Effect of nitrogen fertilizer and weed management on the yield of transplant aman rice

The research work was conducted at the Agronomy Field Laboratory, Bangladesh Agricultural University, Mymensingh during aman season from July to December, 2015 to study the effect of nitrogen fertilizer and weed management on the growth and yield of transplant aman rice cv. BRRI dhan46. The experiment consisted of four fertilizer treatments viz. 0 kg N ha (N0), 40 kg N ha(N1), 80 kg N ha(N2) and 120 kg N ha(N3) and four weeding treatments viz. one hand weeding at 20 DAT (W1), two hand weedings at 20 and 35 DAT (W2), three hand weedings at 20, 35 and 50 DAT (W3), weeding by Japanese rice weeder twice at 20 and 35 DAT (W4) and unweeded control (W5). The experiment was laid out in randomized complete block design with three replications. The highest plant height (113.00 cm), number of total tillers hill (8.74), number of effective tillers hill (6.18), panicle length (21.98 cm), number of grains panicle (114.20), grain yield (4.00 t ha), straw yield (5.25 t ha) and biological yield (9.25 t ha) were recorded in N2 (80 kg N ha) treatment. The lowest plant height (106.00 cm), number of total tillers hill (7.20), number of effective tillers hill (5.00), panicle length (20.70 cm), number of grains panicle (97.60), grain yield (3.52 t ha), straw yield (4.46 t ha) and biological yield (7.97 t ha) were recorded from N0 (No nitrogen fertilizer control) treatment. On the other hand, the highest grain yield 4.23 t ha was observed in three weedings condition because of the highest number of effective tillers hill (6.81), number of grains panicle(111.10). The highest straw yield (5.51 t ha) was also found in three weeding condition. The lowest grain yield (3.40 t ha) was recorded in W5 (unweeded control) treatment. The lowest straw yield (4.31 t ha) was also observed in W5 (unweeded control) treatment because of the smallest plant (106.97 cm) and lower number of total tillers hill (7.20). Therefore, 80 kg N haalong with three hand weedings at 20, 35 and 50 DAT may be used for obtaining the highest grain and straw yields of BRRI dhan46. Introduction Agriculture is the largest employment sector in Bangladesh. As of 2016, it employs 47% of the total labor force and comprises 16% of the country's GDP. The performance of this sector has an overwhelming impact on major macroeconomic objectives like employment generation, poverty alleviation, human resources development and food security. A plurality of Bangladeshis earns their living from agriculture. Rice is the dominant staple food for many countries of the world (Mobasser et al., 2007). It is also the most important food crop and a major food grain for more than one third of the world population (Zhao et al., 2011). Nitrogen is the key element in the production of rice and gives by far the largest response. It is an essential plant nutrient that plays a significant role in growth, yield and quality of rice. The important role of nitrogen fertilizers in increasing rice yields has been widely recognized, particularly after the development of modern varieties. Nitrogen is an integral part of protoplasm, protein and chlorophyll and plays a remarkable role in increasing cell size which in turn increases yield. Excess amount of nitrogenous fertilizer results in lodging of plants, prolonging growing period, delayed in maturity, susceptibility to insect-pests and diseases and ultimately reduces yield (Uddin, 2003). Efficient fertilizer management gave higher yield of crop and reduced fertilizer cost (Hossain and Islam, 2006). Plant growth is seriously hampered when lower dose of nitrogen is applied, which drastically reduces the yield. It is also a fact that improper use of nitrogenous fertilizer, instead of giving yield advantage, may reduce the same. Many workers have reported a significant response of rice to nitrogen in different soils in Bangladesh (Bhuiya et al., 1989, Hussain et al., 1989 and Islam et al., 1990). Weeds are one of the most important agricultural pests. Infestation of weed is one of the most important causes for low yield of rice. There is no doubt that maximum benefit from costly input like fertilizers and pesticides in rice can be fully derived when the crop is kept free from weed infestation. The edaphic and climatic conditions of the country are congenial for the growth of numerous species of obnoxious weeds, which lead to considerable ARTICLE INFO Article history: Received: 20 December 2017 Accepted: 01 January 2018


