The trial was carried out during Rabi season of 2011-12 and 2012-13 (from November to April) in the research farm of WRC, BARI, Nashipur, Dinajpur, Bangladesh. The soil of the experimental field belongs to under the ‘Old Himalayan Piedmont Plain’ designated as Agro Ecological Zone (AEZ) # 1 (FAO/UNDP, 1988) characterized by flood free highland, fine in texture (Sandy loam and Silty loam), poor in organic matter content and strongly acidic (pH ranges from 4.5 to 5.5) (WRC, 2009). It is situated in the northern part of Bangladesh and geographically the area lies between 25°38" N and 88°41" E, and 38.20 m above from the sea level. The treatments were two dates of sowing viz. 30 November (D1), 15 December (D2), 30 December (D3) & 14 January (D4) and four genotypes viz. BARI Gom-26 (V1), BAW (Bangladesh Advanced Wheat)-1151 (V2), BARI Gom-27 (V3) & BARI Gom-28 (V4). The experimental design was Split-plot. Sowing times were accommodated in main plot and the genotypes were assigned to subplot. The unit plot size was 4×4 m. The land was prepared with four times ploughing horizontally with power tiller followed by laddering in 12-15 cm depth. Each of the subplots was fertilized @ 120-30-50-20-1-4.5-5000 kg ha?1 as N-P-K-S-B-Zn-cowdung. Urea, Triple superphosphate (TSP), Muriate of potash (MoP), Gypsum, Boric acid and Zinc sulphate were used as the source of N, P, K, S, B and Zn, respectively. All of TSP, MoP, gypsum, boric acid, zinc sulphate, cowdung and 2/3 3rd of urea were applied as basal dose during final land preparation. Seeds were treated with Provax 200 WP @ 3g/Kg seeds which contains Carboxin and Thiram. After well preparation of land, seeds @ 120 kg ha-1 of each genotype were sown continuously in 20 cm apart rows in 2.5-5.0 cm depth. One sowing was performed on 30 November (Normal growing environment; NS) and another on 30 December (heat stress environment; LS).The rest amount of urea was applied as top dress at crown root initiation (CRI) stage followed by first irrigation (at 20 Days after sowing; DAS). The second irrigation was applied at late booting stage (55 DAS) and another applied at early grain filling stage (75 DAS). The data of numbers of tillers, days to booting, heading, anthesis, physiological and harvest maturity, yield and yield contributing characters {Spike m-2, spike length, spikelets spike-1, grain spike-1, Thousand Grain Weight (TGW) and grain yield} were taken. The number of tillers was counted from randomly selected five lines (each length 1.0 m) in the field standing crop. After full maturity, the crop was harvested plot wise according to treatments. Sample plants were harvested separately with sickle from an area of 2.4×3 m (i.e., 3 m long, 12 middle rows) of each subplot avoiding border effects. The harvested crop of each subplot was bundled separately, tagged and taken to a threshing floor. The bundles were thoroughly dried in bright sunshine until fully dried, then weighed and threshed. Threshed grains of each subplot were again dried with sunshine and weighed; lastly grain yield was converted into t ha-1. To obtain the actual yield of all genotypes, grain yield weight was adjusted at 12% moisture by the following equation (Hellevang, 1995):
Y (M2) = 100-M1 / 100-M2 × Y (M1)
Where, Y (M2) = weight of grain at expected moisture percentage (generally 12% for wheat)
Y (M1) = weight of grain at present moisture percentage
M1 = present moisture percentage
M2 = expected moisture percentage
. Data were analyzed using MSTAT-C. Treatment means were compared for significance by the least significant difference (LSD) test at P ≤ 0.05.