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Options for drowned-out spots in fields

Excessive rainfall this season has led to ponding and flooding in fields, leaving drowned out spots and unplanted areas as the water recedes. What options are there to deal with drowned-out/unplanted areas in fields at this point in the growing season?

1) Do nothing: If at all possible, this is an option to avoid. Left unattended, drowned-out areas can be overrun with weeds, leading to tremendous increases in the weed seedbank. This can impact weed control for years to come. Alternatively, if weeds do not take over and there is little plant growth in the area, fallow syndrome could impact crop yields the following year. Fallow syndrome can occur when there is not enough living root material for beneficial soil mycorrhizal fungi to survive. These “good” fungi facilitate uptake of phosphorus and other nutrients with limited mobility in the soil.

2) Mow areas: Mowing weeds before they produce seed can help prevent contributions to the weed seedbank, but equipment limitations, accessibility, and the ability to mow areas before viable weed seeds are produced are challenges with this option.

3) Use tillage: If you can get tillage equipment into affected areas throughout the season in a timely manner, tillage can help prevent weeds from getting out of control. Leaving these areas void of living plant material, however, does not address concerns about fallow syndrome (see above) or wind or water erosion impacts.

4) Plant a cover crop: Planting a cover crop in drowned-out or unplanted spots can help reduce weed populations, prevent fallow syndrome, and reduce erosion potential. Species selection, seeding method, termination, herbicide interactions, and cost are considerations when determining which cover crop is the best fit.

Cover crop options:

Where soybeans were originally planted, soybeans can be planted as a cover crop. This may be one of the better options if low-cost seed is available, although do not expect any harvestable grain from soybeans planted after mid to late July. Planting soybeans where corn was originally planted is best avoided to prevent having three consecutive years of soybeans if the field is typically planted in a corn-soybean rotation. Soybean-on-soybean areas can lead to hot spots for soybean diseases.

When considering other cover crop options, it is a good idea to avoid cover crops that are poor hosts of soil mycorrhizae to reduce the potential for fallow syndrome. This includes the brassicas (e.g. mustards, radish, rape, and forage turnip). The Midwest Cover Crop Council Decision Tool (http://mccc.msu.edu/covercroptool/covercroptool.php) lists cereal rye and medium red clover (both winter hardy), annual ryegrass, buckwheat, crimson clover and sweetclover as good options when planting in late July/early August.

Annual ryegrass may be one of the best lower cost cover crop options that do not over-winter. If a cover crop that over-winters is desired, cereal rye is an ideal option. Both are relatively easy to use and respond well to broadcast incorporation and aerial seeding (assuming rainfall).

Further information on suggested planting dates, seeding rates and attributes of common cover crops can be found at https://z.umn.edu/ccoptions. The cover crop selection tool at www.mccc.msu.edu is another useful source of seeding information.

Previous herbicide applications may hinder cover crop establishment and prohibit the grazing of a cover crop. Even when rainfall has been high, concentration of a previously applied herbicide can be high enough in the soil to impact cover crop germination and growth since microbial activity is the primary driver in degradation of most products. Consult the herbicide label for guidance. Higher cover crop seeding rates may help offset potential stand reductions, although there are no guarantees. Note that cereal rye has been one of the most tolerant cover crops species to a range of previously-applied herbicides in University trials. For further information about herbicide interactions with cover crops go to: https://z.umn.edu/ccandherbicides.

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