ABSTRACT In the present investigation BR11 and BR28 paddy were grown in Aman and Boro seasons respectively. Three herbicides:
Rifit 500 EC, Ronstar 25 EC and Machete 5G were applied in different doses to
control the weeds and ultimate yield and yield components in both Aman and Boro paddy. Eleven different treatment viz.: T1 (control, no herbicide was used), T2 (only hand
weeding was done), T3 (Rifit normal dose), T4 (double the dose of T3), T5 (half of the dose of T3), T6 (Ronstar normal dose), T7 (double the dose of T6), T8 (half of the dose of T6), T9(normal dose of Machete), T10(double the dose of T9) and T11(half of the dose of T9) were used in the present investigation. A total of 35 weed species of 28 genera and 13 families were found to occur in BR11 paddy grown in Aman season in first (W1), second (W2) and third (W3) weedings. In First weeding, 35 weed species under 28 genera and 13 families were obtained. In first, second and third weedings, the highest number of individual species was found in sp. no. 6. Fimbristylis miliaceae (L.) Vahl. The lowest number. of individual species was obtained in spp no. 4, 28, 29, 31 and 33. In second weeding, 27 weed species under 21 genera and 8 families were found to occur. In second and third weeding spp. no. 4, 5, 28, 29 and 30 were not found. They did not grow after first weeding. In Boro season, the highest number. of individual species was in sp. no. 7 Cyperus difformis (L), sp. no.16 Lindernia rotundifolia L. Alston and sp. no.6. Fimbristylis miliaceae in first, second and third weedings respectively. In first weeding spp. no. 15, 19, 27, 28 and 32 were absent but they were found in Aman season. In second and third weedings spp. no. 4,5,14, 28 and 29 were not found. They were controlled after first weeding. The number of weed species in T4 gradually decreased from W1 to W2 and in W3 no weed species was found. It indicates that double the dose of Rifit at T4 has positive effect in the control of weed species in BR11 Aman paddy. In Boro paddy, T6 was found effective to reduce the number of weed species in W1 and W2. In W3 almost all the weed species were found absent. In this case Ronstar 25 EC at normal dose has positive effect to control the weed species. In both Aman and Boro seasons the number of tillers, length per tiller, length per panicle, area of flag leaves, number of filled grains, percentage of filled grains, straw and grain yield per hector were found maximum at T3 when normal dose of Rifit 500 EC was applied. The number of weed species was also found to control gradually from W1 to W3 in both the Aman and Boro season in T9.