Comments on herbicide carryover potential
Due to the widespread drought conditions experienced over the past year, many growers are concerned about the potential for herbicide carryover injury to this year's crops.
The potential for herbicide carryover injury is driven by two main factors:
• Concentration of available herbicide remaining in the soil at the time of rotational crop planting. This depends on herbicide chemical properties, soil characteristics, and weather.
• Susceptibility of the rotational crop to the herbicide. Some crops are not injured by relatively high concentrations and other crops are highly injured by low concentrations. The stresses that the newly planted crop faces during establishment can also affect response. Emerging plants are more likely to show injury to residual levels of herbicide if other stresses such as compaction or cold, wet soils are also present.
There is little that growers can do at this point to affect the amount of residual herbicide present in their fields.
However, there are a few things that can be done to reduce the risk of crop injury.
• Review spray records for each field and product labels to see what restrictions are indicated. Many labels specify the time required between herbicide application and planting of a rotational crop. Planting sooner than the specified time increases the risk of injury.
• Ensure seedling stresses are minimized. This gives the young plants their best chance of surviving herbicide residues with little damage. This can include making sure soil pH and fertility levels are optimum for the crop, reducing compaction, and avoiding planting into cold, wet soils.
• Change planned crop. In some cases it may be best to plant the same crop as the previous year, or at least a crop for which last year's herbicides are also labeled. This significant step has the most potential to reduce the risk of crop injury.
• Delay planting. Growers could plant suspect fields last to give more time for the degradation to occur.
• Consider tillage. Tillage may dilute the herbicide in the soil profile and provide aeration and faster soil warming to stimulate microbes, but results are mixed on whether this will provide a significant benefit.
• Conduct a bioassay or chemical analysis. It is important to note that carryover injury is impacted by many factors besides just how much herbicide is present.