The heavy rains that many fields received will raise a few key questions regarding herbicide performance and replant considerations. A major question with preemergence corn and soybean herbicides is if the heavy rains leached the herbicide out of the upper soil profile. The rain certainly will have diluted the concentration of the herbicide in the zone where most of the weed seeds are germinating and this dilution is likely to be greater on coarse-textured soils. However, I believe that enough of the herbicide residues will remain in the upper profile to provide adequate weed suppression as the initial flush of weeds was controlled prior to the rains.  Corn fields in particular should be scouted in the next week to confirm control. Otherwise, the growth stage of some corn may be too advanced for safe herbicide applications (i.e. the maximum growth stage for several herbicides is V6 corn) if a postemergence herbicide is needed.

Cautions for Replanting. If farmers are considering to replant flooded fields, they will need to determine how previous herbicide applications and residues in the soil affect replant options. 

Corn scenarios

1.  Only a burndown application of glyphosate or Gramoxone was made: Any crop can be replanted.

2.  A postemergence program was planned, but not applied yet:   Any crop can be replanted.

3.  Burndown, preplant, or preemergence herbicides were applied to corn: Several corn herbicides will not allow replanting of other crops in the same year as the initial herbicide application. A quick way to check on the rotational intervals for several field crops is to review Appendix Table 2, Planting intervals for rotational crops (pages 220-222) in the Pest Management in Wisconsin Field Crops Bulletin, A3646. Next, confirm the replant options on the label for the specific herbicide that you are using.  There are two reasons for these replant intervals. First, the corn herbicide could injure a different crop if planted before the herbicide has degraded in the soil. Second, the corn herbicide may cause illegal residues to occur in the replanted crop. The bottom line is that the restrictions on the herbicide label must be followed.

Some common corn herbicides will severely limit replant options of other crops. For instance, most atrazine labels state “Do not plant to any crop except corn or sorghum until the following year”. This restriction would prohibit planting soybeans, alfalfa, or fall-seeded winter wheat in flooded fields after they drain. Winter wheat could be seeded 4 or 4.5 months after preemergence grass herbicides like Dual II Magnum, Harness/Surpass, Outlook, and Prowl and after preemergence broadleaf herbicides like Camix, Lumax, Hornet, and SureStart. Many other crops will not be allowed to be replanted if a herbicide was applied.

Soybean   A large majority of soybeans are Roundup Ready and are only treated postemergence with glyphosate. Consequently, these fields can be replanted to soybeans or any other crop. Some Roundup Ready and conventional soybean fields may have been treated with a preemergence herbicide and these fields could be replanted to soybeans. As with corn, use no-till or shallow tillage to avoid diluting the remaining herbicide and monitor to see if a postemergence herbicide application is justified. Most preemergence soybean herbicides will not allow immediate replanting of alfalfa or other crops. Winter wheat would be an option for fall seeding in most cases. 

Alfalfa If new seedlings were treated with Raptor or Pursuit, soybeans can be replanted, but alfalfa cannot be reseeded according to label directions. Nearly all established alfalfa fields are not treated with herbicides and any crop could be seeded in this situation.

Weed Management in Areas not Replanted.  If fields or potholes are not replanted with a crop, weeds will have a field day in these areas because there is no crop competition to suppress their growth. Unless these weeds are controlled, weed seed production can be huge and the weed seed bank will increase dramatically unless action is taken. Here are some suggestions for managing weeds in these areas.

In fields with drowned out areas, the first consideration is whether or not a preemergence herbicide had been applied. Preemergence herbicides will suppress weed growth for some time period into the summer so immediate action is not needed. If a preemergence herbicide was applied, tillage may be the best option to control weeds later in the summer after weeds have emerged. Another option is to mow the area, but this likely will not prevent all weed seed production.  Although it may be tempting to plant a cover crop in these drowned areas, consider if the preemergence herbicide will kill the cover crop (e.g. atrazine would likely kill oats). If no preemergence herbicide was applied, no-till a cover crop into the area as soon as the soil is fit.

To control annual weeds in fields where preemergence herbicides were not applied, tillage may seem to be the cheapest and easiest option, but there are drawbacks to fallowing fields with tillage. Tillage may cause soil compaction if the soil is still wet and will leave the soil exposed to further erosion. Tilling for weed control will not prevent later flushes of weeds so additional tillage will likely be needed during the summer, which will add to the cost.

A herbicide burndown treatment is an alternative to using tillage for fallowing. As weeds begin to regrow, an application of 0.75 lb ae/a of glyphosate (1 qt/a of brands with 3 lb ae/gal) would probably be adequate for most weeds that are 6 to 12 inches tall. Even with chemical fallow, additional weeds will emerge because glyphosate lacks residual soil activity. To avoid respraying, the field could be no-till planted with a cover crop to suppress summer annual weeds. Oats would be an inexpensive cover crop to consider. Cover crops may be damaged depending on previous herbicide use and could not be harvested for grain or green-chop if planted before the permitted replant interval.