Experiments were conducted at the Agronomy Field Laboratory of Bangladesh Agricultural University, Mymensingh (Lat. 24075´ N and Long. 9005´ E), during three consecutive seasons of Rabi 2004-05, Kharif-II 2005 and Rabi 2005-06 to study the phenological events of soybean cultivars under variable sowing dates. The land belongs to the Sonatola Soil Series with Non-calcareous Dark Grey Floodplain soil under the Old Brahmaputra Floodplain Agro-ecological Zone (AEZ 9) (UNDP and FAO, 1988). The soil of the experimental land was loamy in texture with pH 6.84 and contained 1.5% organic matter, 0.073% total nitrogen, 12.00 ppm phosphorus, 9.5 ppm sulphur, 0.47 ppm zinc, 0.18% potassium. The experiments included six sowing dates (viz. 4 and 24 November, 14 December, 3 and 23 January and 12 February) and two soybean cultivars (Bangladesh Soybean 4 and BARI soybean 5) in Rabi 2004-05 season, four sowing dates (viz. 25 July, 14 August, 3 and 23 September and three soybean cultivars (Bangladesh Soybean 4, BARI soybean 5 and Shohag) in Kharif-II 2005 and four sowing dates (26 November, 16 December, 5 January and 25 February) and two cultivars (Bangladesh Soybean-4 and Shohag) in Rabi 2005-06 season. All the experiments were laid out in a split-plot design with sowing date as mainplot and cultivar as subplot having plot size of 5 m × 4 m in Rabi seasons and 4 m × 3 m in Kharif-II season. Each unit plot was uniformly fertilized during final land preparation @ 23-69-60-16-1 kg ha-1 N-P2O5-K2O-S-Zn through urea, triple super phosphate, muriate of potash, gypsum and zinc sulphate (BARC, 2005). Seed was inoculated with Rhizobium inoculum supplied by the Bangladesh Institute of Nuclear Agriculture @ 20 g of inoculums kg-1 of seed (BINA, 2004) and sown continuously in 30 cm apart rows (BARI, 1999) manually using @ 40 and 70 kg seed ha-1 for cultivar G-2 and BS-5, respectively on dates as specified in experimental treatment. Weeding was done by hand at 20 and 40 days after sowing (DAS). At each weeding, extra seedlings were thinned to maintain about 5 cm distances between plants in a row. Two irrigation were given at 35 and 60 DAS on 4 November 2004 sown crop (when soil moisture deficit reached about 50% of field capacity). The crop sown on 24 November, 14 December 2004, 3 January, 23 January and 12 February 2005 was irrigated at 31, 57, 37, 17 and 26 DAS, respectively. The cutworm was controlled through hand picking. The cultivar BS-5 sown on 24 November 2005.. The crop was monitored at 3-day intervals throughout the growing season and different phenological stages viz., emergence (E), flowering (R1 stage), seed formation (R5 stage), physiological maturity (R7 stage) and harvesting maturity (R8 stage) were recorded. The time when at least 50% seedlings emerged plot-1 designated as the emergence stage. The flowering stage was the time when 50% of the plants had one open flower at any node of the main stem in any plot and seed formation stage was considered when the seed became 3 (± 0.1) mm long at one of the four uppermost nodes on the main stem. The physiological maturity was the time when 50% plants had at least one mature (brown) pod on the main stem. The crop was harvested when 95% of the pods of a plot became brown and time was recorded as harvesting maturity. The data on climatic parameters for the period from January 2004 to December 2006 were recorded. Daily temperature (minimum, maximum and mean), total monthly rainfall and monthly penman evaporation were collected from records of the Weather Yard (situated 500 m away from the trial site), Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh while data on daily photoperiod and solar radiation were collected from Bangladesh Meteorological Department, Agargon, Dhaka. The photoperiod data showed that the day length was shortest in December (11.45 hours) and the longest in June (14.53 hours). From the temperature data thermal time between any two phenological stages was calculated using the following formula given by Hadley et al. (1984) and Summerfield et al. (1993).
Thermal time (Tt) = Σ [(Tmin + Tmax)/2 – Tb]
Where, Tmin = minimum temperature, Tmax = maximum temperature and Tb is the base temperature is the lowest temperature at which rate of development becomes zero. A base temperature of 6 0C was considered to calculate thermal time (Rahman et al., 2004).