Castor (Ricinus communis L.) tolerance and weed control with preemergence herbicides

U.S. castor production is limited in part due to only one labeled herbicide (trifluralin) for effective weed control. Field studies were conducted in south Texas and in the Texas High Plains in 2008, 2009, 2012, and 2013 to evaluate castor tolerance and weed efficacy following the application of preemergence (PRE) herbicides. During years of little or poor rainfall, no castor injury was noted following the application of PRE herbicides. However, when rainfall occurred soon after application of PRE herbicides, linuron doses of 1.12. kg/ha or greater resulted in stunting of castor plants and reduced stands when compared with the non-treated control. Acetochlor, clomazone, pendimethalin, and trifluralin consistently caused the least injury to castor plants. However, castor bean yield in many instances was not affected by herbicide injury. Diuron at doses of 1.12. kg/ha or greater and the combination of diuron plus linuron at 0.56. kg/ha or greater effectively controlled (> 80%) smellmelon (Cucumis melo L. var. Dudaim Naud.) and Palmer amaranth (Amaranthus palmeri S. Wats), but provided inconsistent control (0-99%) of Texas millet [Urochloa texana (Buckl.) R.Webster]. Results of these studies indicated that castor tolerance to PRE applications of acetochlor, clomazone, and pendimethalin was excellent, thus providing the greatest opportunity for future use in castor production.