It appears that the spring is developing such that there will be the “traditional” mad rush to get the crop in and worry about weed management after the fact.

This is not good and I would again recommend strongly that you consider the difference between killing weeds and managing weeds – the difference is $$$ at the end of the year. Please give serious consideration to applying a preemergence herbicide(s) that will provide early season and residual control of the weeds in your fields that germinate first and in heaviest population densities. I recommend that these treatments be applied prior to planting, but if this is not possible, apply the herbicide immediately after planting. Observe the need for a burndown as the winter annuals are there and in very high populations – they may be small, but they are a problem. Do not think you can eliminate the burndown herbicide treatments in preemergence applications unless tillage has been conducted very close to the planting and spraying operations.

One strategy that has unfortunately become more popular is the delayed application of a residual herbicide(s), typically with glyphosate, after the emergence of weeds and crop. While this can be an effective weed management tactic, timing is critically important, just like with any other post herbicide treatment. If you are too late (not to kill the weeds, but rather to protect crop yield), you lose $$$. Also, there are concerns that these treatments are being marketed as a 1-pass treatment that will provide season-long weed control. This is unlikely and plan on a timely second application of something postemergence.

Last – what about glyphosate rate and the potential for selection of glyphosate resistant populations? The thought is that low rates de facto select for resistant biotypes. This is somewhat correct and somewhat incorrect. Without going into the details about quantitative resistance, polygenic versus single gene resistance, and specific mechanisms of resistance, suffice to say it is best to avoid repeated applications of low glyphosate rates, in part because the remote potential for the selection of resistant weed biotypes but mostly because you may not control the weeds. Use the rate of glyphosate that reduces the risk of poor weed control. Also recognize that low glyphosate rate and resistance selection is not likely the consequence of a onetime application strategy. A repeated application of low glyphosate rates has the potential for the selection of weeds with quantitative glyphosate resistance. However, one application of glyphosate, regardless of rate, has little potential for the selection of resistant weed biotypes.

SOURCE: ISU’s Mike Owen

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