Growth of rice cultivars by direct seeding and transplanting under upland and lowland conditions

Rainfed rice crops are grown under both upland and lowland conditions. While upland crops are almost always direct seeded, both transplanting and direct seeding are commonly practiced for lowland crops in Asia. Direct seeding may be by broadcasting or dibbling. The effects of these various rice cultural practices on shoot and root growth and grain yield were examined using four contrasting cultivars in each of three years at Surin, Northeast Thailand. Under upland conditions, no yield was obtained under severe drought in two of the three years and yield was less than half that under lowland conditions in the third year. Lowland crops did not suffer water stress. Under lowland conditions, direct-seeded crops yielded more than transplanted crops in one year, slightly less in another when establishment was a problem in direct seeding, and similar between the two methods in the other. Direct seeding, particularly broadcasting, produced more total dry matter than transplanting. Root dry-matter growth was small after panicle initiation under all conditions, and was greater in direct seeding than transplanting in lowland conditions. Root growth occurred mostly in the top 10 or 15 cm soil layer in both upland and lowland crops. Root mass below 30 cm depth exceeded 10% of the total root mass at maturity in only one crop in which seeds were dibbled under upland conditions. Cultivar differences in root growth were generally small among all experiments. One notable exception was high root mass near the soil surface for IR20, a cultivar suitable for irrigated conditions, when transplanted under lowland conditions. Although shoot growth was also similar among cultivars, there were often significant cultivar differences in grain yield. The highest yield was produced by IR57514-PMI-5-B-1-2, a semi-dwarf cultivar that produced a large number of panicles in most experiments.