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Biennial Wormwood Control in Dry Bean with Tillage and Herbicides
April 15, 2003

George Kegode
Department of Plant Sciences
North Dakota State University

Biennial wormwood has become an established weed of dry bean in the northern Great Plains states of North Dakota, South Dakota, and Minnesota. Biennial wormwood first became a significant weed problem in dry bean cropping systems in the early 1990s but was routinely misidentified as common ragweed. Consequently, herbicides that were applied to treat these infestations did not adequately control biennial wormwood. A study was conducted in 2002 to evaluate the influence of herbicide application method and timing in combination with inter-row cultivation on control of biennial wormwood in navy bean. Initially, biennial wormwood was the target weed, but due to a low and inconsistent population of this species, redroot pigweed and common cocklebur, which were more abundant, were evaluated instead.

Navy bean was planted in 30-inch rows on June 17, 2002. The original goal was to seed navy bean about June 1, which is a date that is favorable for biennial wormwood emergence, but wet soil conditions from excessive rainfall delayed seeding until July 17. Spartan was applied preemergence (PRE) both broadcast and banded while Basagran plus methylated seed oil (Scoil) was applied broadcast and banded as early (EPOST) and late postemergence (LPOST) treatments. All broadcast treatments were applied using Spraying Systems Co. XR 8002 nozzles that delivered 17 gpa at 38 psi, and all banded treatments were applied using TwinJet 60-4002E nozzles also delivering 17 gpa in a 10-inch-wide band over each of the center two rows of the plot. PRE treatments were applied on June 18. EPOST treatments were applied July 16 when navy bean was 6-inch tall and 3 trifoliolate leaves; redroot pigweed was 1- to 6-inch tall and 4- to 10-leaves and common cocklebur was 3- to 8-inch tall. LPOST treatments were applied July 23 to 6- to 8-inch-tall navy bean with 4 to 5 trifoliolate leaves; redroot pigweed was 3- to 25-inch tall, and the largest plants were flowering; and common cocklebur was 12-inch tall. On July 19 the center two rows of the first seven treatments were cultivated and the entire experiment was sprayed with Select at 16 fl oz plus Scoil at 1 qt/A to control grass weeds. Evaluations of phytotoxicity and efficacy were made Aug. 2 and 23 and the center two rows of each plot were harvested Oct. 23 with a small plot combine. Plots measured 10 by 25 ft, and the experiment was a randomized complete block design with 4 replicates.

Weed control was highly variable. Spartan applied preemergence provided 97% or greater control of redroot pigweed when broadcast-applied but caused on average 23% injury to navy bean. Redroot pigweed control with Basagran was 70% or less, regardless of application method or inclusion of inter-row cultivation. Common cocklebur control by Basagran was 90% by Basagran when broadcast-applied LPOST with inter-row cultivation, but was 68% or less with all other Basagran treatments, with or without inter-row cultivation.

Navy bean yields were variable and largely not significantly different when treatments were compared. Navy bean treated with Basagran generally provided the highest dry bean yields, whereas yields from Spartan treatments were lower probably due to injury to navy bean.


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