The potential for herbicide injury with preemergence herbicides is greater for soybeans than corn. The risk increases with environmental conditions that reduce crop vigor and growth rate, and with heavy rain that moves the herbicide to the depth of the germinating seed or the emerging seedling. Much of Iowa has experienced these conditions this spring. That’s why these symptoms are being found in many fields.

Iowa State University Extension weed management specialist Bob Hartzler says in the vast majority of cases, soil-applied herbicides control target weed species with little to no adverse effect on the crop. However, soybean plants are sometimes injured by these herbicides, he adds. Herbicides vary in their inherent potential to cause soybean injury. Some are rated very low for potential to cause injury, whereas other herbicide active ingredients are rated as having a greater inherent potential to cause damage.

Application rate, weather make a difference
The rate at which a herbicide is applied can influence potential for soybean injury. Also, the environment and weather have a large influence on severity of injury caused by soil-applied herbicides. Hartzler provides the following information and observations.

Sulfentrazone (Authority mixes) and flumioxazin (Valor) are Herbicide group (HG) 14 (PPO inhibitors) — products with a relatively high risk of injury. Injury occurs most often when rain increases herbicide availability, as the hypocotyl of the soybean plant approaches or emerges through the soil surface. Symptoms include necrotic lesions on the cotyledons and hypocotyl, which often are more severe in poorly drained areas of the field. In worse-case scenarios, the hypocotyl can be girdled, resulting in plant death. The herbicide may contact the apical bud, resulting in malformed leaves and occasionally death of the primary stem. 

Since HG 14 herbicides are contact materials, soybean plants usually recover quickly from the injury, except in cases where the hypocotyl is girdled or the apical bud is damaged. The low use rate of saflufenacil (Sharpen) in soybeans reduces, but doesn't eliminate, the risk of injury with this herbicide product.

Other herbicides cause problems
HG 15 amide-type herbicides are less likely to cause soybean injury than HG 14 products. The most common symptom with HG 15 is development of heart-shaped leaflets. Soybeans typically grow out of this very quickly. 

An interaction between HG 15 products and flumioxazin reduces soybean tolerance, therefore increasing the potential for adverse crop response. Fields treated with tankmixes of these products or Fierce (flumioxazin + pyroxasulfone) prior to heavy rains may exhibit symptoms.

Metribuzin (HG 5) causes interveinal chlorosis and necrosis on emerged leaves. Symptoms typically appear on unifoliates and the first trifoliate leaves; later-emerging leaves are usually unaffected. In worst-case scenarios, two or three nodes of leaves on the plant may be killed. The risk increases in fields with high-pH soils due to greater availability of the herbicide.

Pendimethalin and trifluralin (HG 3) herbicides cause swelling of the hypocotyl, reduced root growth and delayed emergence. Injury is most likely where cool, wet soils slow emergence, therefore increasing absorption of the herbicide into the emerging soybean.

Crop injury is a risk associated with using herbicides, and the lower herbicide tolerance of soybean plants compared to corn plants increases the likelihood of damage. Most preemergence products used in soybeans are either the contact-type or xylem mobile herbicides; thus, little herbicide reaches the growing point under most situations.

This increases the likelihood that injured crops will quickly recover from the damage, minimizing the risk of yield loss. Where injury is evident, stand counts should be taken to determine if stand loss has occurred, fields should be closely monitored over the next few weeks to document recovery, and finally, any field operations that could place additional stress on the crop should be avoided until normal growth resumes.

Source: Iowa State University