Water management in irrigated rice - Rice Knowledge Bank ...

Flooded yield (t ha –1 )87A65432102001 2002 2003 2004 2005% Aerobic yield1009080706050403020100ContinuousNewRestored2001 2002 2003 2004 2005YearFig. 3.9. Yield of aerobic rice variety Apo under floodedconditions (A) and relative yield of Apo under aerobicconditions (as % of the Apo yield under flooded conditions)(B). In B, the black columns indicate continuous aerobicconditions, the darkly shaded column indicates yield undernew aerobic conditions after conversion of flooded fields,and the lightly shaded column indicates yield under restoredaerobic conditions after 1 year of fallow or flooded conditions(average is given). Data from Bouman et al (2006a).flooded rice-rice cropping (Roland Buresh, personalcommunication).There are indications that soil-borne pestsand diseases such as nematodes, root aphids, andfungi occur more in nonflooded rice systems thanin flooded rice systems (Sharma et al 2002, Singhet al 2002, Ventura and Watanabe 1978, Venturaet al 1981). Current experience is that under fullyaerobic soil conditions, rice cannot be grown continuouslyon the same piece of land each year (ascan be successfully done with flooded rice) withouta yield decline (George et al 2002). Figure 3.9presents recent data from a continuous aerobic ricecropping experiment at IRRI (Bouman et al 2005,Peng et al 2006). Since 2001, aerobic rice varietyApo has been continuously grown under floodedand aerobic conditions in the same field. Floodedyields in the dry season are usually 6.5−7 t ha −1 ,except in 2001, when diseases depressed yields. InBYield (kg ha –1 )2,5002,0001,5001,0005000060120N (t ha –1 )160200Fig. 3.10. Yield of aerobic rice variety Apo under aerobic soilconditions at IRRI in 2004, at four levels of fertilizer N supply,and under three levels of water input (black column = 1,147mm, darkly shaded column = 772 mm, and lightly shadedcolumn = 632 mm). Unpublished data, IRRI.2001, yield under aerobic conditions was 86% ofthat under flooded conditions, but this graduallydeclined until it was only 45% in 2005. In 2003,half of the flooded fields were converted to aerobicconditions, and aerobic yields returned to 85% ofthe flooded yields, as in the first year, 2001. In 2004,half of the continuous aerobic fields were left fallowor were flooded for the whole year, and returnedto aerobic conditions in 2005. This “restoration”attempt brought aerobic yields back to 65% offlooded yields, and was only partially successful.The mechanisms behind the gradual yield declineand the restoration effect are not yet understood,although high levels of the nematode Melodoigynegraminicola are found in the aerobic rice fields(up to 3,000 counts g −1 fresh root) compared withthe flooded fields (6−400 counts g −1 fresh root;unpublished data).In some field experiments and farmers’ fields,nearly complete yield failure has been observed infields where aerobic rice was established the veryfirst year (Fig. 3.10). Usually, such fields had a lightsoil texture and a history of being partially croppedto upland crops or to rice under nonflooded conditionsbefore aerobic rice got established. Always,nematodes were found when yield failures wereobserved, sometimes aggravated by the presence ofroot aphids, fungi, and/or nutrient disorders. Croprotation is necessary under such conditions, andbreeders are trying to develop aerobic rice varietieswith tolerance of these soil sicknesses.33