- Wheat planting this fall may be delayed in some areas by the late start and end of the season, the later than normal herbicide applications and their potential carryover restrictions.
- Herbicide rotational restrictions must be followed to avoid issues with crop injury on wheat.
- The rate of degradation of residual herbicides play a key role on their potential carryover and injury to the following cash crop.
Winter wheat planting could be delayed this fall due to the late harvest resulting from a delayed start of the season. Later plantings were followed by delays in soil herbicide applications and that ultimately pushed the post-herbicide application further into the summer months. Of concern are herbicide applications made in July and August, because both months were relatively dry.
There are many ways in which herbicides dissipate and are no longer available in concentrations high enough to cause injury to susceptible crops. Two of these factors include the amount of herbicide intercepted by crop foliage or weeds and the environmental conditions following the application.Herbicides intercepted by foliage are often degraded during plant growth processes or by UV light (photodecomposition) and are largely unavailable to the soil environment. Environmental conditions such as soil moisture and moderate temperatures allow the soil microbes to be more active and efficient in degrading herbicides. Soil moisture also allows the herbicide to be in soil solution for uptake of actively growing weeds/plants and allows for leaching through the soil profile. Tillage also exposes the herbicide to a greater volume of soil and microbes, speeding up degradation. In general, the length of herbicide persistence depends on four main factors 1) chemical properties of the herbicide, 2) chemical properties of the soil (mainly soil pH), 3) the soil texture and organic matter content, and 4) the environmental conditions that occur after the application.
When soils dry out, as during a drought, herbicides are not broken down as quickly as under normal soil moisture conditions.If soil moisture is in limited supply, then the herbicide is unavailable for degradation by growing plants and soil microbes, thus increasing the persistence of the herbicide. This year, much of our concern revolves around the use of residual herbicides late in the growing season because of generally low amounts of rainfall during the months of July and August. Therefore, it is important to revisit labeled planting restrictions for wheat and other crops as we approach the fall planting season.
Review the following table.Note that many of the common herbicides have a 3 to 4.5 month rotation restriction for planting wheat. For example if Dual II Magnum was applied in a field on July 1st , wheat cannot be planted in that same field until after mid-November.
Formore information on herbicide rotational restrictions or other wheat plantingconsiderations please contact your local FS crop specialist.