The study was conducted at the Agronomy Field Laboratory, Department of Agronomy, Bangladesh Agricultural University, Mymensingh (240 75/ N latitude and 900 50/ E longitude in the south-west part of the old Brahmaputra plain). The experimental site was a medium-high land with sandy clay loam texture (50 % sand, 23 % silt, 27 % clay) and pH 7.2. The mean monthly maximum and minimum air temperatures were 29.5 and 23.10 C, and 29.6 and 23.40 C recorded during the growing seasons of 2013 and 2014, respectively. The highest air temperature was recorded in July (maximum 32.3o C and minimum 26.8o C in 2013 and maximum 32.5o C and minimum 26.7o C in 2014). Temperature declined gradually from July to November (29.6 to 23.10C) during both years. The total rainfall received during the cropping period (June to November) was 1287 mm in 2013 and 1625 mm in 2014. Sufficient rain water was available in 2013 during transplantation and establishment of rice due to heavy rainfall in July (339 mm). In 2014, comparatively less rainfall (300 mm) was recorded in the month of July than 2013 but no additional irrigation was required at that period for rice transplantation and seedling establishment. The highest rainfall of 2014 was recorded in the month of August (568.6 mm). In the study, ten weed control treatments viz. T1 = weedy check, T2 = Weed-free check (four manual weedings done at 20, 35, 50 and 65 days after transplanting), T3 = Pyrazosulfuron-ethyl fb (followed by) hand weeding (HW) at 25 days after transplanting (DAT), T4 = Butachlor fb HW at 25 DAT, T5 = Pyrazosulfuron- ethyl fb acetochlor + bensulfuron methyl, T6 = Butachlor fb acetochlor + bensulfuron methyl, T7 = Pyrazosulfuron-ethyl fb orthosulfamuron fb butachlor + propanil, T8 = Butachlor fb orthosulfamuron fb butachlor + propanil, T9 = Pyrazosulfuron-ethyl fb orthosulfamuron fb 2,4-Damine, T10 = Butachlor fb orthosulfamuron fb 2,4-D amine during the first year and fifteen treatments viz. T1 = weedy check, T2 = Weed-free check, T3 = Pyrazosulfuron-ethyl, T4 = Butachlor, T5 = Orthosufamuron, T6 = Pyrazosulfuron-ethyl fb butachlor + propanil, T7 = Butachlor fb butachlor + propanil, T8 = Orthosulfamuron fb butachlor + propanil, T9 = Pyrazosulfuron-ethyl fb 2,4-D amine, T10 = Butachlor fb 2,4-D amine, T11 = Orthosulfamuron fb 2,4-D amine, T12= Pyrazosulfuron- ethyl fb orthosulfamuron fb butachlor + propanil, T13 = Butachlor fb orthosulfamuron fb butachlor + propanil, T14 = Pyrazosulfuron-ethyl fb orthosulfamuron fb 2,4-D amine and T15 = Butachlor fb orthosulfamuron fb 2,4- D amine during the second year were studied. The experiment was laid out in a randomized complete block design with three replications. In 2013, acetochlor + bensulfuron methyl (proprietary mixture) herbicide had phytotoxic effect on rice; therefore this herbicide was discarded in 2014.
Before starting the experiment, the existing weeds of the field were initially killed by application of pre-planting non-selective herbicide, Roundup® (glyphosate 41 % SL- IPA salt) @ 75 mL/ 10 L water (2.25 L ha-1) on 12 July 2013 and 10 July 2014. After one week, strip tillage was done in the field by Versatile Multi-Crop Planter (VMP) maintaining 20 cm line spacing (Haque et al., 2016). The land was fertilized with phosphorus, potassium, sulphur and zinc @ 20, 35, 10 and 1.5 kg ha-1 as triple super phosphate, muriate of potash, gypsum and ZnSO4, respectively just before strip-tilled the field. Then, land was inundated to 3-5 cm depth of standing water for 48 hours. After two days, 25- day-old rice seedlings of cv. BINA dhan-7 were transplanted at 15 cm spacing between hills apart from 20 cm strips allocating three seedlings per hill. Nitrogen was applied @ 70 kg N ha-1 as urea into two installments, at 7 and 35 days after transplanting (DAT). Herbicides were applied by hand operated knapsack sprayer fitted with flat-fan nozzle at a spray volume of 300 L ha-1.
Weed density and biomass were taken from three randomly selected quadrats of 0.25 m2 (50 cm x 50 cm) each at 20, 35 and 50 DAT (data at 20 and 50 DAT were not presented as these were less well correlated with grain yield) to evaluate the efficacy of herbicides. The weed density was counted in plants m-2 and the weed dry matter was recorded in g m-2 after oven drying the samples at 70 0C for 72 hrs. Weed control efficiency (WCE) and weed control index (WCI) were calculated using the equations of Devasenpathy et al. (2008).
WCE (%) = (WPc - WPt)/WPc x 100
Where, WPc = Weed population (no. m-2) in
control (weedy) plot and WPt = Weed population (no. m-2) in treated plot
WCI (%) = (DMPc- DMPt)/DMPc x 100
Where, DMPc = Weed dry matter production
in control plot and DMPt = Weed dry matter production in treated plot
Plant height and yield attributing characters were recorded from five randomly selected hills before harvesting the whole plot. Grain and straw yields were recorded by harvesting the crop from the central area 6 m2 (2 m × 3 m) area of the plot and grain yield was adjusted at 14% moisture level. Percent yield increase over control (YOC) was calculated by the following formula (Devasenpathy et al., 2008).
YOC (%) = (TY - WY)/WY x 100 Where, TY = Grain yield in weed control treatment and WY = Grain yield in weedy treatment
Economic analysis was carried out to determine the cost-effectiveness of different herbicide treatments following the procedure by Parvez et al. (2013). Four manual weeding operations were considered sufficient to keep the plots weed-free throughout the growing season. Labour required for one manual weeding and one herbicide spraying ha-1 area were 25 and 2 person day-1, respectively. The cost required for one labour was Taka 250 day-1. Herbicide requirement was calculated by the amount of commercial product ha-1 and the cost of each herbicide was calculated based on their local market price. The net return ha-1 for each treatment was calculated by deducting the total cost (fixed cost + weed management cost) from the gross return.
Data were subjected to analysis of variance (ANOVA) and means were compared by Tukeys’s Honest Significant Difference (HSD) test at P<0.05 using statistical package program ‘Statistical Tool for Agricultural Research (STAR) nebula’ developed by International Rice Research Institute (version 2.0.1, January 2014).