Because of glyphosate-resistant Palmer amaranth, many fields would be rendered unproductive were it not for residual herbicides. However, care must be used when using residual herbicides. Some residual herbicides can, under the wrong conditions, severely injure the crop to which they are applied. For example, a high rate of diuron applied to cotton grown on coarse-textured soil can significantly reduce seedling density. Some soybean varieties are more sensitive to metribuzin (Sencor) than other varieties.
Crop injury from a herbicide applied the previous year is called “herbicide carryover.” Cotton may not be planted for 18 months following an application of Python to corn or soybean. The potential for herbicide carryover was one characteristic of traditional herbicide programs that was forgotten as long as glyphosate was effective on a broad weed spectrum and few other herbicides were utilized.
Crop injury from herbicide residue in the soil is not restricted to persistent residual herbicides applied the previous year. Dicamba applied burndown will severely injure soybean if used incorrectly. Following a dicamba application of eight ounces per acre or less, 14 days is required before planting soybean. An often overlooked stipulation on the dicamba label is that the treated field must receive at least one inch of rainfall before beginning to count the 14 days. Clethodim (Arrow, Select Max, Volunteer), which is a standard burndown herbicide for glyphosate-resistant Italian ryegrass, is used as a postemergence herbicide, but it actually exhibits soil activity. The soil activity of clethodim is low in comparison to other herbicides such as Dual Magnum, but it is sufficient to cause severe injury on grass crops like corn and rice if the specified preplant intervals are not followed.
Most state weed control guides contain sections on recropping restrictions and crop rotation intervals. Recropping information can also be found on the herbicide label, but it can be difficult to locate. Some labels have sections entitled “Rotational Crops”. Other labels list recropping information in sections entitled “General Use Precautions” or “Special Precautions”.
A problem that has become increasingly common in Mississippi and across the Midsouth over the last two years is corn injury from fomesafen (Flexstar, Flexstar GT, Prefix, Reflex) carryover. In most cases, corn injury has occurred in fields where corn is grown in rotation with soybean and multiple applications of Flexstar, Flexstar GT, and/or Prefix were used during the soybean year. However, there have also been cases in corn-cotton rotations where Reflex was used preplant in cotton, and Reflex or Prefix was used at layby. The rotation interval for corn following labeled fomesafen applications is 10 months. In most cases, a labeled rate (0.375 pounds active ingredient per acre) of a herbicide containing fomesafen applied to soybean should not injure corn the following year. However, where excessive rates or multiple applications of herbicides containing fomesafen are utilized, corn injury can be devastating. The weather during the fall and winter may also contribute to the problem if there is insufficient rainfall to help dissipate the herbicide.
Herbicide carryover is real and its potential should not be ignored. Residual herbicides can be utilized effectively with minimal concerns for the current or future crops. However, weed control programs including residual herbicides do require more planning and more thorough recordkeeping than a glyphosate-only program. Keep records of which fields received a residual herbicide and check these records prior to planting the following spring. Be careful during the residual herbicide application. Be sure the correct rate is used and that the sprayer is properly calibrated. Avoid boom overlaps in the field and while spraying the ends of the rows.