atrazine 1.5 lb atrazine 3.0 pt atrazine 4L Breakfree ATZ Lite 5.5L 4.0 lb acetochlor 2.0 qt 2.0 lb acetochlor 2.5 pt Breakfree 6.4E 1.5 lb atrazine 0.75 lb atrazine 1.5 pt atrazine 4L Buctril + Atrazine 1.0 lb bromoxynil 2.0 pt 0.25 lb bromoxynil 1 pt bromoxynil 2E 2.0 lb atrazine 0.50 lb atrazine 1 pt atrazine 4L Bullet 4ME 2.5 lb alachlor 4.0 qt 2.5 lb alachlor 2.5 qt Micro-Tech 4ME 1.5 lb atrazine 1.5 lb atrazine 1.5 qt atrazine 4L Callisto Xtra 0.5 lb mesotrione 24 fl oz 0.09 lb mesotrione 3.0 oz Callisto 3.2 lb atrazine 0.6 lb atrazine 1.2 pt Aatrex 4L Capreno 0.57 thiencarbazone methyl 3.0 oz 0.01 lb thiencarbazone methyl lb tembotrione lb tembotrione - Cinch ATZ 2.4 lb S-metolachlor 2.1 qt 1.26 lb S-metolachlor 21 oz Dual II Magnum 2.67 lb atrazine 1.63 lb atrazine 3.25 pt atrazine 4L 1.88 isoxaflutole lb isoxaflutole 2.6 oz Balance Degree Xtra 2.7 lb acetochlor 3 qt 2 lb acetochlor 36.6 oz Harness 7E 1.34 lb atrazine 1 lb atrazine 1 qt atrazine 4L Distinct 70WDG 21.4 % diflufenzopyr 6 oz 1.3 oz diflufenzopyr 1.3 oz diflufenzopyr 55.0% dicamba 3.3 oz dicamba 6 oz Banvel Iowa State University Extension Weed Science 14