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Dean Riechers Dean Riechers
Assistant Professor
Department of Crop Sciences
(217) 333-9655; riechers@illinois.edu
Kevin Kelley Kevin Kelley
Graduate Research Assistant
Department of Crop Sciences
(217) 244-6882; kelley@illinois.edu

Stressed-Out Soybeans: The Impact of Off-Target Plant Growth Regulator Herbicide Injury

During the past several growing seasons, a significant number of soybean plants have been reported by field crop scouts that exhibited abnormal growth and development of new leaves. These symptoms, most frequently noted on recently developed trifoliolate leaves, have typically been described as leaf cupping, puckering, or crinkling. These symptoms tend to vary widely by year, location, soybean variety, and environment, and closely resemble symptoms that are frequently observed following exposure to plant growth regulator (PGR) herbicides commonly used for weed control in corn. PGR herbicides used in corn for postemergence broadleaf weed control can injure soybeans at extremely low rates. These PGR herbicides include dicamba (found in Banvel, Clarity, Distinct and premixes Northstar, Celebrity Plus, Yukon, and Marksman), clopyralid (found in Stinger and premixes Accent Gold and Hornet), and 2,4-D.

PGR herbicides are very effective for controlling broadleaf weeds, and can also be used in corn to control weeds that are resistant to acetolactate synthase (ALS)-inhibiting and/or triazine herbicides. However, soybeans are very sensitive to PGR herbicides and the route of soybean exposure to PGR herbicides can often be difficult to determine. Soybean herbicide formulations or spray additives may inadvertently remove PGR residues from application equipment and directly apply these residues in conjunction with the postemergence soybean herbicide. Soybeans can also be exposed via application spray drift or from volatilization. Ester formulations of PGR herbicides, such as 2,4-D ester, are more volatile than salt (amine) formulations. Dicamba is also volatile and soybeans appear to be especially sensitive to dicamba. As little as 1/10,000th of a field use application rate of dicamba can cause leaf cupping symptoms on newly emerged soybean leaves.

In addition to causing severe injury to newly emerging soybean leaves, PGR injury to soybeans can also lead to delayed maturity, decreased yield, and decreased germination of harvested soybean seed. Field studies were conducted in 2001, 2002, and 2003 to determine the impact on yield of PGR injury to soybeans at different growth stages. Treatments (2,4-D, Clarity, Distinct, and Stinger) were applied at three different soybean growth stages (V3, V7, and R2), and represented very low levels of these herbicides that might unintentionally come in contact with soybeans through either spray drift or spray tank contamination. An additional study examined spray tank contamination more closely by applying typical soybean postemergence herbicides (Raptor, Pursuit, Flexstar, or glyphosate) alone and in combination with a low rate of dicamba (1% of the field use rate of Clarity in corn). Field data showed an interaction when combining dicamba-imposed stress with Flexstar, Pursuit, or Raptor, in that overall soybean yield and several yield components (including test weight and seeds per pod) were reduced up to 35%, especially at the V7 application timing. When dicamba was included with glyphosate, there was no such interaction and the yield response was basically the same as with dicamba applied alone (5 to 10% yield reduction). This study demonstrates that the stress imposed by dicamba, in conjunction with the stress imposed by postemergence herbicides typically applied in conventional soybeans, can make the foliar injury worse and further decrease grain yield.

In addition to our field studies, we are also working on developing a laboratory diagnostic test to detect the presence of PGR herbicides in soybean leaves. This test will allow for the diagnosis of the causal agent of soybean leaf cupping injury, and could help determine if the cupped soybean leaves that are frequently observed in the field are the result of PGR herbicide exposure, or from one of the common soybean leaf viruses found in Illinois.

Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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