2.1 Description of the experimental location, soil and climate: The experimental field was located at 24°25' N latitude and 90°50' E longitude at an elevation of 18 m above the sea level (FAO and UNDP, 1988). The soil was more or less neutral in reaction with pH value 6.8, low in organic matter and fertility level belonging to the Sonatola series of non-calcareous dark grey floodplain soil under the Old Brahmaputra Alluvial Tract. The land type was medium-high with silty loam in texture.
2.2 Experimental treatments and design: The experimental treatment consisted of two factors. They are as follows:
Factor A: Variety (3)
i. BRRI dhan34 (V1)
ii. BRRI dhan49 (V2)
iii. BRRI dhan52 (V3)
Factor B: Mustard crop residues (5)
i. No crop residues (T1)
ii. Mustard crop residues at 3.0 t ha-1 (T2)
iii. Mustard crop residues at 2.0 t ha-1 + one hand weeding (T3)
iv. Mustard crop residues at 3.0 t ha-1 + one hand weeding (T4)
v. Three times hand weeding (T5) (at 15, 30 and 45 DAT)
The experiment was laid out in a Randomized Complete Block Design (RCBD) with three replications. Thus total number of plots were 45. Each plot size was (2.0 m × 2.0 m).
2.3 Description of the rice variety: BRRI dhan34: BRRI dhan34 was developed by Bangladesh Rice Research Institute (BRRI) and released in 1997. It attains a plant height about 117 cm. Its growth duration is 135 days. These are clean, short, bold and fine aromatic rice. This variety gives an average yield of 3.5 t ha-1 (BRRI, 2015). BRRI dhan49: BRRI dhan49, a variety of aman rice, was developed by the Bangladesh Rice Research Institute (BRRI). This variety is resistant to waterlogging conditions. It takes about 135 days to mature. It attains a plant height of 115 cm. The grains are medium bold to medium slender with light golden husks and kernels are white in color. It gives a grain yield of 5.0-5.5 t ha-1 (BRRI, 2015). BRRI dhan52: BRRI dhan52, is a flood-tolerant rice variety for aman season, released in 2007. It takes about 155-160 days (under 13-15 days submerged condition) and 140-145 days for the non-submerged condition. In submerged condition, it produces grain yield of 4.0-4.5 t ha-1 (if 10-15 days in submerged condition). This variety is capable to produce 4.5-5.0 t ha-1 in non-submerged condition. These are clean rice, grains are medium slender and kernels are white in color (BRRI, 2015).
2.4 Collection and preparation of crop residues: Mustard crop residues were used in this study. Crops were grown at the Agronomy Field Laboratory, Bangladesh Agricultural University and were harvested at the time of the ripening stage to collect crop residues. After collection, the crop residues were dried under shade in the cover threshing floor of Agronomy Field Laboratory of BAU. The studied crop residues were cut as small as possible by using sickle.
2.5 Experimental details: Seeds of the test cultivar (BRRI dhan34, BRRI dhan49 and BRRI dhan52) were collected from Agronomy Field Laboratory, Bangladesh Agricultural University, Mymensingh. The sprouted seeds were sown uniformly in a well-prepared nursery bed on 25 June 2016. Proper care was taken to raise the healthy seedlings in the nursery bed. The field was ploughed with tractor-drawn plough followed by laddering. The layout of the field was made after final land preparation. Weeds and stubbles were removed and cleaned from individual plots. The experimental plots were fertilized with urea, triple super phosphate, muriate of potash, gypsum and zinc sulphate @ 164, 60, 104, 67, 10 kg ha-1, respectively. The entire amounts of triple superphosphate, muriate of potash, gypsum and zinc sulphate were applied at the time of final land preparation. Urea was applied in three installments in equal splits at 15, 30 and 45 days after transplanting (DAT). The prepared mustard crop residues were applied at 7 days before transplanting aman rice at the time of final land preparation. After that crop residues were mixed well to the respective plots. The uprooted seedlings were immediately transferred to the main field. Healthy and similar sized seedlings were selected for transplanting. Seedlings were transplanted in the well-prepared puddle field on 26 July 2016 at the rate of three seedlings hill-1 maintaining row and hill distance of 25 cm and 15 cm, respectively. The crops were harvested at full maturity. Maturity of crops was determined when 90% of the grains became golden yellow in color. Then the harvested crops of each plot except 5 hills plot-1 was bundled separately, properly tagged and brought to the threshing floor. The crops were then threshed and the fresh weights of grain and straw were recorded from an area of l m2 in the middle of each plot. The grains were cleaned and finally, the weight was adjusted to a moisture content of 14%. The straw was sun-dried and the yields of grain and straw plot-1 were recorded and converted to t ha-1.
