Seedling recruitment in direct-seeded rice: weed biology and water management


In rice cropping, water constitutes a powerful selective agent for weed management (Mortimer and Hill 1999). Permanently flooded fields of transplanted rice that are unweeded by 45 days after transplanting (DAT) exhibit a weed flora different from fields where the soil remains saturated. Noticeable differences are also evident across toposequences where land formation imposes different drainage and flooding regimes in rainfed rice (Pane et al 2000). However, it is the timing, duration, and depth of flooding events in association with the method of crop establishment that govern the precise nature of weed suppression by water. Wet seeding onto a saturated puddled soil with subsequent flooding 10–15 days afterward in comparison with transplanting into standing water alters the dominance hierarchy of weeds with the replacement of obligate aquatic dicotyledonous species by semiaquatic species, including competitive grasses. Weed species shifts may be relatively rapid. From observations of farmers’ fields, Azmi and Mashor (1995) recorded that seven seasons of wet seeding resulted in the appearance of 21 new weed species, particularly Echinochloa species and forms of “weedy” rice, and the exclusion of 12 others. Understanding the processes underlying the selective nature of water in governing the recruitment and survival of weed species involves comparative autecological studies of seed germination and seedling and plant growth in relation to flooding regimes. These regimes may vary from permanently aerobic but moist soils, through continuously saturated anaerobic soils, to saturated soils subject to varying water depths arising from periodic flooding and drainage, either as a result of rainfall variation or as a policy of water management. Potentially, plants may experience all of these flooding regimes at different stages in their life cycle and the diversity in rice weed floras is in part because, in many semiaquatic species, the rooting systems of adult plants are tolerant of, or adapted to, submergence in flooded soils. However, controlled studies of weed community dynamics in transplanted and wet-seeded rice emphasize the fact that weeds are recruited early in the life of both crops. In transplanted rice continuously flooded at shallow (< 50 mm) or deeper (approx. 100 mm) depth, recruitment of plants to the weed community occurred only in the first 10–20 DAT and subsequently losses continued until the rice harvest. In wet-seeded rice initially sown on saturated soil and subsequently flooded to these depths by 10–15 days after seeding (DAS), recruitment of sedges and dicotyledonous species was restricted to the same early period in contrast to graminaceous species that increased in overall density up to 60 DAS (Hill et al 2001). Cohort recruitment of weeds in drySeedling recruitment in direct-seeded rice: weed biology and water management


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