It’s another post-emergence application (post) soybean season, and we have largely been receiving questions similar to the past several years. These include how to manage giant ragweed, pokeweed and a few other perennials, vines and marestail (good luck there). It is evident however that many fields were subject to a more comprehensive approach to marestail management, starting with herbicides applied last fall, and marestail problems are generally less frequent. But this is possibly offset by the lack of rain to effectively activate residual herbicides in areas, which results in higher and more varied weed populations to manage with post herbicides. Some things to consider relative to post treatments in soybeans include options for control of marestail, ragweed, other weeds, as well as application timing and plant size.
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With regard to post control of marestail, hiring a crew of people with hoes is probably the most effective option. Or at least cut the top of the plants off so that seed production is prevented. We are hearing mostly about difficulty controlling plants that overwintered in fields not treated last fall (there’s a lesson here somewhere). Plants that have survived one herbicide treatment already are not likely to be controlled by anything. In fields where the population is still sensitive to ALS inhibitors, a high rate of Classic or FirstRate may provide some suppression or control of small plants. Flexstar and other PPO inhibitors have essentially no activity on marestail, and low rates of 2,4-DB are not much better.