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Home , Dicamba, Metribuzin, Saflufenacil

Management of -Resistant Horseweed (Marestail) in No-Till

Horseweed Biology should therefore not expect to obtain effective control with postemergent , including combinations • Horseweed (marestail) has two primary periods of plus Classic, Synchrony® or FirstRate. of emergence – from late March through June and from late summer through late fall. • Horseweed plants remain in the rosette stage through LibertyLink Soybeans — The Most Effective Strategy late March in the southern states to late April in northern ® states, followed by stem elongation (bolting) and rapid • The LibertyLink system is the most effective growth to an eventual height of 3 to 6 feet. Plants that tool for management of herbicide-resistant horseweed, emerge the previous fall will bolt earlier than especially in fields with high horseweed populations. spring-emerging plants. • Use burndown and residual herbicides as outlined on the ® • Horseweed competes with soybeans throughout the next page. Apply Liberty after emergence (29 to 36 oz./A.) growing season and reduces crop yield. Horseweed before horseweed plants exceed 6 inches in height. Follow matures in late summer or early fall, and produces with a second application of Liberty as needed. up to 200,000 seeds per plant, which are readily dispersed by wind. Management Steps

1. Use fall or early spring herbicide treatments in fields Herbicide Activity and Resistance where horseweed seedlings are observed and especially in Horseweed in fields with a history of horseweed-control problems. The primary goal of a fall or early spring treatment is • Herbicide programs must include a spring burndown control of emerged plants. It should not be considered to ensure that the field is free of horseweed at soybean a substitute for a preplant or pre-emergent herbicide planting, and soil-applied residual pre-emergent herbi- treatment later in spring. An application of burndown cides to control horseweed for another six to eight weeks. and residual herbicides is still required closer to planting Failure to follow these guidelines can result in poor control in fields that were treated with burndown herbicides in and reduced crop yield. A recent Ohio State University the fall or early spring. For fall applications, we suggest horseweed study with various herbicide scenarios resulted using 2,4-D as the base herbicide to control horseweed in the following soybean yields: and combining it with one or more of the following — 51 bu./A. where the burndown treatment failed to ensure control of other winter weeds: to control emerged plants — Glyphosate — 57 bu./A. where the burndown treatment was effective, — (can use premix, such as Brash® or Weedmaster®) but there was no residual herbicide — Basis® — 65 bu./A. where the burndown was effective — A low rate of Canopy®/Cloak™ EX or DF and residual herbicides were used — Autumn™ Super, or • Horseweed is most easily controlled when in the seedling, or rosette stage, and spring burndown herbicides should For early spring applications, we suggest a similar approach be applied before stem elongation. using 2,4-D or dicamba as the base, and adding glyphosate and/or a reduced-rate application of a residual herbicide. • Horseweed populations with evolved resistance to Apply the remainder of the residual herbicide closer glyphosate or ALS-inhibiting herbicides (Group 2, such to the time of soybean planting. as Classic® and FirstRate®) are widespread, and many populations are resistant to both sites of action. Farmers Management of Herbicide-Resistant Horseweed (Marestail) in No-Till Soybeans

2. Apply effective burndown herbicides in spring. Do not a waiting interval of 14 days until soybean planting plant into existing stands of horseweed. Start weed-free is required for rates of 8 oz./A. or less, and 28 days at planting by using one of the following preplant for rates up to 16 oz./A.” Refer to the specific product herbicide treatments, applied when horseweed plants label to confirm the interval between application are in the rosette stage. Note: Thorough tillage close and planting. to planting also effectively removes horseweed. • In horseweed populations sensitive to ALS-inhibiting — 2,4-D ester or dicamba plus glyphosate herbicides, the activity of any of the above can be (1.5 1.5 lb. a.e./A.) improved with the addition of a herbicide that contains ™ ® — 2,4-D ester or dicamba plus chlorimuron (e.g. Canopy/Cloak/Fallout , Valor XLT, ® ® (Sharpen®/Verdict™) plus glyphosate and Envive , Authority XL/MAXX) or cloransulam (Gang- ® ® methylated seed oil (MSO) ster , Sonic , Authority First). The addition of metribuzin to any burndown treatment can also improve control — 2,4-D ester plus Gramoxone (3 to 4 pts./A.) of emerged horseweed. plus a metribuzin-containing herbicide — Liberty (29 to 36 oz./A.) or Liberty 3. Include residual herbicides with the preplant-burndown plus a metribuzin-containing herbicide treatment. Add one of the following herbicides or — Saflufenacil (Sharpen/Verdict) plus MSO (1% v/v) herbicide combinations to the burndown herbicides plus either glyphosate or Liberty for residual control of horseweed until the soybean leaf canopy develops. • The mixture of glyphosate and 2,4-D ester or dicamba ® has become more variable for control of horseweed — Flumioxazin - Valor, Valor XLT, Envive, Enlite , ® in some fields. Plants should be in the rosette stage Fierce or Gangster at the time of application for best results. In fields —  - Authority First, Sonic, Authority XL, where this mixture has previously failed to provide Authority Broadleaf, Authority Assist, Authority Maxx effective control, use one of the other burndown or Spartan® treatments listed above. — Metribuzin – Metri™ DF, Tricor®, etc. Use rates of at • Use the highest rate of a 2,4-D-ester product that least 8 oz./A., and preferably 10 to 12 oz./A., but do not is allowed, based on the interval between application exceed recommended rate for soil type. You can add and soybean planting. For all 2,4-D-ester products, rates metribuzin to other metribuzin-containing products up to 0.5 lb. of active ingredient per acre (ai/A) must (e.g., Boundary®, Canopy/Cloak DF, Intimidator™, be applied at least seven days before planting. Rates Matador™, Authority MTZ) to bring total metribuzin between 0.5 lb. and 1 lb. ai/A. should be applied at least rate to 0.38 to 0.5 lbs. ai/A. Sensitivity to metribuzin 30 days before planting, with the exception of some varies among soybean varieties; check with your products, such as E-99, Salvo®, and Weedone® 650 seed supplier for more information. that allow 1 lb. ai/A. to be applied 15 days before — The ALS-inhibiting (Group 2) component of premix planting. Refer to the specific product label to confirm products will not contribute to control of ALS-resistant the interval between application and planting. populations, and product rates should be increased • Dicamba can be more effective than 2,4-D on marestail as necessary to maximize control from the non-ALS in the spring, but risk of injury to soybeans is greater herbicide component. Adding 4 to 6 oz./A. of metribuz- if the plantback guidelines are not followed. Labels in to flumioxazin- and sulfentrazone-based products on dicamba products, such as Clarity®, contain the can improve residual control of these populations. following statement: “Following application of dicamba and a minimum accumulation of one inch of rain,

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Technical editing for this publication was led by Mark Loux, Ph. D., The Ohio State University, in partnership with other universities in the soybean-growing regions of the United States. The United Soybean Board neither recommends nor discourages the implementation of any advice contained herein,and is not liable for the use or misuse of the information provided. Take Action is supported by BASF, , DuPont, Dow, , and Syngenta. ©2013 United Soybean Board