Comparative Study of Weed Control Methods in Rice

Field investigation was made on the second crop (wet season) of 1964 at Chiayi Agricultural Experiment Station to evaluate the effectiveness of 13 herbicides and herbicide mixturers together with weedy and weeded checks for controlling weeds in rice. A total of 15 treatments were arranged in randomized complete blocks with four replications. The effects of herbicide treatments on rice were evaluated by weed counts, cover estimates, weed control ratings, phytotoxicity estimates, weed weight, agronomic eharacters and grain yield. Some findings of this investigation may be summarized as follows: Ametryne and Simetryne gave best control of weeds, particularly of aquatic floating broadleaves which were apparently not killed by other chemicals. When weeds, especially grasses were rather large at time of treatment, they were not controlled by PCP-Na, MCPA and 2, 4-D. Propanil gave poor control of weeds under the conditions of this experiment. Rice was injured severely by Ametryne, PCP-Na and Simetryne. MCPCA was also found to have certain inhibitory effects on the growth of rice. Propanil and Swep were not toxic to rice under flooded condition. Broadleaved weeds, especially those of aquatics dominated the weed population of the experimental plots at the early stage of rice growth, but their position was apparently taken over by grasses at the late stage. The change in weed population was clearly reflected in weed weight which was primarily contributed by grasses. Sedges were not an important problem. Weeds hastened the heading of rice while chemical injury delayed it. Plant height and tillering ability of rice was significantly reduced at the early stage of growth. Panicle length and grain weight were not affected by herbicide treatments but weeds seemed to reduce number of grains per panicle. DBN, MCPCA, PCP-Na, Swep, Simetryne, Hydram and a mixture of Swep+ MCPA produced more grain yields than weeded check. DBN yielded 563kg or 12.0% more than weeded check and 1,320 kg or 33.6% more than weedy check, the latter being significant at 1% level. Weedy check reduced grain yield Ly 757kg or 19.3%, a difference exceeding the 5% level of probability, when compared with weeded check, indicating a serious loss of yield by weed competition. Ametryne gave the lowest yield due to the reduction of stands caused by its toxicity to rice.