Allelopathic activity of crop residue incorporation alone or mixed against rice and its associated grass weed jungle rice (Echinochloa colona [L.] Link)

Weed suppression is one of the several benefits achieved by soil incorporation of crop residues and such suppression is believed to be allelopathic in nature. The allelopathic potential of different crop residues: viz. sorghum (Sorghum bicolor [L.] Moench), sunflower (Helianthus annuus L.), brassica (Brassica campestris L.) was evaluated in rice (Oryza sativa L.) and jungle rice (Echinochloa colona [L.] Link). Chopped crop residues were soil-incorporated alone and mixed at 6 g kg^{- 1} soil (12 t ha^-1) and compared with a control without residues. Soil incorporation of residues substantially delayed germination of jungle rice. The time to start germination, time to 50% emergence, mean emergence time, emergence index, and final germination percentage were all depressed by residue incorporation. Final germination of rice and jungle rice dropped by 11 to 15% and 11 to 27% with residue application alone and by 18 to 22% and 8 to 34% with a combination of crop residues, respectively. Residues were more suppressive to germination dynamics of jungle rice than rice. Crop residues exerted a pronounced negative influence on the shoot (25 to 100% and 14 to 44%) and root lengths (22 to 100% and 10 to 43%) of rice and jungle rice, respectively. Shoot and root dry weight of both rice and jungle rice also decreased significantly. An appreciable quantity of phenolics was recorded in soil amended with sorghum+sunflower+brassica residues. Since soil incorporation of allelopathic crop residues was detrimental to both rice germination and seedling growth, it is suggested that the time of residue application for jungle rice suppression and rice seeding time need to be adjusted so as to minimize rice crop damage.