by Bob Hartzler, Department of Agronomy
A fairly common occurrence in late June and July is the appearance of
distorted leaves on soybean plants. The most common cause of this response is
exposure to a growth regulator herbicide (2,4-D; dicamba; etc.).
Dicamba
is the most commonly used growth regulator herbicide in Iowa crop production,
and is present in numerous products (Banvel, Clarity, Marksman, Distinct,
Status, and many others).
Soybeans may be exposed to
dicamba in three ways: spray particle drift, vapor drift
(volatilization), and sprayer contamination. Injury from spray drift or sprayer
contamination usually is fairly straightforward in diagnosing, but questions
often arise regarding volatilization. Volatilization occurs when dicamba is
sprayed, lands on the target site, but later evaporates and moves from the
treated field. The potential for volatilization is affected by many factors. As
temperatures exceed 85 °F, the threat of volatilization increases.
Dicamba
is more likely to evaporate off corn leaves than soil, so the risk increases
with late applications of dicamba to corn since more product will be intercepted
by corn leaves. Risk of volatilization varies among dicamba formulations, with
the dimethlyamine salt (Banvel) greater than the diglycolamine salt (Clarity) or
sodium salt (Distinct, Status). Studies have shown that the majority of
volatilization occurs within 2-3 days of application. Determining the potential
for yield impact from growth regulator herbicides shortly after exposure is
difficult, but if only two or three leaves show slight distortion, it is likely
yields will not be affected. Studies found that if soybean height at the end of
the season was not affected, then yields usually were not affected by dicamba
exposure.It generally is accepted that occasionally soybeans will develop
symptoms similar to that of growth regulator herbicides in the absence of the
herbicide. These symptoms typically occur during stress or periods of rapid
fluctuating weather. Under these conditions, the symptoms typically appear
uniformly across a field, rather than displaying a spray or drift pattern. With
this type of response, the cupping is relatively minor and limited to one or two
leaves.
Yangzhou pioneer chemical CO.,LTD.
