Climate changes and the cost of irrigation water in semi-arid areas seriously reduce the availability of water for irrigation. The optimal allocation of water resources to irrigation and limit water overexploitation are necessary in these regions. A field trial aimed to evaluate irrigation regimes (60, 80, and 100% based on field capacity, abbreviated as FC60, FC80 and FC100) and organic fertilizer (0, 15, and 30 t ha-1 farmyard manure, abbreviated as FYM) on two rapeseed varieties (Hydromel and Nathalie) in the semi-arid region of Qazvin, Iran. The highest lateral growth (branch number) was observed in the Hydromel cultivar with the application of 15 and 30 t ha-1 farmyard manure (FYM30 and FYM15) under FC100 and FC80 conditions. Comparison of lateral growth between the cultivars showed that Nathalie cultivar was less affected by FYM and irrigation. The chlorophyll content decreased under FC60; however, no significant difference was observed between FC80 and FC100. The maturity of Nathalie was earlier than that of Hydromel. However, the use of FYM significantly extended the days to maturity in Hydromel. Although the silique length of the Nathalie cultivar was 13% less than Hydromel, this component significantly decreased in Hydromel under water deficit conditions (FC80 and FC60). The highest number of siliques was recorded in the Hydromel under FYM30+FC100 and FYM30+FC80. The 60% irrigation regime caused a significant reduction in the number of siliques. Although the seed yield of Hydromel was higher than that of Nathalie, the stability of the seed yield in Nathalie was more evident under FC80 and FC60. Overall, the obtained results showed that the Hydromel variety can produce an acceptable yield under FYM30+FC80 and save 20% in water consumption. The results indicated that the 60% irrigation regime was a stressful deficit irrigation and cannot be recommended for this semi-arid region.
