Integrated weed management programs offer the greatest potential for long-term, sustainable solutions to weed populations demonstrating resistance to herbicides from multiple families. Soil-residual herbicides are important components of integrated weed management programs and provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides. Recent survey data indicate the percentage of Illinois soybean acres treated with soil-residual herbicides has increased during the past few years.
In the vast majority of instances, soil-applied herbicides control target weed species with little to no adverse effect on the crop. However, soybean plants sometimes are injured by these herbicides. Questions about soybean injury caused by soil-applied herbicides recently have been posed, so this article will review some of the factors that can contribute to herbicide-induced soybean injury.
Herbicides vary in their inherent potential to cause soybean injury. Many university-generated herbicide effectiveness rating tables also provide estimates of soybean injury potential. Some herbicide active ingredients, such as cloransulam and clomazone, are often rated as having very low potential to cause soybean injury, whereas other active ingredients are rated as having a greater inherent potential to cause injury. The rate at which the herbicide is applied can influence the potential for soybean injury by increasing or decreasing the amount of herbicide in a given volume of soil.
Most many cultivars are not overly sensitive to any particular herbicide, but other soybean cultivars can vary in their sensitivity to certain herbicides. Data in the scientific literature and company-generated variety trials demonstrate cultivar sensitivity differences to various soil-residual herbicides. Some cultivars demonstrate sensitivity to one active ingredient, whereas other cultivars can be sensitive to more than one active ingredient.
The environment has a large influence on the severity of soybean injury caused by soil-applied herbicides. Environment-induced crop stress, often caused by cool, wet soil conditions, can enhance soybean injury from soil-applied herbicides. In most cases, herbicide selectivity arises from the soybean plant’s ability to rapidly metabolize the herbicide to a nonphytotoxic form before it causes much visible injury. Soybean plants growing under favorable conditions are able to adequately metabolize the herbicide before any injury symptoms are expressed. However, when the soybean plant is under stress, its ability to metabolize the herbicide can be sufficiently reduced to the point at which injury symptoms develop.
Soil physical properties can increase or decrease the potential for soybean injury by impacting how much herbicide is available for plant uptake. Soils with higher amounts of clay and organic matter have a greater ability to adsorb more herbicide onto these soil colloids. Herbicide bound to soil colloids is not available for plant uptake. In contrast, coarse-textured soils have less adsorptive capacity so more herbicide remains available for plant uptake. Labels of soil-applied herbicides often contain precautionary language about the increased potential for soybean injury when the product is applied to sandy soils or soils low in organic matter.
The application timing of soil-residual herbicides also can impact the potential for soybean injury. Applications made immediately before or after soybean planting result in a high concentration of herbicide near the emerging soybean plants. In contrast, a herbicide is often more widely distributed within the soil profile by the time of soybean emergence when applications are made several days or weeks prior to planting.
The soil-applied PPO-inhibiting herbicides, including saflufenacil, flumioxazin, and sulfentrazone, are very effective for control of Amaranthus species. These herbicides (and many others) also can cause soybean injury. Our first experience with soybean injury from soil-applied PPO inhibitors occurred in 1996 while evaluating sulfentrazone for control of herbicide-resistant waterhemp. Soybean injury symptoms caused by these soil-applied herbicides can vary depending on the soybean developmental stage when exposure occurred. The most commonly encountered injury symptoms occur on the hypocotyl and cotyledons (Figure 1), often indicating the plants were exposed to a high concentration of herbicide as they were emerging.
Figure 1. Injury symptoms evident on emerging soybean plant.
Symptoms include necrotic lesions on the soybean hypocotyl near the soil surface and reddish-colored spots or lesions on the hypocotyl and/or cotyledons (Figures 2 and 3). Lesions on the hypocotyl may not always kill the young soybean plants, but can create an area of weakened tissue that may lead to stems breaking during rain or high wind. In severe cases, plants may actually die following emergence of the cotyledons.
Figure 2. Reddish-colored hypocotyl tissue near the soil surface.
Figure 3. Emerged soybean with damage to hypocotyl and cotyledons.
Plants with damage only to cotyledons usually develop normally (Figure 4).
Figure 4. Injury to cotyledons.
Other symptoms can occur after soybean emergence if treated soil is splashed into the soybean meristem by heavy precipitation.
There likely is no solitary reason for the recent instances of soybean injury from soil-applied PPO-inhibiting herbicides. As previously mentioned, our first experience with this type of soybean injury occurred almost 20 years ago and we have continued to observe this type of injury intermittently ever since. These herbicides have become very popular choices for the management of herbicide-resistant Amaranthus populations, and widespread application of these herbicides increases the probability of encountering soybean cultivars that inherently are more sensitive to one or more of these herbicides. In many instances of soybean injury, the herbicide was applied after soybean fields were planted and a precipitation event occurred within a few days of soybean emergence. Cool air and soil temperatures during the same interval can further increase injury potential by slowing the rate of herbicide metabolism. A crusted soil surface can slow soybean emergence, increasing the time the hypocotyl and cotyledons remain in the zone of high herbicide concentration. Once the herbicide is moved deeper into the soil profile, the potential to cause additional injury is greatly reduced.