Management Strategies for Potential Herbicide-Resistant Weeds in Cotton in the San Joaquin Valley of California

Glyphosate-resistant (GR) weeds are changing the cotton (Gossypium hirsutum L.) production systems throughout the US cotton belt. This phenomenon has compelled cotton researchers in the San Joaquin Valley (SJV) of California to search for options before the onset of this problem. One such option could be the combined use of pre-emergence (PRE) residual and postemergent (POST) herbicides prior to planting.   However, these treatments may have phytoxic effects on the crop.  Therefore, the objective of this study was to assess stand loss and crop safety of POST and PRE herbicides applied at 15 and 35 days prior to planting cotton (DPP).

A two-year (2012 and 2013) study was conducted in Five Points, CA on Acala cotton Phytogen 725 RF. Planting date for both years were in early April.  In both years, the early (35 DPP) and late (15 DPP) pre-plant herbicide treatments were applied in early March and late March, respectively. Each plot was 67 feet long with 4 rows of cotton spaced 38 inches apart.  The  treatments included Prowl H₂O (pendimethalin), Caparol (prometryn), Dual Magnum (S-metolachlor), Karmex (diuron), Matrix (rimsulfuron), Sharpen (saflufenacil), Express (tribenuron), Roundup PowerMax (glyphosate), 2,4-D (dimethylamine salt) and Clarity (dicamba). Several of these herbicides are not labeled for cotton pre-plant due to potential stand loss or crop injury. However, these options need to be explored from a resistance management perspective. The herbicide applications were tilled immediately after application.

 Stand counts and crop injury evaluations were made on May and June.  In both years, there were no significant differences between any of the treatments on stand loss, crop injury, fruit retention, and plant height.  Therefore, any of these treatments could serve as an option for cotton growers as a resistance management strategy, should there be an onset of GR weeds in the SJV.