A field experiment was conducted at the experimental farm of the Bangabandhu Sheikh Mujibur Rahman Agricultural University, Salna, Gazipur under wet land condition from November to May, 2013-2014. The soil of the experimental site was silty clay of Shallow Red Brown Terrace type under Salna Series of Madhupur Tract in the Agro Ecological Zone (AEZ) 28. The experimental site is situated in a sub-tropical climatic zone, characterized by heavy rainfall during the months from May to September and scanty rainfall during rest of the year. Boro rice variety BRRI dhan28 was used as test crop. Two herbicides; Pretilachlor (pre-emergence at 5 DAT) & Pyrazosulfuran-Ethyl (post- emergence at 15 DAT) were used in the experiment. There are 12 treatments such as T1 = Pretilachlor @ 1L ha-1 + Nitrogen @ 0 kg ha-1, T2 = Pretilachlor @ 1L ha-1 + Nitrogen @ 50.6 kg ha-1, T3= Pretilachlor @ 1L ha-1 + Nitrogen @ 101.2 kg ha-1), T4= Pretilachlor @ 1L ha-1 + Nitrogen @ 151.8 kg ha-1), T5= Pretilachlor@ 1L ha-1 + Nitrogen @ 202.4 kg ha-1, T6 = Pyrazosulfuron-ethyl @ 150g ha-1 + Nitrogen @ 0 kg ha-1, T7= Pyrazosulfuron-ethyl @ 150 g ha-1 + Nitrogen @ 50.6 kg ha-1, T8= Pyrazosulfuron-ethyl @ 150g ha-1 + Nitrogen @ 101.2 kg ha-1, T9= Pyrazosulfuron-ethyl @ 150g ha-1 + Nitrogen @ 151.8 kg ha-1, T10= Pyrazosulfuron-ethyl @ 150g ha-1 + Nitrogen @ 202.4 kg ha-1, T11= Control (Unweeded + without nitrogen), T12= Weed free (with recommended dose of N). The experiment was laid out in a randomized complete block design (RCBD) with three replications. A fertilizer dose of 77-80-55-5 kg of Triple Super Phosphate, Muriate of Potash, Gypsum and Zinc sulfate respectively was applied in the experimental field (BRRI, 1999). The whole amount of fertilizers was applied as basal dose during final land preparation. Urea was top-dressed in three equal installments at 20, 40 and 60 days after transplanting (DAT), respectively. Forty days old seedlings were uprooted from the seedbed on January 14, 2014. Transplanting was done with one seedling in each hill at a spacing of 20 X 15 cm on the same day. Intercultural operations such as irrigation, pest management and other necessary cultural operations were done accordingly. The crop under weed free treatment was kept weed free by continuing weeding operations every week.
Plant samplings were done at 15 days interval starting from 30 DAT until maturity. Data on different growth parameters, yield components and yield were collected. At each sampling data were recorded on the following parameters: Leaf area index (LAI) was measured from 30 DAT at 15 days interval with Leaf area meter (Model AAM-7 Hayashi Denko Co. Tokyo, Japan). Leaf area index was then calculated by using the following formula as shown below:
LAI= Leaf area for 5 hills (cm2) / Land area occupied by 5 hills (cm2)
Crop growth rate was calculated using the following formula,
CGR= GA-1 {(W2-W1) / (T2-T1)} g m-2day-1
W1= Total dry weight at time T1 (g) W2= Total dry weight at time T2 (g) T1= Initial time (day) T2= Final time (day) GA= Ground area (m2)
Total nitrogen (TN) content was determined by micro-Kjeldahl method (Bremner and Mulvaney, 1982) using this formula,
% N of Grain or Straw= (T - B) x N x 1.401 x S-1 x 100
T = Sample titration value (mL) of standard H2SO4 S = Weight of soil sample in g
B = Blank titration value (mL) of standard H2SO4 N = Strength of H2SO4.
Nitrogen use efficiency (NUE) was calculated by following formula, (Olson and Swallow, 1984)
NUE = (NUna–NUno) / NR
NUna = Nitrogen uptake (kg ha-1) by grain with addition of nutrient NUno = Nitrogen uptake (kg ha-1) by grain without addition of nutrient NR = Rate of added nutrient (kg ha-).
Visual abundance of weeds was calculated through direct eye observation of standing weeds in the experimental plots at 60 DAT. The scores were used as 4 = High; 3 = Medium; 2 = Low; 1 = Very low/negligible.
Weed control efficiency was calculated with the following formula:
Weed control efficiency (WCE) = {(DMC – DMT) / DMC} x 100
DMC = Weed dry matter in unweeded treatment DMT = Weed dry matter in weed control treatment. Crop was harvested at well-matured stage on the date of 14 May 2014. At maturity, five plants were harvested from each plot to record data on yield components and the plants from an area of 2.4 m2 was harvested on grain and straw yields. The data were compiled and tabulated with the help of the computer package Statistix 10 and the means were separated through Least Significance Difference (LSD) test.