Published online by Cambridge University Press: 27 March 2009
Pre-emergence herbicides applied at field recommended doses, 3 days after transplanting (DAT) rice plants, inhibited growth and N2-fixation of Azolla pinnata (Bangkok) and BGA (blue-green algae) inoculated 10 DAT. This inhibition was up to 15 DAT in Azolla and up to 20 DAT in BGA. Butachlor and Oxadiazon resulted in higher toxicity to Azolla and BGA than Benthiocarb and Pendimethalin. The application of 0·5 kg/ha active ingredient of 2,4-DNa did not inhibit growth of Azolla but inhibited BGA growth. However, 2,4-DEE, a post-emergence herbicide, applied 30 DAT showed inhibitory effects on the growth and N2-fixation of both Azolla and BGA. Inoculation of 2·0 t/ha of fresh Azolla 10 DAT produced maximum biomass within 20–25 days of herbicide treatments, depending upon the season. The inoculation of 10 kg/ha of a dry mixture of BGA 10 DAT could produce the maximum biomass 60 and 80 DAT in control and herbicide treated plots, respectively. The biomass and nitrogen produced by Azolla recorded at maximum mat formation were similar in both herbicide treated and untreated plots, but in BGA these were higher in controls than those of herbicide treated plots.
The use of Azolla and BGA biofertilizers along with herbicides increased the grain and straw yields, and panicle number and nitrogen uptake, by rice over no Azolla or BGA treatments. The Azolla and BGA treatments even without weeding increased rice yield up to that of herbicide and biofertilizer treatments.