By Calvin Trostle (ctrostle@ag.tamu.edu);  The growth regulator type herbicide, 2,4-dichlorophenoxyacetic acid, or 2,4-D, has a long history of versatile weed control in U.S. agriculture for many crops. Two main formulations exist for 2,4-D: 1) “esters”, which tend to have a higher level of chemical activity on weeds, but more potential injury for small grains, especially on jointing stage through near boot stage; 2) “amines”, which are softer than ester formulations, with a slightly reduced injury potential, and are less prone to vapor drift. Purdue University’s “Amine or Ester, Which is Better?, http://www.btny.purdue.edu/weedscience/2004/articles/amineester04.pdf, explains well these key differences between the two formulations.

Vapor drift is a concern with nearby sensitive crops, and the “low volatility” formulations of the ester form, LV4 and LV6, are commonly used to reduce the potential for off-target movement of the chemical.

Texas A&M AgriLife Extension has for a long time has taken a conservative approach to the use of 2,4-D products as well as MCPA (2-methy-4-chlorophenoxyacetic acid) in wheat due to injury concerns. Former state extension small grains specialist Dr. Travis Miller discusses 2,4-D in relation to wheat growth and development in “Growth Stages of Wheat: Identification and Understanding Improve Crop Management,” http://varietytesting.tamu.edu/wheat/docs/mime-5.pdf

In this wheat growth guide, Dr. Miller notes that by Feekes growth stage 6.0—first node visible (which means jointing is occurring (Figures 1 & 2)—all applications of phenoxy herbicides like 2,4-D, MCPA, and dicamba should have been applied. These chemicals can be translocated to the growing point (developing head) and cause potential injury. In contrast, sulfonylurea herbicide products are labeled to initial boot stage or even initial flag leaf emergence.

However, most 2,4-D labels as well as MCPA state that wheat can be treated up to just prior to boot stage. Texas A&M AgriLife Extension’s “Weed Control Recommendation in Wheat,” (2008) (http://varietytesting.tamu.edu/wheat/otherpublications/B-6139%202008%20Weed%20Control.pdf) repeats label directions for 2,4-D and MCPA with these prolonged application windows. Several of these labels caution, however, that producers should either refrain from 2,4-D applications or reduce the rate if injury is not acceptable. AgriLife weed scientists and chemical company representatives suggest that MCPA ester is much safer on small grains than either 2,4-D formulation and is a thus a safer choice if using a phenoxy for later applications (but also slightly less effective).

Calvin Trostle, Professor & Extension Agronomist Lubbock, TX ctrostle@ag.tamu.edu