Waterhemp One of the most common Midwest weed species, waterhemp is a member of the pigweed (amaranth) family, with similarities to Palmer amaranth. It’s hardy, prolifi c and germinates throughout the growing season. Resistance history: Waterhemp populations have been documented as resistant to ALS, PPO and HPPD inhibitors; triazines; and glyphosate. Resistance has been documented in 14 states, with some populations developing resistance to two or more herbicide families. Control challenges: Preemergence herbicides with residual control are most eff ective in controlling early germinating waterhemp plants. Those that germinate later in spring are more diffi cult to control and can survive to produce seed. 2 3 4 5 14 MAY/JUNE 2014 Marestail (horseweed) Marestail can germinate through nine months of the year, but typically pops up in fall in the North and in spring in the South. Fall-germinating marestail will overwinter in the rosette stage and bolt in spring. Each plant spews out up to 200,000 tiny seeds, which are spread long distances by wind. Resistance history: Glyphosate-resistant marestail was fi rst documented in Delaware in 2000 and has been confi rmed in 21 states. It also has developed resistance to paraquat, atrazine, ALS inhibitors and diuron. Control challenges: The best control options depend on germination timing. Spring preplant tillage, early spring burndown and preplant herbicide applications are eff ective in controlling spring-germinating marestail. A fall burndown treatment with residual control helps manage fall-germinating marestail. Plants that survive burndown are much more diffi cult to control, especially in soybeans. Kochia Primarily a problem in the western Corn Belt, kochia is considered a summer annual weed, but seedlings can emerge in early spring once soil temperatures reach 40 degrees F. When uncontrolled, kochia plants can grow into cone-shaped bushes that reach 6 feet in height and are poisonous to cattle and sheep. Seeds are often spread when mature, dry plants detach from the root base and are blown along like tumbleweeds. Resistance history: Kochia populations were documented as resistant to triazine herbicides in 1976 and ALS inhibitors in 1987. Control challenges: Preplant tillage is eff ective in controlling kochia because the plant has a shallow taproot. Where tillage is not an option, spring burndown using multiple modes of action is most eff ective. Many postemergence applied herbicides suppress, but don’t control, kochia. Giant ragweed It’s called giant because this ragweed can reach heights of 17 feet, although height depends on what is competing with the plant for sunlight. Typically emerging from March through May, but as late as July, giant ragweed blooms from July through October. Two plants per 110 square feet can reduce corn yields by 13 percent and a plant in a soybean fi eld can produce more than 5,000 crown-shaped seeds. While found throughout the Midwest and East, giant ragweed is most problematic in the eastern Corn Belt. Resistance history: Giant ragweed resistance to atrazine was found in 1990, resistance to ALS inhibitors was documented in 1996 and glyphosate resistance was found in 2004. Control challenges: Control of light populations is possible with preemergence herbicide applications, but moderate to high populations require a combination of pre-and postemergence applications. Fields with ALS-resistant giant ragweed should be rotated to corn and wheat when possible. Get Help with Herbicide Plans Weed control and resistance management have become complicated and very technical topics. A new website provides answers to common questions about weed species, herbicide classifi cations, modes of action, application techniques and herbicide resistance. TakeActiononWeeds.com was developed by the United Soybean Board in partnership with several universities, commodity organizations and crop protection companies. Growers can use website tools to create a weed management plan to reduce the risk of developing herbicide-resistant weed populations. CHSINC.COM