Transplanting young seedlings in irrigated rice fields: Early and high tiller production enhanced grain yield

Transplanting rice seedlings 20 days old or older has been commonly reported to generate an increase in grain yield as a result of higher tiller production. A series of experiments was conducted at the IRRI farm during the dry and wet seasons to quantify, in a range of plant types, the impact of even younger seedlings and contrasting nursery management on grain yield and to identify plant traits supporting high performance under a given establishment technique. Seedling age at transplanting, ranging from 7 to 21 days, and contrasting nursery types (seedling tray, dapog, mat nursery, and traditional wet-bed seeding) were evaluated for an elite line, a new plant type and hybrid rice. To avoid any confounding effect, sowing date in the nursery, seed rate and crop management in the main field were all the same. In the two seasons, and for all genotypes and nursery types, transplanting older seedlings induced a delay in the onset of linear dry matter accumulation and tiller emergence, while the rate of dry matter accumulation and tiller emergence was unchanged. This delay reduced nitrogen content in the seedlings. Plants recovered quickly, however, after transplanting. The delay also reduced maximum tiller number, and extended crop duration with delayed maximum tillering, flowering and maturity. Grain yield was consistently higher for younger seedlings, with, in some cases, a difference as large as 1tha-¹ between 7- and 21-day transplanting. This result was valid for the four genotypes evaluated, with a higher impact during the dry season. In contrast, no significant difference was observed for the influence of nursery type on the timing of tiller emergence and on grain yield. Some differences in seedling vigor (plant dry weight, specific leaf area, N content), higher in the case of dapog and wet bed, and in maximum tillering, higher in the case of the seedling tray, however, were observed. But these differences did not have a significant impact on the late increase in crop dry matter and on panicle number at maturity. No significant interaction between seedling age and nursery management for all genotypes and for all the parameters measured was found. Promoting early tiller emergence as a response to transplanting young seedlings increased grain yield in all cases despite the associated decrease in tillering efficiency. Extended growth inside the nursery, rather than transplanting shock per se, appeared to be the main reason for delayed tiller emergence in lat