Descurainia sophia (flixweed) is an annual weed widely distributed in cultivated crops, forage and rangelands throughout the world. This weed, in particular its seed, is used for food, medicinal and industrial purposes. The objective of this study was to investigate the effects of irrigation management and different application rates of nitrogen on morphological traits and yield components of this weed as a means of enhancing production and economic returns. A two-year field experiment was conducted in a semi-arid region in Iran during the 2009–2010 and 2010–2011 growing seasons. The experiment was a split plot in a randomized complete block design with three replicates, with irrigation treatments as main plots and nitrogen rates as sub plots. The three irrigation regimes consisted of 0.1 maximum allowable depletion or deficiency (MAD) of available soil water (ASW), 0.2 MAD of ASW, 0.4 MAD of ASW in 2010 and 0.2 MAD of ASW, 0.4 MAD of ASW and 0.8 MAD of ASW in 2011. The plants were grown at three nitrogen rates of 0, 200, and 300 kg N ha−1. Generally there were significant increases in seed yield, biomass, straw yield, harvest index, number of siliques (pods) per plant, seed weight, plant height, time to maturity, water use efficiency, protein concentration in seed, straw N concentration, nitrogen uptakes and nitrogen harvest index each growing season by applying nitrogen at all irrigation treatments. There was a decreasing trend in number of plants per m2, nitrogen utilization efficiency and oil concentration with increasing nitrogen application under all the irrigation treatments. Seed yield response to irrigation treatments and N rates seemed to be more related to number of plants per m2 and number of siliques per plant. The highest seed yield was obtained at the plant population of 686 plants per m2. The results obtained here suggest that nitrogen application can improve the seed yield and seed quality of D. sophia grown under different irrigation regimes. Averaged over both years, the application of 300 kg N ha−1 under 0.2 MA

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