Weed community response to crop rotations in western South Dakota

Producers in the semiarid Great Plains are exploring alternative crop rotations, with the goal of replacing winter wheat-fallow. In 1993, a study was established to compare performance of eight rotations comprised of various combinations with winter wheat (W), spring wheat (SW), dry pea (Pea), safflower (Saf), corn (C), sunflower (Sun), proso millet (M), or fallow (F). After 8 years, we characterized weed communities by recording seedling emergence in each rotation. Seventeen species were observed, with downy brome, kochia, horseweed, and stinkgrass comprising 87% of the community. Rotations with the least number of weed seedlings were W-F and SW-W-C-Sun; in comparison, weed density was six-fold higher in W-M. Density of downy brome and kochia was highest in W-M compared with other rotations, whereas stinkgrass and green foxtail were prominent in proso millet of the W-M and W-C-M rotations. Horseweed established readily in safflower and dry pea. In the semiarid Great Plains, designing rotations in a cycle of four that includes cool- and warm-season crops can be a key component of integrated weed management.