With the temperamental weather we have experienced this spring, stressed crops and herbicide injury are always possible, particularly in corn from PRE applications. The primary ALS inhibitor corn herbicides used PRE in our region include products that contain flumetsulam (Hornet and Surestart II), thiencarbazone (Corvus and TriVolt), and rimsulfuron (Basis Blend and Resolve products). The potential for ALS injury increases with rapid uptake of the herbicides during warm wet weather conditions.
Due to the rainy and cloudy weather in some areas, the crops have been struggling to grow and likely will be more sensitive to postemergence applications. Certain formulations, such as EC (emulsifiable concentrate) formulations, compared to encapsulated formulations (CS and ME), can cause more injury. For example, Degree Xtra and Prowl H2O are generally safer on emerged corn than their counterparts Harness Xtra and Prowl 3.3EC, respectively. There is also an increased crop injury potential when combining residual herbicides with loaded glyphosate products or adding additional adjuvants to the spray mix.
Regarding application timing after environmental stress, the general rule of thumb is to allow a few sunny days to pass after coming out of a rainy, cool period before applying herbicides (but we may not have that luxury this season). Since the plants are stressed, this allows them time to build up a thicker leaf surface and get their metabolic processes functioning faster to detoxify the herbicide. With all the moisture, warmer temperatures, and eventual sunlight, the plants will be growing very quickly and are succulent, so consider using nonionic surfactant (NIS) instead of crop oil concentrate (COC) or methylated seed oil (MSO) as the spray additive.
As always, it is rarely the herbicide alone that produces crop injury. Consider the weather (temperature and moisture), planting technique (seed depth and coverage, slit closure, etc.), herbicide rates, combinations, and timing relative to the crop, and other environmental, soil type, and management factors that may contribute to the problem. Thoroughly consider the possible reasons for the symptoms before you draw conclusions. Here is a summary of the potential for herbicide injury and the associated symptoms.
Group 15
This group of herbicides includes Dual II Magnum, Harness/Surpass/Keystone, Outlook, Zidua/Anthem Maxx and their associated atrazine and other premixes, as well as many generic products. These herbicides are classified as shoot inhibitors, and injury is typically seen as leafing out underground or tightly wrapped leaves that fail or have difficulty unfurling. Emerged plants may appear buggy-whipped. The acetamides do not generally impact the root system.
The injury potential for the acetamide herbicides to corn is generally not great; we rate them as "Good crop tolerance" in the Agronomy Guide. As with most soil-applied herbicides, the injury potential increases on lighter, lower organic matter soils under cool wet conditions with shallow planted corn. The injury potential can also increase depending on a tank-mix partner with herbicides such as 2,4-D, dicamba, and pendimethalin (Prowl), sometimes increasing the severity of the injury.
Group 4
Dicamba is the active ingredient in Clarity, various other formulations and generic products, and a component of Status and Yukon. Dicamba is classified as a growth regulator herbicide. The potential for injury with soil applications of dicamba increases with lighter, coarse/sandy/shaley, low organic matter soils, cool, wet weather, shallow planted corn, a seed that is not adequately covered during the planting operation, and heavy rainfall soon after application. Corn should be planted a minimum of 1.5 inches deep if dicamba is applied preemergence. Dicamba injury from soil applications may range from stunting to pruned and thickened roots, twisting, swelling, and onion leafing of shoots. 
The injury potential from foliar applications of dicamba generally increases with warmer temperatures and increasing corn maturity. Foliar applications of dicamba should be applied by V3/V4 to reduce the risk of crop injury. Crop and methylated seed oils as well as rates above 0.25 lb ai/acre (½ pint Clarity) can increase the injury potential with foliar applications. (Many of these previously mentioned concepts apply to 2,4-D injury as well.)
Group 3
Pendimethalin is the active ingredient in Prowl and several generic brands. Pendimethalin is classified as a root inhibitor herbicide. Pendimethalin should be applied to corn after planting and to corn planted at least 1½ inches deep. Pendimethalin injury potential increases with cool wet weather, shallow planted corn, light, lower organic matter soils, and with corn seed that is not adequately covered by soil (open slits). The Agronomy Guide rates pendimethalin as "Good crop tolerance" as long as proper management practices are used.
The symptoms of pendimethalin injury include pruned roots, swollen root tips, and stunted plants or seedlings. Emerged plants may appear purple due to the secondary effects of phosphorus deficiency as a result of inhibited root growth. Soil crusting can enhance injury. Acetamide herbicides are frequently used in combination with pendimethalin and may contribute to the observed injury symptoms.
Group 2
The primary ALS inhibitor corn herbicides used PRE in our region include products that contain thiencarbazone (Corvus and TriVolt), rimsulfuron (Basis Blend and Resolve products), and flumetsulam (Surestart II and Hornet) as well as a few generic products. As with the several other soil-applied herbicides already mentioned, the potential for Group 2 injury increases with cool, wet weather, shallowly planted corn (plant at least 1½ inches deep), and lighter, lower organic matter soils. The injury often appeared worse where soil property/conditions also exist, such as on clay knobs, and areas planted when wet, producing compaction and cloudiness. The Agronomy Guide denotes ALS injury potential to corn as "Fair."
ALS herbicides can stunt both corn root and shoot growth. The roots can be "bottle brushed" but are more often simply reduced in lateral growth and development. The roots may also appear slightly thickened and swollen. The shoots may appear stunted and purple to yellow with interveinal chlorosis.
The potential for ALS injury increases with rapid uptake of the herbicides during warm, wet weather conditions. Much of this injury appeared as stunted, somewhat chlorotic corn, with our injury ratings ranging from 10 to 25%. This type of injury is frequently seen with abundant rainfalls during May, followed by the hot weather of early June, which stimulates herbicide uptake above the tolerance level for corn. If good growing conditions occur during the weeks following injury, the corn usually recovers with little or no impact on grain yields.
Group 27
Balance Flexx, Corvus, TriVolt (isoxaflutole), and mesotrione (Callisto and a component of Acuron, Lumax, Lexar, Storen, and others) are pigment inhibitor herbicides preventing the development of chlorophyll in susceptible species turning plants white. Soil applications of Balance/Corvus/TriVolt hold many of the same restrictions and concerns as pendimethalin and ALS herbicides ("Fair-Good" rating). Cool wet weather, shallow-planted corn, open seed furrows, lower organic matter, lighter textured soils, and higher use rates increase the possibility of injury.
The potential for mesotrione injury from soil applications is usually less than isoxaflutole ("Good" rating). Mesotrione applied post can occasionally injure corn, and like many foliar applications, warm, humid weather generally increases the potential for injury. With mesotrione, corn generally quickly recovers from any post-applied yellowing or bleaching-type symptoms. Mesotrione has a greater risk of injury to corn with certain OP and carbamate insecticides as well as in applications that include methylated seed oils.
