1. Review spray records for each field and review product labels to see what restrictions are indicated.
Many labels specify the time required between herbicide application and planting of a rotational crop (see Table 1). Planting sooner than the specified time increases the risk of injury. In addition to a time interval, labels may also list conditions under which a particular crop may or may not be planted, so scrutinizing the “fine print” is key. After a drought year it is probably best to err on the conservative side regarding plant-back times.
2. Ensure seedling stresses are minimized to give the young crop 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. Other stresses that the seedling experiences can exacerbate response to herbicide residues (and to the herbicide applied in the current year).
3. 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 and is worthy of consideration in high-risk fields.
4. Delay planting.
In drought years, herbicide degradation rates are typically slower than normal, so more time than normal may be required for sufficient degradation. With spring moisture and warming soil temperatures, the microbes will start to act again to degrade herbicide residues. However, it is unlikely that this will have a significant impact in early spring, and the yield potential gained with earlier planting could be lost to herbicide injury.
Growers could plan to plant suspect fields last to give more time for degradation to occur. Seedlings in later-planted fields often experience lower stresses and faster development than those in early-planted fields, which could help the crop outgrow putative carryover injury more quickly.
5. Consider tillage?
The jury is out on whether tillage impacts carryover potential. 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. Growers in long-term no-till who try to reduce carryover potential by tilling will sacrifice many of the soil quality benefits accrued from no-till over the years, possibly without a major impact on crop response this year.
6. Conduct a bioassay or chemical analysis.
Some growers plan to sample fields and plant their intended crop in greenhouse pots to see if any symptoms appear (Figure 7). However, to be valuable this must be done with care, and interpretation of the results can be difficult or misleading. Laboratory analyses, while fairly accurate, are costly and only tell you the concentration of herbicide present. As discussed previously, carryover injury is impacted by many factors besides just how much herbicide is present. Differences in the inherent susceptibility of different crops to each herbicide affect how concentration results should be used.