Soil-active herbicides can greatly improve weed control in food plots as they control weed seedlings before they compete with planted species. However, soil-active herbicides can have similar, damaging effects on your future plantings if the active ingredient controls the planted species. Knowing the plant-back intervals for soil-active herbicides can mean the difference between a lush or failed food plot.
The plant-back interval refers to the length of time a particular herbicide remains active in the soil at a high enough concentration that it may influence a future planting. Many broadleaf- and grass-selective herbicides, and all pre-emergence and preplant-incorporated herbicides, have residual effects that may impact future seedling establishment. (Glyphosate, the most common herbicide used to prepare food plots for planting, control weeds in glyphosate-tolerant soybeans or corn, or manage undesirable plants in fields and woods, is not soil-active.)
Plant-Back Intervals: Read the Label
Many food plot managers use broadleaf-selective, grass-selective, and broad-spectrum selective herbicides, and a growing number are learning the benefits of pre-emergence and preplant-incorporated herbicides. Residual soil activity varies widely based on the herbicide, use rate, soil, and precipitation. Some herbicides have short residual activity, whereas others may remain active for many months.
For example, clethodim is a grass-selective herbicide that remains active in the soil for approximately seven days. On the other hand, trifluralin is a broad-spectrum selective herbicide that may remain active in the soil for at least 20 months. Application rate obviously influences residual activity of many herbicides, with greater rates having a longer residual effect. Soils with higher organic matter, as well as regions with reduced rainfall and temperature, tend to result in longer residual herbicide activity.
All of this probably sounds complicated, and it can be! However, there’s good news: nearly all herbicide labels have information in their label on plant-back intervals. If you needed another reason to read the label, the label tells you exactly when you are able to plant a particular crop following an herbicide application. Always read the label!
Another excellent source of information about plant-back intervals is the appendix of Dr. Craig Harper’s “Wildlife Food Plots and Early Successional Plants,” a book that belongs on the shelf of anyone interested in planting food plots.
Common Scenarios in Food Plots
Although herbicide labels provide the information you need to know about plant-back intervals, there are several common scenarios for food plot growers. Several herbicides have short plant-back intervals, but ignoring these can lead to major problems.
For example, 2,4-D has a 15- to 30-day residual activity (depending on rate) on broadleaf plants, including soybeans. Managers using 2,4-D in burndown applications should plan accordingly, as crop damage may result if broadleaf plants are planted too soon afterward.
Similarly, managers wishing to drill winter wheat into established perennial clover or alfalfa should be aware of residual activity of clethodim, a grass-selective herbicide, if they are planting soon after spraying in the fall. Even with only one week of residual activity, damaged plots may result if consideration is not given to plant-back intervals. Several herbicides have much longer rotations, and these can be very problematic if records are not kept regarding when a plot was sprayed with a particular herbicide.
Perhaps the most common herbicide used in food plots with a long residual life is imazethapyr, which often is used to control various broadleaf and grass weeds in clover, cowpea, soybean, and jointvetch food plots (trade names include Pursuit and Thunder). Imazethapyr has a four-month plant-back interval for clover, alfalfa, and wheat, as well as an 18-month plant-back interval for oats and grain sorghum. See the chart below for more recommended wait times between spraying and planting.
| Future Crop | Minimum Wait After Imazethapyr |
|---|---|
| Brassicas | 40 months |
| Sorghum | 18 months |
| Sunflowers | 18 months |
| Oats | 18 months |
| Wheat | 4 months |
| Rye | 4 to 18 months* |
| Clover | 4 months |
| Alfalfa | 4 months |
| Pea/Bean | Anytime to 4 months** |
| Soybeans | Anytime |
If you plan to double-crop a particular field, these intervals may limit your options for a cool-season planting if imazethapyr is used in the spring. That is not to suggest you avoid using imazethapyr. I use it all the time! Rather, you should consider future plans for a plot before treating it, and select another herbicide with a shorter plant-back interval if necessary.
Planning for Crop Rotations
To reduce future problems with residual herbicide damage, it’s important to plan rotations in advance. Thoroughly reading herbicide labels is critical, as some herbicides may cause damage to new plantings more than a year later. Some herbicides are even damaging to seedlings of plants they can be safely applied to later in life.
For example, if you plan to frost-seed perennial clover, it is important to make your final imazethapyr application at least four months in advance, as residual imazethapyr will damage seedling clover. Another strategy is to incorporate fallowing into your rotation, as this will allow the herbicide to degrade in the soil while allowing native plants that deer select to grow in a particular field.
How Are Herbicides Degraded in the Soil?
What exactly happens to herbicides after you spray an area? This question might not be one you have previously considered, but herbicides certainly don’t last forever in the soil. Microbial degradation is the most common pathway, and this includes breakdown from fungi, bacteria, and protozoans.
Warm, fertile, neutral pH soils tend to have shorter herbicide residual life because they are better for microbials that break down herbicides. Sunlight also can lead to degradation and is the reason some herbicides (such as trifluralin) must be incorporated into the soil following application. Some herbicides also are degraded through chemical reactions in the soil. Regardless of the pathway, it is important for managers to understand these chemicals are broken down over a relatively short period of time and generally don’t leave the application site.
Learn It, Know It, Live It
Herbicides are our best tool to reduce weed pressure in food plots, but it’s important to keep plant-back intervals in mind when treating food plots. By carefully reading the label and considering crop rotation, you can use residual herbicides effectively while preventing them from damaging your planting.