Weed management following prevented planting

There are a few weed management situations to consider depending on how prevented planting fields were managed in 2020.

Given the current dry conditions across North Dakota, it seems likely that last year’s prevented planting acres will return to crop production this year. There are a few weed management situations to consider depending on how prevented planting fields were managed in 2020:

  • If prevented planting fields were not even touched in 2020, then one could expect a lot of weed species went to seed. There could be many different weeds species that produced a lot of seed, so these fields should be managed with a broad spectrum herbicide program in 2021 in anticipation of many grass and broadleaf weeds. Furthermore, if these fields will be no-till in 2021, then there will also be many winter annual weeds that need to be controlled with burndown applications prior to planting this year’s cash crop. A higher rate of glyphosate (at least 1 pound acid equivalent per acre) will take care of many winter annual weeds. However glyphosate-resistant weeds like marestail/horseweed will need additional or alternate products for control prior to planting.
  • If prevented planting fields were managed with a few applications of glyphosate throughout the summer, then the weed species that would have produced the most seed would be glyphosate-resistant weeds like waterhemp or marestail. These fields should have management plans tailored to those glyphosate-resistant weeds that survived and produced abundant amounts of seed in the absence of competition from other plants. For marestail, this means a lot of overwintered plants will need to be managed with either tillage or an aggressive burndown program. For waterhemp, management should start with an effective preemergence herbicide with multiple effective sites of action on waterhemp. Timely postemergence applications will be required to control the plants that emerge after the soil residual herbicide breaks.
  • If prevented planting fields were managed with a few tillage passes throughout the season, then there was likely minimal weed seed produced. One exception could be when the first tillage pass occurred after July 1. Waterhemp and early emerging weeds like common lambsquarters and kochia had already produced some seed by July 1 last year. Overall these fields will be in good shape for weed control in 2021, but there may be some additional seeds of those weeds to battle this year.
  • If prevented planting fields were managed intensely for weed control in 2020, then I would not expect any additional weed challenges compared to previous years. One thing to consider in more intensely managed fields is the presence of a living cover crop this spring. If there are cover crops like cereal rye in these fields, then termination timing may take special consideration this year given the dry conditions. Early termination of these cover crops may be desired to conserve moisture for the cash crop. Herbicides for cover crop termination will work best once plants break dormancy and are actively growing. This typically takes a few warm days in a row, with overnight lows remaining above freezing for best control. If we do not get several warm days in a row, then make sure to use full rates of glyphosate (at least 1 pound acid equivalent per acre) and do not skimp on ammonium sulfate in order to give glyphosate the best chance to control covers.

(Joe Ikley is an NDSU Extension specialist and assistant professor of weed control. He can be reached at .)

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