2021 Herbicide Guide for Iowa Corn and Soybean Production Volunteer Corn in Storm-damaged Fields The 2020 derecho damaged millions with soybean, applications should of acres of corn in Iowa, which creates be targeted for when the corn is less Contents • Volunteer Corn in Storm-damaged the potential for large populations of than 12 inches in height, especially Fields .....................................................1 volunteer corn in 2021. While not when high densities are present. • Dicamba Update..................................2 persistent, it is highly competitive Herbicide rates should be based on • Designing Resilient Weed with both corn and soybean. Weather size and density of the volunteer Management Programs .....................3 conditions last fall resulted in corn. Use the higher range of rates • Corn Herbicide Effectiveness significant germination of corn seed when treating large plants or high Ratings ..................................................7 in many fields prior to soils cooling densities. Tank-mixing a Group 1 • Soybean Herbicide Effectiveness down; however, most fields will still herbicide with broadleaf herbicides can Ratings ..................................................8 have sufficient corn seed remaining to result in reduced grass control due to • Grazing and Haying Restrictions ......9 produce problems this season. antagonism between products. Most • Herbicide Package Mixes ...............10 • Herbicide Sites of Action .................18 Group 1 labels provide instructions An important step in developing • Herbicide Site of Action and Typical on minimizing this risk, whether by management plans for 2021 is Injury Symptoms................................21 increasing rates or using sequential identifying herbicide resistant traits applications. If an Enlist corn hybrid in the hybrids planted in 2020. was planted in 2020, use either Glyphosate (Roundup) and glufosinate clethodim or sethoxydim since Enlist options, placing the 2021 crop at risk if (Liberty) are two effective options to corn provides resistance to fluazifop high populations of volunteer corn are manage volunteer corn, but many corn and quizalifop. present. hybrids contain resistance traits for one or both of these herbicides. Based on the trait package of the prior One small caveat. While availability is year’s corn, glyphosate or glufosinate likely limited for 2021, another option As with other weeds, controlling may provide an alternative to Group 1 would be to plant an Enlist corn hybrid emerged volunteer corn at planting herbicides. Since glufosinate is a contact since the Enlist trait package provides to ensure a weed-free start is critical. herbicide, two applications generally resistance to 2,4-D, glyphosate and Delaying planting can reduce are required for complete volunteer certain Group 1 herbicides (only ‘fops’). populations by allowing some of corn control. Assure II (quizalofop) is registered for the corn seed to germinate prior to use on Enlist hybrids, and provides planting. No preemergence herbicide Management in corn effective control of volunteer corn. will provide effective control of Caution is advised when planting Other ‘fop’ herbicides are not registered volunteer corn in either corn or corn in fields with significant storm for use on these hybrids. soybean. damage due to limited control options for volunteer corn. Again, the traits Management in soybean of last year’s corn dictate available Rotating fields to soybean or some options; determine whether glyphosate This publication is prepared by Bob other broadleaf crop provides the best or glufosinate are a viable treatment Hartzler, professor in agronomy and options for managing volunteer corn. due to the absence of one, or both, of extension weed scientist at Iowa Several Group 1 herbicides (ACCase these traits. If last year’s corn had both State University, and Prashant Jha, inhibitors) are available that provide glyphosate and glufosinate resistance, associate professor in agronomy and effective control in soybean. Due to there are no postemergence control extension weed specialist at Iowa the competitiveness of volunteer corn State University. WC94 Revised February 2021 Dicamba Update Dicamba had a tumultuous 2020 with What has changed What remains the same a court ruling in June that immediately • The requirement for use of a • Apply only one hour aft.er sunrise vacated the labels of three products volatility reducing agent (VRA) through two hours before sunset. used in dicamba-resistant soybean or pH buffering agent is probably • Apply when wind speeds are (XtendiMax w/VGT; FeXapan w/VGT; the most significant change on the between 3 and 10 MPH; do Engenia). This ruling was overridden labels. The VRAs are pH buffers not apply