WEED MANAGEMENT FOR UNPLANTED ACRES

• Heavy spring rains can result in wet or flooded fields that may delay planting, prevent planting, or drown out already planted fields.
• Once the decision has been made to leave a field unplanted, then agronomic decisions need to be made involving weed management and cover crops.
• Those filing claims under prevented planting crop insurance should discuss conditions and requirements with a crop insurance professional.

Removing weeds prior to seed set can reduce increases to the weed seed bank. Also, this strategy can help minimize the waste of nutrients intended for crop growth. There are several options for weed control including one or more of the following: herbicides, tillage, cover crops, and mowing. Any of these tools should be employed prior to weeds setting seed.

There are different herbicide options depending on if a cover crop will be planted. If considering a cover crop, then a burndown and possibly an in-crop application may be options. If no cover crop will be planted, then additional herbicide options are available and annual maximum use rates have increased relevance.

Due to effectiveness and economics, a Roundup® brand glyphosate-only agricultural herbicide is a common herbicide option for unplanted acreage. Dicamba products such as XtendiMax® herbicide with VaporGrip® Technology (a Restricted Use Pesticide) and DiFlexx® herbicide are also common tank-mix partners for an additional site-of-action for control of broadleaf weed species. The addition of a residual herbicide approved for use in fallow land may also be considered, however in the absence of a crop to shade out emerging weeds, season-long control may not be achieved.

If considering a cover crop, tillage alone can be highly effective on small weeds and has no plant-back restrictions. If weeds are larger, tillage can be used in conjunction with a burndown herbicide to increase control. For most perennial weeds and weeds under stressful conditions, waiting 5 to 7 days after the herbicide application to perform tillage can help improve weed control by allowing time for translocation. Good growing conditions can reduce the time needed for herbicide translocation. If tillage has been completed, time has passed, and a burndown herbicide will be applied, note that large weeds may not have been controlled with tillage, but rather injured and may regrow. This can result in misjudging weed height due to part of the weed being buried below ground.

Using tillage for season-long weed control may be detrimental to soil health. Tillage can break up compaction in the tillage zone, but it can also create a layer of compaction underneath the tillage zone. Also, repeated trips across the field with heavy equipment may increase compaction. Leaving the field fallow and weed-free with tillage also increases the risk for wind and water erosion, as well as deterioration of organic matter as the soil is constantly being exposed to the elements. Additionally, with minimal weed growth, the risk for fallow syndrome in corn is greater the following season. Fallow syndrome is primarily characterized by phosphorus deficiency symptoms and slow early growth. 

Larger weeds can be managed with mowing versus tillage, but weeds should still be controlled prior to setting seed. Mowing can be used in conjunction with tillage or herbicides. As with tillage, waiting 5 to 7 days after the herbicide application to mow can help improve weed control by allowing time for translocation. Good growing conditions can reduce the time needed for herbicide translocation. Mowed weeds will be older and more hardened off than what the height would indicate so rates should be adjusted accordingly. The risk for erosion and fallow syndrome in corn would likely be less with mowing versus tillage. The key to mowing is making sure it is completed prior to weeds setting seed.

A cover crop can often aid in weed control, help minimize erosion, and help minimize the effects of fallow syndrome, which may be observed in corn that is planted into fields that were flooded or fallow the previous season. If a cover crop will be used, it is very important to check rotation restrictions of burndown herbicides for the specific cover crop(s) planted.

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