The experiment was conducted at the research field of the Department of Agronomy, Sher-e-Bangla Agricultural University, Dhaka, Bangladesh. The site is located at 23o 77’N (latitude) and 90o 33’E (longitude) at an altitude of 8.6 m above sea level. The soil texture was clay loam, with soil pH ranging from 6.1 to 6.3 and an organic matter content of 1.29%. The area has a sub-tropical climate, characterized by high temperatures (12–40 to 31–40 °C), high relative humidity (68–78%) and heavy rainfall (30–110 mm) with occasional gusty winds during Kharif season.
Time and Plan of the Study
Four rice varieties (two inbred varieties, viz. BR11 and BRRI dhan39; and two hybrid varieties, viz. ACI 1 and ACI Shera) and four transplanting dates (20 July, 4 August, 19 August and 3 September) were used in the experiment. BR11 is slightly photosensitive whereas the rest of the varieties are not responsive to photoperiod. The growth duration of BRRI dhan 39 is 122 d whereas BR11, ACI 1 and ACI Shera have around 145 d each.
The experiment was laid out in a randomized complete block design with three replications. There were 16 treatment combinations with a total of 48 plots. The size of each plot was 4.0 × 3.0 m, and the spaces between plots and replications were 1 m and 1.5 m, respectively.
Crop Establishment and Cultural Operations
Seeds of BR11 (Mukta) and BRRI dhan39 were obtained from the Bangladesh Rice Research Institute (BRRI) at Joydebpur, Gazipur, and the hybrid rice seeds from the ACI Seed Company. Viable seeds were selected by following the specific gravity method. The seedbed was prepared by puddling with repeated ploughing and then by laddering. Selected inbred and hybrid varieties of rice seeds were sown in a nursery seedbed 30 d before transplanting.
Land Preparation
The experimental field was first ploughed with the use of a tractor-drawn disc plough. The field was then irrigated and prepared by three successive ploughings and cross ploughings with a tractor-drawn plough, and subsequently leveled by laddering.
Fertilizer Application
The experimental plots were fertilized with 120, 80, 80, 20 and 5 kg ha-1 of N, P2O5, K2O, S and Zn in the form of urea, triple superphosphate (TSP), muriate of potash (MOP), gypsum and zinc sulphate, respectively. The entire amount of triple superphosphate, muriate of potash, gypsum and zinc sulphate was applied as a basal dose during final land preparation. Urea was top-dressed in three equal installments, i.e., after seedling recovery, during the vegetative stage and at 7 d before panicle initiation.
Seedling Transplant
Seeds of each of the inbred and hybrid cultivars were sown from June to July, 2011 in nursery beds and 30-d-old seedlings were transplanted on 20 July, 4 August, 19 August and 3 September into the experimental plots. The seedbeds were irrigated by application of water 1 d prior to uprooting the seedlings to minimize mechanical injury of the roots. Seedlings were transplanted per BRRI and ACI recommendations. Gap filling and other inter-cultural operations were carried out as needed.
Harvesting and Postharvest Operations
The crop was harvested when 80–90% of the color of the grains had turned golden yellow. The inbred varieties BRRI dhan39 and BR11 were harvested on 23 January and 26 January, respectively, and the hybrid varieties ACI 1 and ACI Shera on February 02 and February 11, respectively. One m-2 area from the middle portion of each plot was separately harvested for grain and straw yield. Threshing was carried out using a pedal thresher. The grains were cleaned and sun-dried to a moisture content of 12%. Moisture was determined using a moisture meter model (G-Won Hitech Co., Gk. Korea). The straw was oven dried. Finally, grain and straw yields plot-1 were determined on an oven dry basis and converted to t ha-1.
Data Gathered
Experimental data such as plant height, number of tillers per hill, leaf area index, and plant biomass were recorded at 15-d interval after transplanting, and continued till harvest days to flowering was recorded when 50% of the plants within a plot produced flowers. Agronomic characters were based on 10 selected hills from each plot in the field at 15-d interval and at harvest.
Biological yield (dry-basis) was calculated as follows:
Biological yield = Grain yield + Straw yield
Harvest index, denoted by the ratio of economic yield to biological yield, was calculated as follows:
Harvest index (%) =Grain yield/Biological yield *100
Statistical Analysis
All data were analyzed following the analysis of variance procedure using the MSTAT-C package. Differences between means were compared by Least Significance Difference at 5% level of significance (Gomez and Gomez 1984).