Introduction
Agriculture is the largest employment sector in Bangladesh.As of 2016, it employs 47% of the total labor force and comprises 16% of the country's GDP.The performance of this sector has an overwhelming impact on major macroeconomic objectives like employment generation, poverty alleviation, human resources development and food security.A plurality of Bangladeshis earns their living from agriculture.Rice is the dominant staple food for many countries of the world (Mobasser et al., 2007).It is also the most important food crop and a major food grain for more than one third of the world population (Zhao et al., 2011).Nitrogen is the key element in the production of rice and gives by far the largest response.It is an essential plant nutrient that plays a significant role in growth, yield and quality of rice.The important role of nitrogen fertilizers in increasing rice yields has been widely recognized, particularly after the development of modern varieties.Nitrogen is an integral part of protoplasm, protein and chlorophyll and plays a remarkable role in increasing cell size which in turn increases yield.Excess amount of nitrogenous fertilizer results in lodging of plants, prolonging growing period, delayed in maturity, susceptibility to insect-pests and diseases and ultimately reduces yield (Uddin, 2003).Efficient fertilizer management gave higher yield of crop and reduced fertilizer cost (Hossain and Islam, 2006).Plant growth is seriously hampered when lower dose of nitrogen is applied, which drastically reduces the yield.It is also a fact that improper use of nitrogenous fertilizer, instead of giving yield advantage, may reduce the same.Many workers have reported a significant response of rice to nitrogen in different soils in Bangladesh (Bhuiya et al., 1989, Hussain et al., 1989and Islam et al., 1990).
Weeds are one of the most important agricultural pests.Infestation of weed is one of the most important causes for low yield of rice.There is no doubt that maximum benefit from costly input like fertilizers and pesticides in rice can be fully derived when the crop is kept free from weed infestation.The edaphic and climatic conditions of the country are congenial for the growth of numerous species of obnoxious weeds, which lead to considerable yield reduction.High competitive ability of weeds exerts a serious negative effect on crop production causing significant losses in crop yield (Mamun et al., 1993).Poor weed control is one of the major factors for yield reduction in rice, the extent of which depends on type of weed flora and their intensity of infestation.Yield losses due to weed infestation are greater than the combined losses of insect pests and diseases.In Bangladesh, weed infestation reduces grain yield by about 70-80% in aus rice, 30-40% for transplanted (T) aman rice and 22-36% for modern boro rice cultivars (BRRI, 2008;Mamun, 1990).There are studies about nitrogen and weed management in T. aman rice but for a specific variety BRRI dhan46 it is rare.Thus the best weeding needs to be adopted by the farmers with a view to reducing weed infestation and maximizing rice yield.Considering mentioned points, the study was conducted to optimize nitrogen and weed management in transplant aman rice.

Materials and Methods
The experiment was conducted at the Agronomy Field Laboratory, Bangladesh Agricultural University (BAU), Mymensingh during the period from July to December, 2015 to study the effect of nitrogen fertilizer and weed management on the growth and yield of transplant aman rice.The experimental site belongs to the Old Brahmaputra Floodplain Agro ecological Zone (AEZ-9).Non calcareous dark-grey floodplain soil is generally predominant in the site.The land type was medium high and the soil was silty loam in texture.The pH value of the soil was 6.8 and the soil is low in organic matter content.The variety BRRI dhan46 was used as the test crop in the experiment.BRRI dhan46 with photoperiod sensitivity is suitable for late transplanting in aman season after the recession of flood water.Its average grain yield is 4.7tha -1 .The experiment consisted of 4 levels of Nitrogen viz.0 kg N ha -1 (N 0 ), 40 kg N ha -1 (N 1 ), 80 kg N -1 (N 2 ) and 120 kg N ha -1 (N 3 ) and five weeding treatments viz.One hand weeding at 20 DAT (W 1 ), two hand weedings at 20 and 35 DAT (W 2 ), three hand weedings at 20, 35 and 50 DAT (W 3 ), weeding by Japanese rice weeder twice at 20 and 35 DAT (W 4 ) and unweeded control (W 5 ).The experiment was laid out in a randomized complete block design with three replications.Each block was divided into twenty unit plots of size 4.0m×2.5meach.Thus, the total number of unit plot was 60 (20×3).The distance maintained between two unit plots was 0.5m and between blocks was 1m.Nitrogen was applied accordingly experimental specification in the form of urea at three splits application.Five hills were randomly selected and marked with the bamboo sticks in each unit plot excluding border rows to record the data on yield contributing characters and yield.Recorded data were analyzed statistically using "Analysis of Variance Technique and the differences among treatment means were adjudged by Duncan's Multiple Range Test (DMRT).

Effect of nitrogen fertilizer
Nitrogen fertilizer significantly influenced crop characters, yield and yield contributing characters of T. aman rice cv.BRRI dhan46.It was observed that 80 kg N ha −1 produced tallest plant (113.00cm),highest number of total tillers hill −1 (8.74) and highest number of effective tillers hill −1 (6.19).These findings corroborate with those reported by Uddin et al. (2013).N 0 (no nitrogen fertilizer control) treatment produced the shortest plant height (106.00 kg N ha −1 ), lowest number of total tillers hill −1 (7.20) and lowest number of effective tillers hill −1 (5.00) (Table 1).It was observed that number of total tillers hill −1 decreased progressively with the decrease in the amount of nitrogen fertilizer and became lowest at N 0 (Control) treatment (Table 1).The highest number of grains panicle −1 (114.20) and highest weight of 1000-grains (25.85 g) was found in N 2 (80 kg N ha -1 ) treatment and the lowest one was obtained from N 0 (no nitrogen fertilizer control) treatment (Table 1).Grain yield varied from 3.52 to 4.00 t ha −1 .The highest grain yield (4.00 t ha −1 ) was obtained from N 2 (80 kg N ha −1 ) treatment and the lowest one (3.52 t ha −1 ) was obtained from N 0 (Control) treatment (Table 1).The second highest grain yield was found in N 3 (120 kg N ha −1 ) treatment followed by N 1 treatment.The increased grain yield with 80 kg N ha −1 might be due to the cumulative effect of the highest number of effective tillers hill −1 and grains panicle −1 obtained from the supply of nitrogen for the plants.Similar results were found elsewhere (Singh et al., 2000 andSalahuddin et al., 2009).The highest straw yield (5.25 t ha −1 ) was obtained from N 2 (80 kg N ha −1 )treatment and the lowest one (4.46t ha −1 ) was obtained from N 0 (Control) treatment (Table 1).Application of nitrogenous fertilizer encouraged vegetative growth of rice in terms of plant height and number of total tillers hill −1 , which ultimately resulted in the increase of straw yield (Mishra et al., 2003).The biological yield and harvest index also affected significantly due to application of nitrogenous fertilizer.