during temperature by state regulatory agencies and the that reduce conversion of dicamba inversions. EPA, allowing use of the products to the volatile form. Only VRAs during 2020, but not without described on the product’s website • Do not apply if sensitive crops are significant confusion following the can be used. in an adjacent downwind field. ruling. • Dicamba cannot be applied • Maximum boom height of 24 Late in 2020 EPA granted new five- in soybean after June 30. The inches and ground speed of year labels for Xtendimax with VGT, XtendiMax label also prohibits 15 mph. Tavium plus VGT, and Engenia. This application after the R1 soybean • Only use nozzle/pressure article will provide a short overview of stage of soybean, whereas Tavium combinations approved on product major changes in the labels, but users restricts applications aft.er the V4 website. must always reference product labels soybean stage. The date restriction and websites for specific language and will limit applications later in the • Only mix with products that are requirements. season when temperatures favoring approved on the product website. volatility are more likely. • Dicamba-specific training is still • The 110-foot downwind buffer required for applicators, although within fields for spraying fields there are slight changes in this adjacent to sensitive areas has been requirement. increased to 240 ft. The labels still Both dicamba and 2,4-D pose a greater prohibit application when adjacent risk of off-target injury than other downwind areas contain sensitive herbicides due to their ability to induce crops (including non-dicamba- plant responses at lower percentages of resistant soybean), fruits and application rates. It is the applicator’s vegetables, and other susceptible responsibility to properly set up plants. The labels differ in how sprayer and avoid applications when residential areas (i.e. farmsteads) conditions increase the likelihood of are classified; carefully read labels off-target movement and damage (e.g. or consult with manufacturers to susceptible plants in adjacent areas, clarify restrictions. high winds, temperature inversions, • The downwind buffer for spraying high temperatures). in counties with endangered species has increased from 110 ft. to 310 ft.; the omnidirectional buffer remains at 57 ft. The counties within Iowa with restrictions have also changed for 2021. Visit the EPA website Bulletins Live! (https:// www.epa.gov/endangered-species/ bulletins-live-two-view-bulletins) for current listings. 2 2021 Herbicide Guide for Iowa Corn and Soybean Production Designing Resilient Weed Management Programs Herbicide-resistant weeds continue to It is important to recognize that • Approaches: This type of program spread rapidly across Iowa, threatening relying solely on herbicides will fail at typically relies on a combination the current cropping system that is winning the struggle against herbicide of herbicides to provide broad- based on herbicidal weed control. resistance. This article provides a brief spectrum weed control. The Weeds with multiple resistances pose overview of the types of herbicide herbicides must be persistent the biggest threat. More than 90% of strategies used in corn and soybean and be used at full-rates in order waterhemp populations collected in production, including the description to extend control late into the Iowa fields in 2019 are resistant to of their strengths and weaknesses. The season. Split applications of the three herbicide groups. Giant ragweed article concludes with a description preemergence program often and horseweed/marestail are the other of alternative strategies that provide are used in conservation tillage major resistant weeds in the state, but opportunities to enhance weed control systems. Typically, 50-60% of the other weeds will continue to evolve and protect the value of herbicides. product is applied several weeks resistance to herbicides as long as we ahead of planting to control weeds Total preemergence are heavily dependent on chemical that emerge prior to planting and program weed control. reduce the need for timely rainfall, • Advantages and the remainder is applied at or For most growers, adjusting herbicide - Offers the opportunity for a one- shortly after planting. This strategy programs will be the simplest and pass program. Not appropriate for can extend the activity of the most important strategy for managing most fields. herbicide later into the season than herbicide resistance. There are • Disadvantages if it all was applied early. numerous approaches that can provide - Inability of most herbicides to effective control while reducing the risk Total postemergence sustain effective control until the of selecting resistant weed biotypes. program crop canopy