Field Pea Guide to Herbicide Carryover and Herbicide Efficacy
Field pea is often described as an excellent rotational crop that can be effectively integrated into a variety of crop rotations. In semiarid western Nebraska, field peas are typically a fallow alternative in a wheat-corn-fallow or wheat-fallow rotation. In more humid (or irrigated) regions of the state, field peas may be grown as an alternative to soybeans and double-cropped with short-season crops, forages and/or cover crops.
Additional benefits of growing field peas include improvements in soil health, suppression of troublesome weeds (e.g., Palmer amaranth), minimizing pesticide and fertilizer inputs, and more efficient cropping system water use.
When integrating field peas into a crop rotation, herbicide carryover is one of the most important things to consider as it impacts replant options and causes rotation restrictions. This problem can be two-fold:
- The herbicides applied in last year’s crop may damage this year’s field peas.
- The herbicides applied in this year’s field peas may affect next year’s crop. More importantly, herbicides can affect grazing of this year’s annual forage, cover crop, or fall-planted cash crop (e.g., wheat).
The goal of this guide is to help avoid potential crop injury due to herbicide carryover and to help design an effective herbicide program for your weed management. Tables 1-7 offer information useful when planning herbicide programs around field peas in crop rotations:
- Table 1. Corn herbicides that can cause serious carryover injury in field peas. These herbicides should not be used or should be used with caution when field peas follow corn.
- Table 2. Corn burndown and PRE herbicides that will not cause carryover injury in field peas.
- Table 3. Corn POST herbicides that will not cause carryover injury in field peas.
- Table 4. Wheat herbicides that will not cause carryover injury in field peas.
- Table 5. Field peas – weed response to selected herbicides.
- Table 6. Field peas – rate per acre, application time, and comments for selected herbicides.
- Table 7. Field peas – rotation restrictions for selected field pea herbicides.
Carryover Injury in Field Peas Following Applications of Commonly Used Corn Herbicides
Herbicide labels provide guidelines on the required time interval between herbicide application and the planting of susceptible crops. The potential for herbicide carryover injury in field peas depends on complex interactions that can occur among herbicides, soil type, soil moisture, and the susceptible crop during that interval. Many farmers have observed that despite rotational restrictions, some herbicides in Table 1 appear to be safer than others.
The most concerning corn herbicides are certainly those containing mesotrione, such as Callisto (Table 1). The use of mesotrione-based products (Table 1) in last year’s corn will cause serious injury to field peas this year (Figure 1). The solubility of mesotrione is very high (up to 3000 ppm), which means that this product can be reactivated with a small amount of water (0.25-0.5 inch) or can easily leach through the soil profile, especially on lighter soils. For instance, when field peas are planted on sandy ground that received high amounts of precipitation and/or irrigation during the season, it is possible for the crop to grow out of mesotrione injury and finish with relatively minimal to moderate yield losses. On the other hand, in a heavy clay soil with limited water, mesotrione would persist for a while and then reactivate with rainfall and injure the field peas.
A somewhat less concerning corn herbicide is isoxaflutole (Table 1; e.g., Balance Flexx®). We have received farmer reports that isoxaflutole-based products (Table 1), did not cause carryover injury in field peas. Depending on the product labels, rotation restrictions are based on either a 17-18 month rotational interval or a minimum of 15-30 inches of cumulative precipitation.
Atrazine is also of concern but not as much as mesotrione and isoxaflutole. Atrazine is less water-soluble (30-300 ppm); however, it still moves with water (e.g., rainfall). Atrazine is prone to enhanced microbial degradation, especially in soils where it has been used in the past (Kurtz et al., 2010). Many farmers reported little to no atrazine injury on field peas, especially when applied in the spring at lower rates (less than 1 lb ai/ac).
According to North Dakota State University recommendations, field peas may be planted the next cropping season if atrazine rates are less than 0.38 lb ai/ac. The University of Wisconsin recommends maintaining a nine-month rotation interval for field peas following the application of Harness Extra (acetochlor + atrazine premix). According to the Nebraska Extension Guide for Weed, Disease, and Insect Management in Nebraska, field peas may be planted in the second field season. Severe injury was observed in field peas receiving the full rate of atrazine (2 lbs ai/ac) in the fall after grain sorghum or corn harvest (Figure 1). In summary, most atrazine-based products have field pea rotation restrictions of two cropping seasons, which is highly rate dependent.
If you plan to incorporate field peas in your rotation, we encourage you to select corn and wheat herbicides that provide efficacy equivalent to mesotrione-based, isoxaflutole-based, and atrazine-based products, but do not cause carryover injury in field peas (Tables 2-4).
Carryover Injury in Forages and Cover Crops Following Applications of Field Pea Herbicides
A good PRE herbicide program is a critical part of field pea production. Using PRE herbicides to control early season weed pressure can substantially increase the competitive ability of field peas to form the canopy and avoid any POST herbicides or harvest aid applications. This can be achieved with herbicides that provide lasting and broad-spectrum weed control (Table 5). In our studies, the most effective herbicides were those that contained active ingredients for both broadleaf weed and grass control such as Spartan Elite®/BroadAxe XC® (Spartan® + Dual II Magnum® premix) or tank mixing Sharpen® + Prowl® (Table 5).
If your intention is to plant a multi-species cover crop after field pea harvest in mid-July, it is important to understand the components of PRE herbicides and their potential carryover injury on species in the cover crop mix (Table 7). For example, Spartan® and Sharpen® typically provide broadleaf weed control while Dual II Magnum® and Prowl® help control grasses. The potential of carryover injury in your cover crop will follow a similar pattern. If you have a lot of grasses in your cover crop mix, Dual II Magnum® and Prowl® can be damaging. You should consider not using these two herbicide components (especially if grass pressure is low) or consider cutting back their rate to reduce the potential for carryover injury on grassy species in the cover crop mix. Among the broadleaf herbicides, Spartan® has a higher potential for carryover injury to broadleaf species then Sharpen®. Table 7 provides specific rotation restrictions.
It is important to mention that chemical companies will often only evaluate major crops for carryover injury of a particular herbicide and will use a default interval (18 months or greater) for many minor crops. Herbicide degradation in the environment is a complex process and rotation restriction intervals may be different than labeled in your field.
One of the most practical and inexpensive ways to evaluate whether herbicide carryover may affect your crop is a bioassay. In short, a bioassay includes collecting representative soil samples from the field suspected of having herbicide residue, and then planting seeds of your crops (bioassay species) into collected soil. This will allow you to evaluate the potential for herbicide injury. This method is very helpful but it is not always a complete proof of injury potential as herbicides tend to move laterally in the soil profile when suspended in soil moisture.
Check this Nebraska Extension NebGuide, A Quick Test for Herbicide Carry-over in the Soil (G1891). Also check the Replant Options and Rotational Restrictions table in the Guide for Weed, Disease and Insects Management in Nebraska (EC130).
Kurtz, J.L., D.L. Shaner, and R.M. Zablotowicz. 2010. Enhanced degradation and soil depth effects on the fate of atrazine and major metabolites in Colorado and Mississippi soils. Journal of Environmental Quality, 39:1369-1377.
North Dakota State University’s Herbicide Carryover guide.
University of Wisconsin’s Herbicide Rotation Restrictions in Forage and Cover Cropping Systems