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 are grown as an alternative to soybeans, providing opportunities to double crop or integrate a cover crop grazing operation extending into the growing season. Adding field peas can help reduce soil erosion, suppress troublesome weeds (e.g., Palmer amaranth), and minimize cost of crop production inputs.

When implementing field peas into a crop rotation, one of the most important things to consider is the herbicide carryover and restrictions. The potential challenges can be two-fold:

• 1) Herbicides applied in last year’s crop may damage this year’s field peas.
• 2) Herbicides applied in this year’s field peas may affect grazing of this year’s grazing annual forage, cover crop, fall-planted cash crop (e.g., wheat), or next year’s crop.  

This guide will help you avoid these carryover injury problems and design an effective herbicide program for your weed management. Use these tables when planning herbicide programs around the use of field peas:

• 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 remarks for selected herbicides
• Table 7. Field peas — Rotation restrictions for selected field pea herbicides
• (Set of Tables 1-7)

Carryover Injury in Field Peas Following Applications of Commonly Used Corn Herbicides

Although herbicide labels provide guidelines on intervals between herbicide application and the planting of susceptible crops, the potential for herbicide carryover injury in field peas depends on a complex interaction between herbicide, soil, and the susceptible crop during that interval. Many farmers have observed that despite rotation restrictions, some herbicides in Table 1appear to be safer than others.

The most concerning corn herbicides are certainly the ones containing mesotrione (Table 1,; e.g., Callisto). If you applied any mesotrione-based product (Table 1) in last year’s corn, it is almost certain that this year’s field pea crop will die after emergence (Figure 1). Solubility of mesotrione is very high (up to 3000 ppm), which means that this product moves with water. In rare cases, when field peas are planted on sandy ground that received high amounts of precipitation and/or irrigation during the season, mesotrione injury may be moderated and field peas may produce reduced grain yield. On the other hand, in a heavy clay soil with limited water, mesotrione would remain in its active form for a longer time.

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.

The least concerning corn herbicide is atrazine. Unlike mesotrione (e.g., Calisto) and isoxaflutole (e.g., Balance Flexx), atrazine is less water soluble (30-300 ppm) and it doesn’t move much with water. Atrazine, however, is prone to enhanced microbial degradation, especially in soils where atrazine 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).  Severe atrazine injury was observed in field peas receiving the full rate of atrazine (2 lbs ai/ac) in the fall after grain sorghum harvest (Figure 1). In general, most atrazine-based products have field pea rotation restrictions of two cropping seasons.  

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 the 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 is commonly done by using herbicides that provide lasting and broad spectrum weed control (Table 5). In our studies, the most effective herbicides in achieving this goal were those that contained active ingredients for both broadleaf weeds and grasses 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). In the aforementioned PRE herbicide mixes, Spartan® and Sharpen® typically provide broadleaf weed control while adding Dual II Magnum® and Prowl® helps control grasses; thus, the potential of carryover injury will follow the similar pattern. If you have a lot of grasses in your cover crop mix, Dual II Magnum® and Prowl® can be very damaging. Therefore, you should consider not using these two herbicide components (especially if grass pressure is low) or cutting back their rate so the carryover injury on grass species in the cover crop mix is reduced or negligent. Among the broadleaf herbicides, Spartan® has a higher potential for carryover injury in broadleaf species then Sharpen®. Table 7 provides specific rotation restrictions.

Bioassay

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, inexpensive, and effective 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, planting and growing bioassay species, and visually evaluating herbicide injury. For more information, check this Nebraska Extension NebGuide A Quick Test for Herbicide Carry-over in the Soil (G1891).

Resources

For more information, check this Nebraska Extension NebGuide, A Quick Test for Herbicide Carry-over in the Soil (G1891).

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. Available at https://pubag.nal.usda.gov/download/45083/PDF

North Dacota State University (NDSU) Herbicide carryover guide. Available at https://www.ag.ndsu.edu/weeds/weed-control-guides/nd-weed-control-guide-1/wcg-files/15-CO.pdf

University of Wisconsin (UW) Herbicide rotation restrictions in forage and cover cropping systems. Available at http://mccc.msu.edu/wp-content/uploads/2016/10/WI_2015_Herbicide-Rotation-Restrictions.pdf