Probability of Existence of Pre-Emergence Herbicide-Moisture-Variety Interaction Adverse to Direct Seeded Rice on Uplands¹

In the kharif (wet season of) 1975 eight pre-emergence herbicides were tested at two dosages for weed control in upland direct seeded rice. Severe toxicity to rice seedlings was observed on the 14th day in the herbicide sprayed plots but not in others. The extent of toxicity to rice (complete death of seedlings) was 86 to 90% with dinitramine, 87 to 88% with butachlor (N-butoxymethyl-a-chloro-2‘, 6‘-diethylacetanilide). 79 to 91% with C. 288(piperophos4-dimethametryn), 66 to 89% with benthiocarb (S-4-ChlorobenzyI diethylthiocarbamate), 43 to 52% with nitrofen (2, 4-dichloro-4‘-nitrodiphenyl ether), and 38 to 62% with fluchloralin. The weed control efficiencies- with the above herbicides (calculated on the basis of dry weed weight in the treatment in comparison with that under handweeded plots at 14 days) were 74 to 76%, 83 to 89%, 81 to 93%, 69 to 78% 85 to 86% and 74 to 84% respectively. A rainfall of 136 mm received between 2 to 5 days after spraying of herbicides was suspected to be the cause for this toxicity to rice, because, in earlier years when no such high toxicity was noticed, the rainfall during a week after herbicide spray was much less. The study was repeated in Kharif, 1976 with five of the above herbicides under flooded and unflooded conditions using two varieties i.e. CR. 125-12-8 (which was used in 1975) and Annapurna. The results again showed severe toxicity in the case of dinitramine, piperophos-f dimethametryn (C. 288) and butachlor; moderate toxicity with nitrofen and low toxicity with benthiocarb though weeds were controlled efficiently in all cases. Further, the extent of toxicity to a particular herbicide varied with varieties.

The above studies indicate probability of existence of adverse herbicide-moisture-variety interactions which need extensive studies for confirmation particularly because of the high intensity of weed infestation (3470 kg/ha in the present trial) in upland direct seeded rice, indicating the need for efficient, cheap and harmless herbicides.