Effect of glyphosate on the castor plant Ricinus communis L.

The presence of ricin in castor seed has impeded the re-introduction of castor in the US. One concern is the potential of castor to contaminate nearby fields. Since most major crops in the US are resistant to the herbicide glyphosate, we evaluated the susceptibility of castor plants to 0.62% glyphosate, suited to that for corn plants resistant to glyphosate. Younger castor plants were more susceptible to the herbicide, with half of the plants and all the inflorescences dead after one treatment. The 9-week-old castor plants required two spray treatments resulting in significant defoliation with 15% of the plants being necrotic or entirely defoliated. Of 148 inflorescences, 5 bore apparently normal seed pods. However, many seeds in these pods were below normal in mass and oil content. Only 2% germinated vs. > 90% for untreated castor seed. Thus, even on maturing castor plants, glyphosate treatment can be effective in preventing volunteer castor plants.