How to control weeds on prevented planted acres

Prevent planting brings up lots of other issues, including weed control. Here are some ideas on what to do from Mark Loux, Ohio State University Extension weeds specialist.

As we get closer to decisions about cropping vs. prevent plant, consider weed control. The season-long weediness of bare areas that occur in some crop fields from flooding and crop loss give a good idea of what can be in store on prevent plant acres. Here are some considerations.

The Goals for Unplanted Acres Are To:

Prevent annual weeds from going to seed and increasing the soil seedbank. Anything that goes to seed will have to be dealt with in the future.
Treat any perennial weeds in the appropriate growth stage to reduce their population.

Winter annual weeds have already gone to seed or are in the process of doing so. Summer annuals will keep emerging in a bare ground area for much of the summer, depending upon rain. Later on, though, emerging summer annuals will run out of time to mature and develop much seed before frost. The overall goal is to control them from now until then.

Multiple passes of mowing, tillage, or herbicide, or a combination of these, can control them.

When calculating whether to plant a crop or take prevent plant, assume a cost of at least two passes. Weeds that survive/regrow following tillage or mowing are more difficult to kill with herbicides. Thus, herbicides may be best used in the first pass of a combination strategy.

Tillage is best used for relatively small weeds, as large ones are difficult to completely uproot. A single mowing may be best used later in the season, when any weed regrowth can be clipped by frost prior to producing seed.

Herbicides

If opting for a foliar-applied herbicide for the first pass, apply soon while weeds are small enough to be controlled. Follow up with a second one later in the summer as needed.

The most cost-effective approach is probably glyphosate plus 2,4-D, although other growth regulator herbicides that contain dicamba or clopyralid can also be added. This approach may not kill large marestail, but can stop most of the seed production.

Gramoxone or glufosinate could be substituted for glyphosate in some fields, but mostly on small weeds. Large grasses can be a problem.

Cover Crops

Planting a cover crop can help suppress weeds and reduce reliance on herbicides alone. This will most likely not eliminate the need for herbicides, and a burndown treatment or tillage will be necessary to allow planting.

Planting a grass cover will allow use of growth regulator herbicides to control broadleaf weeds. If an unplanted field was previously treated with residual corn or soybean herbicides, check to make sure it’s safe to plant the intended cover after use of those herbicides.

Residual Herbicides

A common question that surfaces whether residual herbicides can replace or minimize the need for foliar-applied herbicide, or extend the time until they are needed. In our opinion, it is difficult to make the case to spend money for residual herbicides. The ones we use on corn and soybeans struggle to provide enough control in a bare ground situation, and most are not labeled for use in noncrop areas.

The herbicides used in industrial vegetation situations that will provide enough control will also persist long enough to mess up crop rotations. The goal for residual herbicide use would be a minimal investment for herbicide(s) that provide broad spectrum control for a month or more. The only possibilities we could find that have labels for true noncrop areas are: