The discovery of glyphosate-resistant Palmer amaranth in central Arizona cotton requires the use of a diverse suite of weed control tactics.

The use of a dinitroaniline (DNA or “yellow”) pre-emergence residual herbicide at the beginning of the cotton season provides a foundation for season-long weed management. Along with tillage and other practices, DNAs are important in the struggle to delay or avoid developing herbicide resistant weed populations.

The DNAs (pendimethalin and trifluralin) are inexpensive and selective in cotton.

These suppress the emergence of small-seeded broadleaf and grass weeds and stunt the growth of larger seeded weeds including morningglory which emerge. This provides more time to spray small weed escapes and improves the effectiveness of post-emergence herbicides.

DNA herbicides are most effective when applied and mechanically incorporated with a light disk or field cultivator prior to listing and bed formation.

However, these operations require an extra pass over the field and incur additional operation costs. To avoid the extra operation, DNA herbicides can be applied with a mulcher after listing although the effectiveness is slightly reduced.

Pre-emergence herbicides can be applied after cotton emergence. DNAs or metolachlor can be sprayed on the soil surface (avoiding contact with cotton foliage and stems) and immediately incorporated with a rolling cultivator or Lilliston.

A less reliable substitute for mechanical incorporation is to simply irrigate and rely on water infiltration to incorporate the herbicide. This leaves the beds and cotton seed line without residual herbicide. Subsequent cultivations can throw soil into the seed line.

Pre-emergence herbicides will not control weeds already emerged at the time of application.

The Prowl H2O formulation of pendimethalin can be broadcast sprayed over the top of cotton alone or mixed with post-emergence herbicides including glufosinate and glyphosate and incorporated.

However, do not tank mix Prowl H2O with metolachlor, pyrithiobac, Sequence (glyphosate, Smetolachlor), prometryn, diuron, or fluometuron herbicides.

Metolachlor can also be sprayed over the top of cotton with or without glufosinate or glyphosate, but only provides about 25 days of Palmer amaranth suppression in contrast to the DNAs which last longer.

Soil texture affects the rates of DNAs needed for control. Finer soil textures (i.e., more clay and less sand) require higher rates of DNAs. Consult product labels before applying herbicides.

For the full report on this issue, including photos and charts, click on this link:

http://ag.arizona.edu/crops/cotton/files/DNA_Herbicides.pdf.