Effect of weed management
Weed management significantly influenced the crop characters, yield and yield contributing characters except plant height (Table 2).W 3 (three hand weedings at 20, 35 & 50 DAT) treatment showed superiority in terms of number of total tillers hill −1 (8.61), number of effective tillers hill −1 (6.81), panicle (21.92 cm), number of grains panicle −1 (111.10),1000-grain weight (25.52 g) while the lowest values were in unweeded control plots.The highest grain yield (4.23 t ha −1 ), straw yield (5.51 t ha −1 ) and biological yield (9.74 t ha −1 ) was obtained from W 3 (weeding at 20, 35 and 50 DAT) treatment.The weeds competed with the crop plants for nutrition, water, air, sunlight and space thus reducing yield.The increased yield in weeded crops was contributed by higher number of effective tillers hill −1 and higher numbers of grains panicle −1 over no weeding treatments.This might be due to the fact that weeding kept the land clean and the soil was well aerated which facilitated the crop for better absorption of nutrients, moisture and solar radiation for higher yield.Effective weed management enhanced production of effective tillers hill −1 , grains panicle −1 which ultimately increased grain yield of rice.Therefore, rice should be kept weed free as much as possible through the life cycle to obtain good yield.Yield variations in rice due to weeding were also observed by many researchers (Chowdhury et al., 1994;Ahmed et al., 1998;Hossain et al., 2002;Islam et al., 2003, Liu et al., 2016).

Interaction effects of nitrogen fertilizer and weed management
Interaction effect of nitrogen fertilizer and weed management did not show significant effect on final plant height, panicle length, no. of grains panicle −1 , 1000-grain weight and harvest index.The highest number of total tillers hill −1 (9.44) and effective tillers hill −1 (7.83) was obtained from N 2 ×W 3 (80 kg N ha −1 × weeding at 20, 35 and 50 DAT) combination and the lowest one was observed in N 0 ×W 5 (no nitrogen fertilizer control × unweeded control) combination (Table 3).Grain yield of transplant aman rice cv.BRRI dhan46 was significantly influenced by the interaction between nitrogen fertilizer and weeding (Table 3).Grain yield varied from 3.25 to 4.50 t ha −1 .The highest grain (4.50 t ha −1 ) and straw yield (6.10 t ha −1 ) was obtained from N 2 ×W 3 (80 kg N ha −1 × weeding at 20, 35 and 50 DAT) combination and the lowest one was obtained from N 0 ×W 5 (no nitrogen fertilizer control × unweeded control) combination (Table 3).Biological yield was significantly influenced by the interaction between weeding and nitrogen fertilizer(Table 3).The highest biological yield (10.60 t ha −1 ) was obtained from N 2 ×W 3 (80 kg N ha −1 × weeding at 20, 35 and 50 DAT) combination and the lowest one (7.35t ha −1 ) was obtained from N 0 ×W 5 (no nitrogen fertilizer control× unweeded control) combination (Table 3).In a column, figures with same letter (s) or without letter do not differ significantly whereas figures with dissimilar letter differ significantly (as per DMRT),** =Significant at 1% level of probability; * =Significant at 5% level of probability.N 0 = Control; N 1 = 40 kg N ha −1 ; N 2 = 80 kg N ha −1 and N 3 = 120 kg N ha −1 .

Conclusion
For the result of the present study it can be concluded that 80 kg N ha −1 along with three hand weedings at 20, 35 and 50 DAT may be used to obtain the highest grain and straw yields of transplant aman rice cv.BRRI dhan46.Farmers will follow this suggestion because it will help them to obtain a satisfactory yield.It will also help to determine the optimum nitrogen level, to find out the appropriate weed control treatments and to find out the effect of interaction between nitrogen fertilizers and weed management on the growth and yield of transplant aman rice cv.BRRI dhan46.

Table 3 . Interaction effects of nitrogen fertilizer and weeding on yield and yield contributing characters of T. aman rice cv. BRRI dhan46
In a column, figures with same letter (s) or without letter do not differ significantly whereas figures with dissimilar letter differ significantly (as per DMRT).