PhD

Weedy rice is the complex of morphotypes of Oryza species widely distributed in the commercial rice fields in more than 50 countries of Asia, Africa and Latin America, especially in areas where farmers have switched to direct seeding due to labour shortage and high cost. Taxonomically, Indian weedy rice is identified as Oryza sativa f. spontanea which belongs to the indica group. Invasion and spread of weedy rice in the cultivated rice fields have forced the farming community to abandon rice farming, leading to socio economic and ecological impacts in the rice bowls of Kerala.
Management of weedy rice infestation is complex mainly because of its morphological similarities to cultivated rice and lack of herbicides for selective control of weedy rice in cropped fields. Hence, the present research programme was undertaken to estimate the extent of infestation of weedy rice in rice bowls of Kerala, to study the biology and ecology, and to chalk out strategies for integrated management of weedy rice.
Survey conducted in the major rice belts of Kerala, viz., Kole, Kuttanad and Palakkad, revealed that more than 65 percent of the area has low to severe weedy rice infestation. In the major rice bowls of Kerala there was severe, moderate and low infestation of weedy rice in 26, 36 and 49 panchayaths, respectively,. The extent of infestation was more in Kuttanad in Alleppey, Kottayam and Pathanamthitta districts and Kole lands of Thrissur district. In Palakkad region, the infestation was more in panchayaths of Alathoor and Nemmara blocks. Survey revealed the presence of weedy rice variants with respect to morphological characters like number of tillers per plant, height of plant, length of ligule, panicle characters, colour of grains, and length and colour of awns.
Studies conducted on the germination of weedy rice seeds revealed that there was no significant difference between the germination of weedy rice in the presence and absence of rice seeds in the soil. It was noticed that seed germination was higher at the surface and 2 cm depth of the soil and there was drastic reduction in the germination from deeper layers of 4-8 cm. Investigations on dormancy revealed that hull induced dormancy in weedy rice could be broken by exposing seeds to 20ºC for 48 hours, treating with 0.6% nitric acid for six hours, rupturing seed coat, or maintaining electrical conductivity of 5 dS m-1 for six hours in the growing media.
Scanning electron microscope studies confirmed that lesser overlapping of lemma and palea contributed to quicker germination of half mature straw coloured weedy rice grains whereas, slow germination in black coloured mature seeds is due to the clear overlapping of lemma and palea. Presence of more number of stomata both on adaxial and abaxial surface, silica deposits and hairy out-growths on leaf, and high root volume in weedy rice contributed to its early vigour, growth and competitiveness.
Management options found effective for the control of weedy rice in direct seeded puddled rice include (1) pre sowing surface application of oxyfluorfen @ 0.2 kg ha-1 three days before sowing in thin film of water (2) selective drying of weedy rice panicles with nonselective broadspectrum herbicides glufosinate ammonium or glyphosate @ 15 % - 20 % concentration at 60-65 DAS using specially designed wick applicator/wiper device (3) stale seed bed technique with (wet tillage in between two stales under heavy infestation) application of broad spectrum herbicide followed by flooding for exhausting soil seed bank (4) straw burning after harvest of infested fields followed by zero tillage in succeeding crop and (5) soil solarisation for more than 30 days during summer months using 100 micron transparent polythene sheets.
A novel hand held wiper device was developed for the direct contact application of nonselective broadspectrum herbicides for selective drying of weedy rice panicles at 60-65 days after sowing, when rice and weedy rice plants maintain height difference of 20 cm. The device was filed for Indian Patent at Patent Office, Chennai (Application No. 1763/CHE/2014 dated 01.04.2014).
Effective and economical management of weedy rice is possible by the integrated use of different options for management depending on the intensity of infestation. Various weedy rice management options standardized in the present study were demonstrated in farmers’ fields in a participatory mode. The demonstrations have given good results and many farmers have already adopted the package for the integrated management of weedy rice.