2.6 Data collection at different growth stages: Weed data: Weed data were taken carefully. Weed population: Data on weed population (30 days after transplanting age) were collected from each plot of the rice plants by using 0.25 m × 0.25 m quadrate as per method described (Cruz et al., 1986). The weeds within the quadrate were counted and converted to number m-2 multiplying by four. Weed dry weight: After counting the weed density, the weeds inside each quadrate were uprooting, cleaned, separated species-wise and dried first in the sun and then in an electric oven for 72 hours at a temperature of 800c. The dry weight of each species was taken by an electric balance and expressed in gm-2. Percent inhibition: Percent inhibition of weed was calculated using the following formula: % Inhibition =[ (Dry weight of weed at control – Dry weight of weed from treatment)/Dry weight of weed at control] ×100
2.7 Data collection of yield contributing characters: Data on grain and straw yields were collected from an area of l m2 in the middle of each plot. The data on other crop characters were randomly sampled from the region outside 1 m2 area (excluding the border hills) which was kept for taking data on grain and straw yields.
i) Plant height (cm): Plant height was measured from the base of the plant (ground level) to the tip of the longest panicle.
ii) Number of total tillers hill-1: Tillers that had at least one visible leaf were counted including both panicle bearing and non-bearing tillers.
iii) Number of effective tillers hill-1: The tiller which had at least one visible grain in the panicle was considered an effective tillers.
iv) Number of non-effective tillers hill-1: The tillers without panicle were counted and considered as non-effective tillers.
v) Length of panicle (cm): Measurement was taken from basal node of the rachis to the apex of last grains of each panicle.
vi) Number of grains panicle-1: The presence of any food material in the spikelet was considered as grain and such spikelet present on each panicle was counted.
vii) Number of sterile spikelets panicle-1: The spikelet that lacked any food material inside was considered as sterile spikelet and such spikelet present on each panicle was counted.
viii) Weight of 1000 grains (g): One thousand clean dried grains were counted from the seed stock obtained from five sample hills of each plot and weighed by using an electric balance. The weight was adjusted at a seed moisture content of 14%.
ix) Grain yield (t ha-1): Grains obtained from each unit plot were sun-dried and weighed carefully. The dry weight of grains of central 1 m2 areas were added to the respective unit plot yield to record the final grain yield plot-1 and finally converted to t ha-1.
x)Straw yield (t ha-1): Straw obtained from each unit plot including the straw of central 1 m2 areas of the respective unit plot was dried in the sun and weighed to record the straw yield plot-1 and finally converted to t ha-1.
xi) Biological yield (t ha-1): Grain yield together with straw yield was regarded as biological yield and calculated with the following formula:
Biological yield = grain yield + straw yield
xii) Harvest index (%): It indicates the ratio of economic yield (grain yield) to biological yield and was calculated by the following formula:
Harvest index (%) =[Grain yield/Biological yield]×100
Statistical analysis:
Data recorded for different parameters were compiled and tabulated in proper form and subjected to statistical analysis. The Analysis of variance was done with the help of the computer package MSTAT-C program. The mean differences among the treatments were adjudged by Duncan's Multiple Range Test (DMRT) as laid out (Gomez and Gomez, 1984).