Response of grain yield and other agronomic characteristics of two native and high- yield rice (Oryza sativa L.) cultivars to nitrogen fertilizer rate in Rasht

Nitrogen makes up about 2 to 5% of the dry matter and its insufficient restricts the yield of crops such as rice more than other mineral nutrients. High demand of crops to nitrogen, decreasing the soil nitrogen supply, high potential of nitrogen loss and also considerable increase in yield potential of new cultivars of rice has led to significant increase in nitrogen fertilizer application in the production of rice and other crops. Therefore, optimizing the rate of nitrogen fertilizer application is one of the important management strategies for yield improvement, reducing the costs of production and maintaining the quality of environment.
Material and To investigate the response of grain yield and other important agronomic characteristics to fertilizer nitrogen rate, and to determine the optimum fertilizer nitrogen rate in two native and improved (high-yield) rice cultivars, an experiment was conducted at research field of Rice Research Institute of Iran located in Rasht in 2013. The experiment was carried out in a randomized complete block design as a factorial experiment with four replications that rice cultivar (native cultivar Hashemi and high-yield cultivar Sepidrood) and fertilizer N rate (0, 40, 80, 120, 160 kg ha-1 of urea fertilizer) were experimental factors.
Analysis of variance showed a significant effect of cultivar, fertilizer N rate and their interactions on the grain and biological yield. Results showed that grain yield was variable from 1.92 ton per ha in control to 5.5 ton per ha in 120 kg N ha treatment in Hashemi cultivar and grain yield was varied from 4.13 ton per ha in control to 8.87 in 160 kg N ha treatment in Sepidroud cultivar. The results of regression analysis showed a different response of grain yield to increase in nitrogen fertilizer rate in two rice cultivars. The results also showed a significant linear relationship between the nitrogen fertilizer rate (X) and grain (Y = 4418.5 29.3X, R2= 0.97) and biological (Y = 7344.0 74.9X, R2= 0.99) yield (Y) in Sepidroud, while, quadratic equation had more accuracy to describe the relationship between the nitrogen fertilizer rate and grain (Y=1.6221 0.0344x -8E-05x2) and biological (Y= 2.5107 0.0973x- 0.0002x2) yield in native cultivar (Hashemi).
The results of this experiment showed that in both Hashemi native and Speedroud high-yield cultivars, the amount of fertilizer nitrogen affects grain yield, significantly. The average grain yield of Hashemi was increased by increasing the amount of fertilizer nitrogen up to 120 kg N per hectare, but increasing the amount of nitrogen fertilizer from 120 to 160 kg per hectare reduced the grain yield. However, increasing nitrogen fertilizer rate from 0 to160 kg per hectare led to linear increase in grain yield in Sepidrood cultivar. These results indicate the inability of the soil to supply crop nitrogen requirements and the necessity of the use of fertilizer nitrogen to achieve high yields, more effective use of fertilizer nitrogen in Sepidrood, and more fertilizer nitrogen rate in this cultivar to maximize grain yield. Based on these results, it is likely to achieve higher yields than maximum yield obtained in this study using fertilizer nitrogen more than 160 kg per hectare.