Effect of Preceding Crop Type and Different Amounts of Nitrogen Fertilizer on the Yield and Quality of Rapeseed Oil

Rapeseed needs a sufficient amount of nitrogen for optimal seed yield, but the efficiency of this plant in using nitrogenous (N) fertilizers is low, which causes an increase in N consumption in the cultivation of this plant. The need to maintain a high grain yield necessitates the use of a suitable timely program and manage the consumption of N fertilizers. Thus, a field experiment was conducted at the Research Station of Agriculture and Natural Resources of Khuzestan University during 2020-2021 growing season to evaluate the effect of preceding crop and N fertilizer on grain yield of rapeseed. The experiment was carried out as split plots based on a RCBD with three replications. Preceding crops (clover-barley mixture, rapeseed, corn, fallow, mung bean, rice and wheat) were allocated to the main plots and N levels (0.0, 100, 160 and 220 kg ha-1 N from the source of urea fertilizer) were assigned to the sub plots. Results indicated that the grain yield of rapeseed depends mostly on grain number, but the grain yield components (including pods number per plant, grain number per plant, grain number per pods) were mainly affected by the main effect of preceding crop and nitrogen. With the application of 100, 160 and 220 kg ha-1 N the number of grains per pod increased by 34%, 39% and 33%, respectively. The number of grains per plant was also increased by the application of nitrogen fertilizer, where all three levels of nitrogen fertilizer resulted in more than 200% increases in this grain yield attribute. The highest grain yield of rapeseed (3877 kg ha-1) was observed when mung bean was used as preceding crop and 220 kg ha-1 N fertilizer was applied, though it was not significantly different from the treatments of mung bean preceding crop and application of 160 kg ha-1 nitrogen fertilizer (with the production of 3105 kg ha-1 grain) and fallow preceding and 160 kg ha-1 N fertilizer. The lowest grain yield of rapeseed was obtained when corn and rapeseed were used as preceding crops and no N fertilizer was applied, as these preceding crops resulted in only 488 and 497 kg ha-1 grain of rapeseed, respectively. Rapeseed had the highest oil yield (1645 kg ha-1) with mung bean being the preceding crop and 220 kg ha-1 N application. It seems that for achieving the highest oil yield of rapeseed, mung bean preceding crop and 220 kg ha-1 N can be focused on for the next researches.