371 [ I Control of Chara with oxadiazon and copper sulphate in waterlogged rice fields in India P. Guha* Agricultural Engineering Department, Indian Institute of Technology, Kharagpur-721302, India Abstract Keywords Experiments were carried out during 1982-1984 in Chara-infested areas under farmer's-field conditions. Under the worst crop-weed competition, the alga accumulated ~ 1060kg dry matter ha-t and removed 21.14 kg N ha- t causing a reduction in rice yield o f ~ 41%. For safe, effective and economic control of Chara, a pre-cmergence spray application ofoxadiazon at 2 kg ha- t was found to be highly effective and comparable to the efficacy ofcopper sulphate at 16 kg ha- i. The dose ofcopper sulphate could be reduced to 0.75 kg ha- i when applied in combination with an equal amount of oxadiazon. Combined application of these two herbicides was found to be much more effective than their single application. Postemergence application of the herbicides by means of mud bails, sand mix or the usual spray was ineffective. Chara; oxadiazon; copper sulphate: herbicides; rice; combined application Introduction In India ~ 80% ofthe 19.5 x 106 ha ofrainfed lowland rice fields are subjected to waterlogging ofbetween 5 and 49 cm (Rao and Biswas, 1979). Such fields are often infested with a variety of aquatic weeds, among which the green algal genus Chara predominates. Mukherjee (1968) reported that out of 4.4x106ha of lowland rice fields, ~0.4× 106ha in the coastal areas of West Bengal are infested with this alga, causing a 10-20% reduction in rice yield every year. Yield losses in rice ranging from 10 to 100% have also been reported from India and Thailand (Guha, Mittra and Chatterji, 1984). Control of this weed by manual methods has several limitations, besides being costly. Moreover, the clones inadvertently left after hand weeding may intensify the infestation through prolific vegetative regeneration. Chemical control therefore appears to be the only means of suppressing this weed but attempts with many chemicals have failed to control this hardy alga (Guha, 1987). However, fortunately oxadiazon has shown some promise (Vongsaroj, Sangtong and Notaya, 1981). Although copper sulphate has also been recommended for inhibition of the algal growth, it needs either a high dose or repeated applications, which may be conducive to development of resistant lines (Mukherjee and Sengupta, 1964) and crop toxicity. In view of these constraints, experiments were carried out to find out a safe, effective and economic method of control of Chara in waterlogged rice fields. *Address correspondence to: Dr P. Guha, ZH 3/203, liT, Kharagpur (WB), India Materials and methods Field experiments were conducted during three consecu- tive 'kharif' seaspns at two different villages located in the chronically Chara-infested areas of Midnapore district of West Bengal. The first and third experiments were carried out at Nachinda with 11 and 12 treatments, respectively, during 1982 and 1984 (Tables 1 and 3) and the second experiment was carried out at Sukunia with 12 treatments during 1983 (Table2). All the trials were laid out in randomized block design with three replications under farmer's-field conditions. The herbicidal treatments were applied to stagnant water in the fields during the morning hours with different doses of zineb (Dithane Z-78 75% WP), oxadiazon (Ronstar 25EC) and copper sulphate (CuSOa.5H20, Analar grade, taken as 100% a.i.). These chemicals were applied by spraying as pre-emergence treatments, whereas for postemergence treatment three different methods of application were adopted: these included application with dry sand (80kgha-1) or dry mud balls of ~ 0.5 g (50 kg ha- ~) soaked with an aqueous solution of the chemicals, or the usual direct spray (4001 ha- ~). Two local rice varieties, 'Medi' and 'Patnai', were cultivated according to local practice as test crops in the first and the other two experiments, respectively, using a fertilizer dose of 40:20:20 kg NPK ha- t and a spacing of 20 x 20 cm. The phytotoxicity of the herbicides on the rice seedlings was recorded at 15 and 30 days after treatment with visual ratings in the scale of 0-10, where zero was no effect and 10 was complete kill. The weed biomass present in each,, of the net plots (4 m × 5 m), the harvest area, was collected before crop harvesting, washed to remove mud, oven-dried at 60°C for 48 h and the dry weight subsequently recorded. 0261-2194/91/0510371- 04 © 1991 Butterworth-Heinemann Ltd