The study was conducted at the Agronomy Field Laboratory, Bangladesh Agricultural University, Mymensingh from November 2013 to March 2014. The experimental site is located at 24°75/ N latitude and 90°50/ E longitude in the south-west part of Old Brahmaputra under the agro-ecological zone Old Brahmaputra Floodplain (AEZ-9) belonging to non-calcareous dark grey floodplain soil. The experimental field was a well drained medium high land with sandy clay loam soil texture (sand-50.0%, silt- 23.4% and clay-26.6%) having a pH of 6.8. The climate of the experimental area was under the subtropical region and characterized by high temperature, high relative humidity and heavy rainfall with occasional gusty winds during the kharif season (April to September) and scanty rainfall associated with moderately low temperature during rabi season (October to April). Weather information regarding monthly average maximum and minimum air temperature and monthly total rainfall prevailed at the experimental site during the study period.
The study was conducted by following bioassay method to evaluate the residual effect of herbicides applied in unpuddled transplanted aman rice on the succeeding crops, such as wheat, lentil and sunflower. Each of these crops was sown in the same plots after harvesting of unpuddled transplanted aman rice. In unpuddled transplanted aman rice, 4 pre-emergence herbicides, 1 early post- and 3 post-emergence herbicides were applied in combinations of 18 herbicide treatments along with a weedy and a weed free control viz, T1=Weedy control, T2= Weed free (4 hand weeding), T3=Pendimethalin followed by (fb) hand weeding (HW) at 25 days after transplanting (DAT), T4=Pyrazosulfuron-ethyl fb HW at 25 DAT, T5=Butachlor fb HW at 25 DAT, T6=Pretilachlor fb HW at 25 DAT, T7=Pendimethalin fb
(acetochlor + bensulfuron methyl), T8=Pyrazosulfuron- ethyl fb (acetochlor + bensulfuron methyl), T9=Butachlor fb (acetochlor + bensulfuron methyl), T10=Pretilachlor fb (acetochlor + bensulfuron methyl), T11 =Pendimethalin fb orthosulfamuron fb (butachlor + propanil), T12=Pyrazosulfuron-ethyl fb orthosulfamuron fb (butachlor + propanil), T13=Butachlor fb orthosulfamuron fb (butachlor + propanil), T14=Pretilachlor fb orthosulfamuron fb (butachlor+propanil), T15= Pendimethalin fb orthosulfamuron fb 2,4-D amine, T16=Pyrazosulfuron-ethyl fb orthosulfamuron fb 2,4-D amine, T17=Butachlor fb orthosulfamuron fb 2,4-D amine and T18= Pretilachlor fb orthosulfamuron fb 2,4-D amine. The experiment was set up in a randomized complete block design (RCBD) with three replications. Unit plot size was 3 m x 4 m. All herbicides were applied in unpuddled transplanted aman rice at their recommended rate (Table 1) from 25 July to 16 August 2013 (Pendimethalin, pyrazosulfuron-ethyl, pretilachlor and butachlor were applied at 3 days after rice transplanting (DAT), orthosulfamuron at 15 DAT, acetochlor + bensulfuron methyl, butachlor+propanil and 2,4-D amine at 25 DAT).
Aman rice was harvested on 04 November 2013 and after harvesting, pre-planting knockdown herbicide glyphosate @ 2.25 L ha-1 was applied on 16 November 2013 to kill the existing weeds of the field. Then wheat, lentil and sunflower were grown in 1m x 1m micro-plots within each of the main plots on 26 November 2013. Before seeding, micro-plots were prepared through required number of spading and other earth work and then the land was fertilized by phosphorus, potassium and sulphur @ 32, 25, 17 kg ha-1, respectively in the form of triple super phosphate, muriate of potash and gypsum, respectively. Nitrogen fertilizer was applied @ 46 kg ha-1 into two installments at 0 and 15 days after sowing of wheat, lentil and sunflower in the form of urea. 100 seeds of wheat (var. BARI Gom-26), lentil (var. BARIMasur-6) and sunflower (var. BARI Surjomukhi-2) were sown in each of the micro-plots by dibbling. The crop was monitored frequently from date of sowing and data were collected from each of the micro-plots up to 25 days after sowing (DAS). Germination of the three crops was counted from 5 DAS to 25 DAS. Seedling shoot length was measured from the base (ground level) of the randomly selected five plants to the tip of the longest leaf. These plants were uprooted and cleaned with distilled water; oven dried at 70o C for 48 hrs and then shoot dry matter was recorded. In case of Leaf chlorophyll content, data were taken from two spots of the young, tender leaf of randomly selected five plants from each micro-plot by SPAD meter and the average value was taken. The collected data were analyzed statistically and means were compared by DMRT using MSTAT-C.