2.1 Description of the Experimental Site The experimental site is located at 24o75´N latitude and 90o50´ E longitude at an elevation of 18 m above the sea level belonging to noncalcareous dark grey floodplain soil under Sonatala series of Old Brahmaputra Floodplain Agro-ecological zone (AEZ-9) of Bangladesh. The experimental field was a medium high land with silty clay loam soil texture having pH value of 6.8. The climate of the experimental area is characterized by high temperature accompanied by moderately high rainfall during April to September, and low temperature during October to March. During the growing season (November 2015 to March 2016), monthly average temperature, relative humidity, air pressure, wind speed, solar radiation, dew point and water temperature were 18.0–25.5 °C, 75-84%, 1009.1–1013.8 mb, 2.33–5.17 km h-1, 194.0–239.9 W m-2, 14.1–20.0 °C and 16.1–24.8°C, respectively, while monthly total rainfall and sunshine hours were 0–104.8 mm and 84.7–200 h, respectively. The soil temperature at a depth of 5, 10, 20 and 30 cm were 19.0–27.5, 19.6–27.4, 19.9–26.8 and 18.7–25.4°C, respectively.
2.2 Experimental Treatments and Design The experiment comprised two weeding regimes viz., (a) weed-free and (b) weedy condition, and ten high yielding Bangladeshi wheat varieties e.g., (i) BARI gom 21, (ii) BARI gom 24, (iii) BARI gom 22, (iv) BARI gom 23, (v) BARI gom 25, (vi) BARI gom 26, (vii) BARI gom 27, (viii) BARI gom 28, (ix) BARI gom 29, (x) BARI gom 30. Plots without wheat (weed monoculture) were also maintained to study the natural growth of weeds in absence of wheat.
2.3 Crop Husbandry The land was first opened with a power tiller and subsequently leveled by laddering. Weeds and stubbles of the previous crop were collected and removed from the field. The land was fertilized with urea, triple superphosphate (TSP), muriate of potash (MoP) and gypsum at the rate of 200, 160, 50 and 120 kg ha-1, respectively. The entire amount of TSP, MoP and gypsum and two-third of urea were applied at the time of final land preparation. The rest one-third of urea was top-dressed at crown root initiation (CRI) stage, i.e., 20 days after sowing (DAS). Seeds were sown on 15 November 2015 in 20 cm apart lines @ 120 kg seed ha-1. Care was taken to avoid bird damage up to 15 days after sowing. Irrigation was given twice at 20 and 50 DAS.
2.4 Data Collection
2.4.1 Weed-related data A quadrate of size 0.5 m × 0.5 m was placed randomly in two places of each weedy plot for collecting weed samples. Weeds were clipped to ground level, identified and counted by weed species, and separately oven-dried at 70 ºC to constant weight. According to a study, dominant weed species were identified using the summed dominance ratio (SDR).