MSc

Pokkali is a unique and sustainable rice farming system in coastal saline soils of Kerala, where rice - prawn rotational system is the traditional system followed. Pokkali system though unique in the world, is vanishing due to many practical problems and socio - economic constraints.
The weed flora of Pokkali rice fields is different from that of other rice ecosystems. Weeds survive and dominate in Pokkali ecosystem by developing morphological and anatomical adaptive mechanisms. Aquatic and semi aquatic weeds occur during the crop season and get dried up and decay in the off-season (high saline phase). Hence no weed management practices are taken up during the period of crop production. With time, deviations in soil fertility status and variations in many other climatic factors like mean rainfall and number of rainy days have been observed. As a result, the weed spectrum has also undergone changes. However, not many studies have been conducted on the weed spectrum or contributing factors to weed shifts in Pokkali areas.
The present study entitled “Weed dynamics in acid saline Pokkali ecosystem” was conducted in three parts. The first part was survey in the five major Pokkali growing areas in Ernakulum and Alappuzha districts viz., Paravur, Palluruthy, Vypin, Kalamassery and Pattanakad blocks. Two panchayaths from each block were selected for surveying. The predominant weeds reported in the Pokkali ecosystem were Diplachne fusca and Eleocharis dulcis.
A total of 36 weeds were identified from Pokkali ecosystem during low saline phase and that included the weed flora in the actual rice fields, bunds and water channels. The newly reported weeds of the ecosystem include Rhizophora mangle,
Hygroryza aristata, Hydrilla verticillata, Najas graminea, Fuirena umbellata, Schoenoplectus lateriflorus, Hygrophila schulli, Cleome burmanii, Ipomoea sp., Digitaria sanguinalis, Lindernia sp., Sesbania sp., Pennisetum clandestinum, Cyperus exaltatus, C. javanicus, Achrostichum aureum, and Exoecaria agallocha. Echinochloa cruss-galli, a dominant weed reported in earlier weed surveys has now confined to a very few locations only and the frequency has now reduced from 80 per cent to 12.5 per cent
When weeds in the field alone were considered, Vypin block was found to have highest species richness (S) and diversity index (H). Similarity coefficients between the blocks indicated that Pattanakad and Kalamassery blocks were highly similar, followed by Vypin and Paravur blocks. When total weeds in the ecosystem were considered, Palluruthy block was seen to have highest species richness and diversity indices.
The second part was a pot culture study of three predominant weeds found during the survey. Experimental design was CRD with three dominant weeds at two salinity levels. Presence of micro-hairs and higher K+ / Na+ ratio were the tolerance mechanisms observed in D. fusca. For E. dulcis and S. zeylanica, higher K+ / Na+ ratio and presence of more aerenchymatous voids were observed.
The third part was the field study to assess the extent of crop loss due to weed infestation in Pokkali system. Factorial RBD was the experimental design used with two rice varieties (Vyttila -6 and Chettivirippu) and three weed management practices (unweeded control, weeding at 20 DAT and weed free condition) as the two factors. From the biometric characters recorded it was observed that the plant height and tiller number per m2 were found significantly higher in weed free conditions. Yield attributes like panicles per m2 and grains per panicle were also found significantly higher in weed free plot. Grain yield in weed free treatment had 19 per cent yield advantage over the
unweeded control check. However the increase in yield of weed free treatment over single weeding was only 4 per cent.
The highest benefit - cost ratio was recorded in weed free treatments of both varieties. However, the ratio of single weeded treatment was found comparable with the weed free treatment. Thus, it can be concluded that a single weeding at 20 DAT is an inevitable practice which can be recommended to farmers for better returns