Corn brittleness and growth regulator herbicides
by Bob Hartzler

July 6, 1998 - Last week's storms resulted in widespread breakage of corn stalks.  It is well known that growth regulator herbicides (2,4-D; dicamba) can increase the brittleness of corn stalks, therefore increasing the potential for green snap.  One of the questions that is often asked is how long after herbicide application does this herbicide-induced brittleness last?  Although there isn't a lot of published research on this, Earl G. Rogers (Iowa State College) conducted research on this topic in the early 1950's.   The research was published in Plant Physiology.

In this study, 2,4-D was applied to corn at several growth stages (two leaf to 17 leaf) at rates of 0.5 and 1.0 lb/A.  Brittleness was tested  2, 5, 11 and 18 days after application by tying a cord 2 to 12" above the soil surface to two adjacent plants and measuring the force required to snap the stalks by pulling on the cord.  Two inbreds and two single cross hybrids were studied in the research.  The main findings of the research were:

1) Differences in brittleness were observed in the four genetic lines studied, and the lines responded differently to 2,4-D.

2)  Brittleness was severe only in plants treated with 2,4-D at the 6 to 11 leaf stage.

3)  Stalk brittleness following application was observed 2 days after treatment, peaked at 5 to 11 days after treatment, and had largely disappeared after 18 days.

The results correlate well with observations in the field.   The four lines varied widely in their potential for green snap, and they were most susceptible during periods of rapid growth.  Most important was the aspect of the study looking at the duration of the brittleness.  The increase in brittleness due to 2,4-D was relatively short-lived, with a maximum effect observed approximately one week after application and declined rapidly within two weeks of application. 

Applications of a growth regulator herbicide made early in the season while the corn is in the 2 to 4 leaf stage should have minimal impact on stalk brittleness.  Later applications significantly increase the risk of green snap, but the effect of the herbicide on brittleness is not long-lasting.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University