The Impact of Pigweed (Amaranthus retroflexus L.) Population on Oil Yield and Fatty Acid Composition of Castor Bean (Ricinus communis L.)

Castor bean, as an important commercial oil-based by products are used in the manufacture of several commercially important commodities like surfactants, coatings, greases, pharmaceuticals, cosmetics, polyesters, polymers. Castor bean is a non-edible oilseed crop which produces an oil rich in Ricinoleic acid, commonly over 80%. The seeds of castor consist of 40-60% by weight of the most useful natural plant oil. Interactive effects among species on inter-species populations are called interference. Interference is one of the main issues on the eco-physiology of plant populations where weeds impose negative effects by approaching the plant to compete in light, water and nutrient elements availability and results in reduced growth and yield. Various reasons have been attributed for the low productivity of crops, among them weed competition is the most important. Redroot pigweed belongs to the Amaranthaceae family, as an important competitor to slow growth of the castor bean plant, can deprive it from nutrients. The purpose of this research was determining the effects of redroot pigweed plant density interference on the yield of oil and fatty acids composition of castor bean plant to increase quantity and quality of oil. To evaluate the effects of Pigweed interference on yield, oil content and fatty acid composition of castor oil plant, a factorial experiment was conducted based on randomized complete block design with three replications at the Center for Agricultural Research in West Azerbaijan (Urmia) (Saatlu station: 37°44´18״ N Latitude and 45° 10´ 53״ E Longitude, at 1338 m above sea level) with silty-sandy soils. The first factor included Castor plant densities (3, 4, 5 and 6 plants per square meter) and the second was Amaranth densities (zero, 5, 10 and 15 plants per square meter). Castor seeds cv. Mobarakeh were planted with red root pigweed simultaneously on 14 June 2013. After Soxhlet extraction, fatty acid analysis was performed by Gas Chromatography. Data were analyzed with SAS 9. 1 software. The results showed the significant effects of castor and amaranth densities, and their interaction effects on the castor oil yield, but only significant effect of amaranth density on the fatty acid composition. According to the results, increasing Amaranth density caused to reduction of 20-38% in castor grain yield, 39% in oil yield compared to pure cultures (without weed). Due to the unique characteristics of ricinoleic acid, it was reduced 19% by increasing pigweed density, but the palmitic acid (74%) and stearic acid (108%), linoleic acid (12%) and Oleic acid (64%) were increased compared with pure cultures (weed free). The results of the survey indicated that the pigweed at densities of 10 and 15 plants per square meter is one of the most important factors affecting yield, oil content and fatty acid composition. In this experiment, Amaranth weed caused to significant reduction of the yield of castor grain, oil and major fatty acids. In this regard, pigweed control at the beginning of the growing season will be effective to reduce seed production, in order to reducing crop yield loss and reduce the scope of its release in the next years. In general, despite the importance of castor plant there has been limited research work carried out on the crop in Iran to increase the crop competitive ability. In this present study, the oil content, oil yield and fatty acids composition of Iranian castor bean oil have been outlined. The results of this experiment showed that changes in oil content and fatty acid composition is depending on environmental conditions such as competition with weed and the Pigweed is a competitive weed which significantly reduces the quantitative (seed and oil) and qualitative (oil composition) yield. Increasing pigweed density decrease grain yield, oil yield and fatty acids composition of castor bean. The amount of this reduction was related to pigweed density. On the other hand, 6 plants/m2 of Castor bean can be recommended for reducing pigweed damage in Castor bean.