RICE RATOONING - IRRI books - International Rice Research Institute

170 RICE RATOONINGnoted that deep N placement in the ratoon crop produced more panicles per plant,more spikelets per panicle, and more vigorous ratoon plants, resulting in asignificantly higher yield than did an equal amount of broadcast N.Water managementAlthough only about 60% of the water normally required for the main crop is needed(10), water is very essential to a successful ratoon crop. In India and Taiwan,irrigated ratoon crops yielded higher than nonirrigated ratoons (18, 23). Dry-seededrices produced more ratoon tillers and higher yields when water was abundant thanwhen it was only moderate (33) (Table 2).However, availability of water, per se, does not assure rice ratooning success.Time of flooding the field is equally critical. Some researchers have suggested thatthe field be kept drained but moist for 2 wk (31) or at least 10 d (4, 25) after harvest ofthe main crop to promote sprouting. Others suggest that flooding be withheld untilratoon tillers are 10-15 cm high (2). Irrigating the field immediately after harvest maycause rotting of stubble, preventing ratooning (20). Hsieh and Young (25) reportedthat fewer hills produced shoots in flooded than in nonflooded ratoon plots.A significant interaction existed between flooding time and cutting height.When stubble was cut lower, delaying irrigation 4-6 d was better than flooding 1 dafter cutting (4, 21, 36, 38). However, many hills died when the crop was cut atground level and flooded 5 cm deep (27). Bahar and De Datta (4) reported thatplants cut at ground level and immediately flooded gave significantly lower yieldsthan plants cut at 15 cm with the same water regime (Table 9). Percentage of missinghills increased as time between flooding and cutting height shortened, significantlyso in the ground-level cutting. Differences between ground-level and 15-cm cuttingwere significant from 0 to 12 d flooding after harvest.Plant growth regulatorsPlant growth regulators (PGR) have been reported to affect plant growth anddevelopment (32). Although studies on using PGR to improve the performance ofrice ratoons are very limited, preliminary trials indicate that PGR enhanced growthand increased grain yield of ratoon crops.Hou (22) observed that benzyladenine (BA), kinetin, gibberellic acid (GA 3 ),and B9 at certain concentrations increased percentage of sprouted buds overuntreated ratoons (Fig. 3). BA and kinetin produced the most sprouted buds. Budsprouting response to PGR treatment was generally highest at the milk stage.There were also indications of changes in panicle number, spikelet sterility,and yield (22) (Table 10). Again, BA and kinetin gave the most panicles. BAincreased panicle number when applied at 25 or at 40 d after transplanting (DT) andkinetin did so when applied 25 DT, followed by 40 DT, regardless of PGR rate.Spikelet sterility increased with increased concentrations of GA 3 , irrespective ofapplication time. However, BA generally produced more sterile spikelets than GA 3 .Kinetin (50 ppm) applied at the milk stage and B9 (600 ppm) applied 40 DT gave thehighest yields. Except for B9, all PGR gave higher grain yields when applied at themilk stage than at other stages.