Herbicide injury to soybeans is showing up in more fields than usual this summer in Iowa. Iowa State University Extension field agronomists are answering farmers’ questions about determining the source and reasons for the damage.
ISU weed management specialist Bob Hartzler and field agronomist Meaghan Anderson offer the following tips on identifying herbicide injury they’ve seen this summer in soybean fields.
Careful assessment of field history, field topography and field edges helps distinguish between different sources of herbicide injury. Also, consider how environmental conditions may influence the crop’s ability to tolerate the herbicide.
4 key reasons injury occurs
1. Carryover. Carryover injury is typically associated with soil types or may be found in streaks or odd-shaped field areas where overlap occurs. Delayed applications and a dry summer in 2019 have increased problems with herbicide carryover.
2. Misapplication. Misapplication injury from tank contamination usually results in field-wide injury, but symptoms may be worse in areas with overlaps. Field corners might be missed by a sprayer, resulting in asymptomatic plants.
3. Poorly cleaned boom. Injury from a spray boom that was not cleaned out properly usually begins near a field entrance and often takes on a characteristic W, M or V pattern as the product works its way out of the spray boom.
4. Drift. Drift injury from nearby fields or non-crop areas typically has a pattern of declining injury as one moves from the source. Herbicides that move via volatilization or products applied during inversions may produce field-wide injury. Look carefully for protected areas that might show symptoms (behind trees, terraces or tall fence lines).
Analysis of where symptoms appear on a plant can narrow the timing when plants were exposed to the herbicide. Vegetative stage soybeans typically produce a new leaf every five days (range 3 to 10), whereas flowering soybeans produce a new leaf approximately every three days (range 2 to 5). Careful observation of where symptoms occur on the plant may help determine the approximate timing of herbicide incident and help identify the source.
A basic understanding of the mode of action of the herbicide groups used in a cropping system is critical in diagnosing problems believed to be caused by herbicides. For photos of these various injury symptoms on plants and in fields, visit Identifying Common Herbicide Symptoms in Soybean.
Herbicide Group 4
The Herbicide Group 4 products mimic the activity of auxin, a plant growth regulator. They translocate to growing points, and primarily affect the development of leaves initiated following exposure.
Several HG 4 products are used in Iowa crops or adjacent areas, which can complicate pinpointing the source of injury. Picloram and aminopyralid are primarily used in pastures and non-crop areas and can persist in soil, hay and manure. Placing hay bales in fields where cattle are kept during the winter frequently results in herbicide injury if the field is planted to soybeans.
Aminopyralid is also labeled for application with dry fertilizer to pastures, creating risk of movement to soybean fields via future dry fertilizer applications. Clopyralid is used in corn and can carry over to the soybean year in certain conditions. Use of both 2,4-D and dicamba have increased with the introduction of herbicide-resistant crops, increasing the risk of misapplications or drift onto susceptible soybeans.
Soybeans exposed to HG 4 products present in the soil at planting typically produce symptoms by the V1 stage of soybean growth. Exposure with postemergence herbicide applications or drift from other areas often occurs after the soybeans have produced one or more normal leaves.
Severe injury from a misapplication of high rates of any HG 4 herbicide product causes epinasty or twisting of the stems within a few days of application; at lower doses epinasty may be absent. High rates of HG 4 products may also cause soybeans to release axillary meristems.
At low doses typically associated with drift, the common symptom of HG 4 products is strapped or cupped leaves. Generally, plant growth regulator injury on leaves is very symmetrical, meaning all three leaflets of an individual soybean leaf will be injured similarly. Note that 2,4-D often causes more of a strapped appearance to leaves, making them longer and skinnier with parallel veins. Callus tissue or cracks may form on stems as well.
Dicamba often causes leaf cupping, but veins at the tips of the leaves may be parallel and stretched. Residues of clopyralid, aminopyralid or picloram in the soil may result in a combination of symptoms including axillary meristem release and cupped, fiddle-necked, or strapped leaves. Caution is needed when assigning blame to a particular herbicide since one of the products may cause symptoms typically associated with a different herbicide. For example, dicamba causing elongated leaves rather than cupped leaves.
Herbicide Group 5
The HG 5 herbicides interfere with photosynthesis, resulting in chlorosis and necrosis of leaves. They move in the xylem and accumulate in expanded leaves. Both metribuzin and atrazine cause similar injury symptoms on soybeans, and injury may occur as a result of carryover or drift (atrazine) or direct application (metribuzin). Soybean plants have a relatively low tolerance to metribuzin; thus, injury may occur when conditions favor activity.
Both atrazine and metribuzin are more likely to injure soybeans on high pH soils. Atrazine carryover was observed this spring, as well as drift from adjacent cornfields. Injury is typically most noticeable on unifoliate and first trifoliate leaves, and it appears as interveinal chlorosis and necrosis.
Herbicide Group 14
The HG 14 herbicides, also called PPO inhibitors, interfere with photosynthesis. Due to rapid destruction of tissue, there is little translocation, and symptoms primarily occur on tissue directly contacted by the herbicide.
Both preemergence and postemergence HG 14 injury is common in soybeans. Injury from preemergence HG 14 products occurs when herbicide is splashed or washed onto tissue either at or just after soybean emergence; tissue contacted by the herbicide develops necrosis.
It is typically most noticeable on soybean hypocotyls (stems) and cotyledons. Under certain conditions, hypocotyls may be girdled, resulting in plant death. Injury from postemergence HG 14 products appears as speckling or necrotic tissue on leaves contacted by the herbicide.
Herbicide Group 15
The HG 15 herbicide products interfere with lipid synthesis, disrupting formation of cell membranes. There is little movement within plants.
Both preemergence and postemergence applications can cause heart-shaped leaflets; postemergence applications may cause other distortions to the margins of leaflets. These symptoms typically occur on the one to two leaves that emerge after the application and are often not symmetrical across all three leaflets of a leaf. Injury from postemergence applications may be confused with dicamba, but the lack of symmetry with HG 15 is helpful in differentiating these herbicides.
Herbicide Group 27
The HG 27 herbicides, also known as HPPD inhibitors or bleachers, interfere with pigments, resulting in chlorosis and bleaching. They move within the phloem, resulting in symptoms appearing on new growth.
Both carryover and drift from HG 27 products onto soybeans have been observed this year. While carryover from HG 27 products is uncommon in most years, the increased problems are likely due to late applications in 2019 combined with a dry summer. The HG 27 injury appears as bleaching or yellowing of leaves that emerge after exposure.
Symptoms may vary from nearly entire leaf bleaching to just the leaf margins. Note that HG 27 products may also cause crinkling of leaves or may appear to mimic very minor HG 4 injury, but the chlorosis distinguishes it from the HG 4 injury. Due to the common herbicide mixes used in corn, you often see HG 27 injury coupled with HG 5 or HG 4 injury.
The spread of herbicide-resistant weeds has led to an increase in herbicide application rates and the types of herbicides used in corn and soybeans. These products may have a low margin of crop safety, or may be persistent, therefore increasing the likelihood of herbicide injury. Determining the source of injury often requires a knowledge of how different herbicides interact with plants, determining what products were used in the area and identifying patterns of symptoms within the field.