Biennial wormwood (Artemisia biennis) postemergence control in soybean

Biennial wormwood has become an important weed for soybean producers in the northern Great Plains states of the United States. Research was conducted at Fargo, Leonard, and Wyndmere, ND, in 2000 and 2001 to evaluate postemergence control of biennial wormwood in soybean. Biennial wormwood densities at Fargo, Leonard, and Wyndmere were 290, 290, and 130 plants/m(2) in 2000 and 22, 670, and 88 plants/m(2) in 2001, respectively. Across years, biennial wormwood control with postemergence herbicides 4 wk after treatment was 81 to 97% at Fargo, 5 to 89% at Leonard, and 2 to 95% at Wyndmere. Biennial wormwood control was greater at Fargo than at the other locations probably because of smaller seedling size at the time of treatment. Bentazon split applied at the rate of 560 g ai/ha followed by 560 g/ha approximately 12 d after the first treatment generally provided the least amount of injury to soybean, the greatest control of biennial wormwood, and the fewest escaped plants. Lactofen at 220 g ai/ha or split applied at 110 g/ha followed by 110 g/ha approximately 12 d after the first treatment provided 84% or greater biennial wormwood control at Fargo but less than 40% control at Leonard and Wyndmere. The 4.5:1 bentazon plus acifluorfen premix full-rate and split treatments provided slightly better control than the 2:1 bentazon plus acifluorfen premix full-rate and split treatments. High levels of biennial wormwood control can be achieved with a better understanding of seedling emergence patterns and by targeting postemergence herbicide applications to seedlings less than 5 cm tall.