By Alyssa Essman, Weed Scientist, Adapted from C.O.R.N. 2022-25
As in years past, we are hearing reports of soybean damage caused by off-target movement of plant growth regulator (PGR) herbicides. Off-target movement can be classified as primary or secondary. Primary herbicide movement takes place at the time of application, also referred to as particle drift. Nozzle type, droplet size, sprayer speed and other management factors affect particle drift, along with wind speed. Particle drift is not influenced by herbicide formulation. Plant injury from primary movement typically has a distinct pattern, often occurring along field edges closest to the treated field and becoming less noticeable farther from the source. Secondary herbicide movement occurs after the time of application and is often used in reference to vapor drift (volatility) or wind erosion. This source of off-target spread is extremely problematic and can be very difficult to predict. There is not always a tell-tale pattern of injury. The growth regulator herbicides 2,4-D and dicamba can be particularly volatile due to their chemical makeup and high vapor pressure. Formulation greatly affects the volatility of 2,4-D. Higher temperatures and lower humidity generally increase the potential for secondary movement of these herbicides. Contamination of shuttles or sprayers with a growth regulator is another source of injury. This can resemble off-target movement but is typically more uniform over the treated area. Injury from growth regulators often appears within 7 to 14 days following an off-target event or contaminated application. Symptoms of PGR (group 4 & 19) herbicide injury include leaf cupping, leaf strapping, epinasty (stem twisting), and plant stunting. Soybean tolerance to sublethal rates of these herbicides varies between the different active ingredients even within the same mode of action classification.
Soybean is extremely sensitive to dicamba, with injury symptoms occurring at rates as low as 1/20,000x of a labeled application rate. The introduction of crops tolerant to dicamba greatly increased use of this herbicide by allowing for POST applications later in the season. Newer dicamba formulations are purported to reduce the risk of secondary movement but have not eliminated this. In 2021, there were 34 official reports of dicamba injury in Ohio, compared to 28 in 2017 (the first year for legal over the top applications). These reports likely underestimate actual damage to soybean, ornamental, horticultural and residential areas. Most instances of injury from off target movement are not reported. Typical dicamba injury includes leaf cupping with a white or yellow leaf tip and plant stunting, but there can be a wide range of symptoms.
PGR damage can also occur later in the season as a result of other dicamba products used in corn, such as DiFlexx, and Status (dicamba + diflufenzopyr; group 4 + 19). Other PGR’s such as clopyralid (group 4; Stinger) or Enlist One/Duo (group 4; 2,4-D) can cause injury similar in appearance. All of these herbicides can cause dicamba-like symptomology when used near sensitive soybean. Soybean sensitivity varies based on herbicide active ingredient. According to research at the University of Illinois, soybean sensitivity to PGR herbicides is as follows: dicamba > dicamba + diflufenzopyr > clopyralid > 2,4-D (see fact sheet #2). The addition of diflufenzopyr does not seem to increase injury to soybean relative to dicamba alone. Clopyralid injury can take place due to residue carryover or off-target movement. Clopyralid has a long half-life in the soil and risk of injury is increased in years with low rainfall. Typical PGR symptoms can occur following carryover or off-target movement of clopyralid, but injury may be less severe with more rapid recovery, compared with dicamba. Soybean is least sensitive to 2,4-D relative to the other PGR herbicides discussed. Whereas dicamba causes severe leaf cupping, 2,4-D tends to result in more leaf strapping (plant veins parallel) and callus formation on stems. Several years ago, OSU was involved in a multi-state study looking at dicamba and 2,4-D injury at various soybean growth stages. Injury from 2,4-D was occasionally undetectable, and higher rates were necessary to cause similar yield reduction that occurred with lower rates of dicamba (see fact sheet #3). Product and rate used, as well as weather conditions following off-target movement influence damage severity and yield potential.
Some speculation has occurred over the years about other sources that can cause injury that appears to be from PGRs. Leaf malformation (crinkling) can result from postemergence applications of acetochlor (Warrant). There is no evidence that AMS (ammonium sulfate), glufosinate (Liberty), or PPO inhibitors (Flexstar) cause the leaf cupping associated with plant growth regulator injury. We have also heard of dicamba-like symptoms (leaf cupping, spike-like appearance of upper soybean stem) following application of 2,4-D to Enlist soybeans. Per EPA guidelines, generic 2,4-D can have up to 250 ppm of dicamba contamination, and only 100 ppm are required to cause visible injury to soybean. Dicamba applications to tolerant soybean systems are not legal past the June 30 cutoff. Other potential sources of off-target injury can occur from PGR applications to ditches, pastures, lawns, and contaminated water movement.
Some fact sheets worth reviewing:
Bish M, Oseland E, Bradley K (2021) Off-target pesticide movement: a review of our current understanding of drift due to inversions and secondary movement. Weed Technol. 35: 345– 356. doi: 10.1017/wet.2020.138