The sowing date is often used to overcome environmental constraints on crop production. Information on the effect of this agronomic technique on the oil quality of castor bean (Ricinus communis L.) is scarce. An experiment was conducted on the basis of randomized complete blocks with three replicates for two years (2008, 2010) to evaluate the effect of different sowing dates on the oil content and fatty acid composition of castor bean under the semi-arid conditions of Tehran-Iran. The treatments were six sowing dates including 5th and 20th of April, 5th and 20th of May and 5th and 20th of June. The results showed that the seed yield, oil content and fatty acid composition were influenced by the sowing date. An earlier sowing date resulted in higher seed yield, oil content and oil yield in either year. Based on the results of the fatty acid analysis by gas chromatography (GC), fifteen fatty acids were observed in castor oil with ricinoleic acid (18:1 Δ9c-12OH) being the major oil fatty acid. The highest content of this fatty acid was obtained at the 20th of June, followed by the 5th of April, as sowing date. A delay in sowing decreased the oleic and stearic acid content but increased the ricinoleic, linoleic and linolenic acid levels in both years. Conclusively, plants sown at the 5th of April encountered favorable environmental conditions resulting in significantly high economic yield compared to other sowing dates.

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