study of optimum transplanting date and nitrogen application in a double-cropping system of rice (oryza sativa l.) for ‘kuhsaar’ cultivar in mazandaran

background and objectives: recently, double-cropping of rice in mazandaran due to favorable climate condition is highly interested by the rice farmers. so that, after harvesting of main crop of rice, farmers can grow again one crop of rice during summer, especially in the years which climate conditions are suitable for double cropping of rice in a rice-rice growing system. therefore, development of suitable field practice and management to produce optimum level of grain yield of cultivars which released for double cropping rice system is crucial. hence, this experiment was carried out to develop a suitable field practices, including transplanting date and nitrogen (n) application rate on kuhsaar cultivar in a double - cropping system of rice in mazandaran, amol during 2012. materials and methods: with this experiment rice cultivar so called kuhsaar was used and the effect of two factors studied in this experiment including: 1) transplanting date in three levels (10 days interval started from 9th august 2012), and 2) nitrogen fertilizer rates (0, 25 and 50 kg/ha) were tested in a factorial experiment arranged in randomized complete blocks design (rcbd) with 3 replications. results: results of analysis of collected data in this experiment indicated that the effect of transplanting dates was significantly different for traits such as days to 50 percent flowering, harvest index, number of filled grain/panicle, and grain yield (α < 0.05), and plant height, flag leaf area, un-filled spikelet /panicle (α < 0.01). also, effect of nitrogen rate was significant for effective tillers/plant (α < 0.01), and interaction of transplanting date/ nitrogen rate was also significant for two traits including effective tillers/plant and harvest index. traits mean comparison through least significant differences (lsd) method showed that 1st transplanting date (august 9th) performed a higher number of grain/ panicle (74.93), as the main component of grain yield, with a maximum grain yield (4194.7 kg/ha). we found that traits such as panicle/plant and panicle harvest index (phi) were in maximum level in 1st and 2nd dates (19th and 29th august) of transplanting. conclusion: according to the results of testing two factors of interest, including transplanting dates and nitrogen application rates we found that through transplanting of this cultivar earlier on 9th august in summer that climate factors affecting plant growth such as temperature and radiation which are in optimum condition could be suitable field practices in a rice-rice cultivation system, will produce the optimum amount of grain yield.