June 6, 2008

Bad weather and excessive rainfall have many growers facing replant decisions. We have received many questions on replant options in fields where preemergence herbicides were applied to corn. There are two primary issues to address: 

• How to kill the surviving corn
• What crop(s) can be replanted if a prepreemergence corn herbicide was previously applied

Killing a Poor Corn Stand

Planting corn into existing stands is not desirable because a field of varying plant sizes and stages will be difficult to manage and some plants will act as weeds, using water and not providing harvestable yield. Several options are available for killing the existing stand.

Mechanical Control. Cultivation can work effectively for any type of corn, but waiting for dry enough conditions to work the soil may not be desirable.

Chemical Control. Another option is to use herbicides to kill the existing stand. If the existing stand is a conventional or LibertyLink hybrid, glyphosate is the best herbicide option. If the corn is a Roundup Ready hybrid, killing the poor stand becomes more complicated.

There are three chemical options to control Roundup Ready corn, but each has some drawbacks. Select Max (clethodim) is slow acting but very effective at controlling a poor stand for a replant scenario (e.g., it will kill all the plants remaining). The drawback is a six-day waiting period between herbicide application and replanting. If the waiting period is ignored, the replanted corn may be stunted.

Liberty (glufosinate) and Gramoxone (paraquat) also can be used. Neither has soil residual activity that would delay planting; however, both options have produced inconsistent results in UNL trials. Immediately after spraying these herbicides, the corn will look ill, but unless conditions are just right — warm, humid, and the plants are large enough for the growing point to be killed — often the plants will survive the application and send up new leaves. Gramoxone activity may be enhanced by tank-mixing it with atrazine or metribuzin. If Liberty is used, add AMS to maximize effectiveness.

Replant crop options when a corn preemergence herbicide has been applied

The logical replant options at this point in the growing season are a short season corn hybrid, a shorter maturity soybean variety or grain sorghum. Depending on the preemergence herbicide used in corn, it may not be possible to replant to any crop except corn. Table 1 lists corn, grain sorghum and soybean replant intervals for some of the more common preplant and preemergence corn herbicides.

Corn: Corn can be replanted at any point, unless Select has been used to kill the existing stand. In most instances, producers can then use the postemergence program they had initially intended for subsequent weed control this year.

Soybeans: If atrazine was applied preplant or preemergence, soybeans should not be planted in that field until next year. This is clearly stated on the label of any herbicide that contains atrazine, regardless of the rate used. Given the time of year, from a practical standpoint some producers may try to replant soybeans where lower rates of atrazine were applied. The producer assumes all crop injury risk in this scenario since this is prohibited by the label. Consider the following points if this is attempted:

• Replanting soybeans where atrazine has been applied and the soil pH is above 7 will likely result in severe injury. Atrazine persistence in the soil increases when the pH is above 7.
• Using tillage to dilute the atrazine concentration in the soil may be beneficial. The emerging soybean plant will not encounter areas of high atrazine concentrations.
• Avoid using herbicides with metribuzin (Sencor and others) for weed control in these replanted soybeans. Soybean injury potential increases when atrazine residues are present and metribuzin is used.

Sorghum: There is more flexibility when replanting sorghum where corn preemergence herbicides have been used.

Replanting sorghum where common corn preemergence herbicides containing atrazine, dimethenamid-P (Outlook), or S-metolachlor (Dual II Magnum, Bicep II Magnum, Cinch, etc) were applied should not present any problems. If products containing acetochlor (Breakfree, Degree, Keystone, etc) were applied, replanting sorghum is not allowed according to product labels. Consult Table 1 for details.

If Lumax or Callisto were applied preemergence in corn, replanting sorghum should cause few if any issues. Both these products were labeled for preemergence applications in grain sorghum in 2008. Lumax or Callisto can cause some bleaching or whitening of the emerging and young sorghum seedlings; however, this generally occurs only when the products are applied within seven days of planting. Our experience has shown that the bleached sorghum grows out of these symptoms quickly and develops normally with little to no impact on yield. If Lumax or Callisto were applied to the corn several weeks ago, there is little potential for injury to replanted sorghum.

In conclusion, this article has provided some general guidelines to consider. Producers should consult the label of the product(s) applied this spring for specific details and directions.

Lowell Sandell
Extension Educator, Weed Science
Mark Bernards
Extension Weed Scientist