Productivity enhancement through rice–fish farming using a two-stage rainwater conservation technique

Agricultural Water Management 67 (2004) 119–131

Productivity enhancement through rice–fish farming using a two-stage rainwater conservation technique A. Mishra∗ , R.K. Mohanty Water Technology Centre for Eastern Region (ICAR), Chandrasekharpur, Bhubaneswar 751023, India Accepted 2 February 2004

Abstract In order to increase the productivity from an unit volume of rainwater, a two-stage rainwater conservation technique was conceptualized and tested, in which part of the rainwater was conserved in rice field up to the weir crest level and the remaining in a refuge for rearing of fish (Catla catla, Labeo rohita, Cirrhinus mrigala and Cyprinus carpio) and prawn (Macrobrachium rosenbergii). Experimental study for 3 years (1999–2001) was conducted in Bhubaneswar, India. Swarna variety of rice was grown in plots of 300 m2 each with weir heights of 10, 12.5 and 15 cm as treatments having three replications each. Weir height of 12.5 cm, provision of peripheral trench on three sides of rice fields (0.5 m width and 0.3 m depth), a refuge occupying 9% of field area and fish stocking density of 25,000 ha−1 were found suitable for this system and resulted in rice equivalent yield of about 4.4 t ha−1 without application of pesticides. A net profit of Rs. 10,781.00 ha−1 was obtained from this dual production system. © 2004 Elsevier B.V. All rights reserved. Keywords: Two-stage rainwater conservation; Rice–fish farming; Productivity; Weir height; Rainfed rice lands

1. Introduction The eastern India is blessed with plenty of rainfall most of which occurs during June– September. During this period about 50% of annual rainfall comes from few intense storms resulting into high runoff losses. Added to this is the erratic nature of monsoon rain that creates water stress at various growth stages of rice, which is a predominant crop during monsoon. Rice suffers from excess water during heavy downpours and shortage during long drought spells. Due to this, the productivity of rainfed rice is considerably lower than that of the irrigated rice. The area cropped with rice in the major states of eastern India (Orissa, ∗

Corresponding author. Tel.: +91-674-2300060; fax: +91-674-2301651. E-mail address: [email protected] (A. Mishra). 0378-3774/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2004.02.003

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West Bengal, Bihar and Assam) is about 17.87 M ha which is 41.2% of the 43.42 M ha rice lands in India. However, the average rice productivity of these states is about 1586 kg ha−1 , which is 309 kg ha−1 lower than the national average productivity (1895 kg ha−1 ). Further, the percentage of irrigated rice area in these four states and in the country is about 34.2 and 50.1%, respectively (MOA, 1999). The productivity gap can be minimized by creating a favorable water regime and better water management strategy for rice crop grown in eastern India. In the rainfed rice lands, dike height around the rice fields is a deciding parameter in conserving the rainwater in situ. Determination of optimum dike height for conservation of rainwater, sediment, nutrient, control of declining ground water table and improvement in rice yield has drawn attention of researchers in the recent past (Islam and Mondal, 1992; Mishra et al., 1998; Khepar et al., 2000). A study on in situ conservation of rainwater in rainfed rice fields by Mishra et al. (1998) revealed that weir height of 6 and 30 cm could store 57 and 99% of the rainwater, respectively. An exponential relationship was observed between the runoff and weir heights. On an average, four irrigations with an application depth of 6 cm per irrigation were required for weir height of 6 cm. Weir heights of 10–14 cm required three supplemental irrigations while weir heights of 18–30 cm required only two supplemental irrigations in a cropping season. Thus, in situ conservation of rainwater minimizes the supplemental irrigation water requirement during dry spells and drainage needs of the catchment. Based on the results of this study, a weir height of 20–25 cm is considered optimum for in situ conservation of rainwater, nutrient and sediments. Further, to obtain increased productivity from an unit volume of rainwater, a two-stage rainwater conservation technique was conceptualized and tested in which part of the rainwater is conserved in rice field up to the weir crest level and the remaining in a refuge for rearing of fish and prawns. Rice–fish farming is an age-old practice in India, however, it has not flourished due to several technical and social constraints. Out of 43 million ha of rice cultivated land in India, about 20 million ha is suitable for adoption of integrated rice–fish production system (Rao and Singh, 1998). However, only 0.23 million ha is presently under rice–fish culture (Radheyshyam, 1998) and the carrying capacity of these suitable lands has not been utilized to the fullest extent. If the area under integrated rice–fish system could be increased, it would help to compensate the economic losses in rice production brought about by natural calamities. It would also enhance the use of land and water resources without bringing about environmental degradation, as fish culture in rice fields improves dissolved oxygen, soil quality and fertility (Likangmin, 1988). In addition to enhanced productivity, this system also generates employment, increases farm income and provides nutritional security. In situ conservation of rainwater by using optimum weir height overlooked another possibility that could increase the value of conserved water. Thus, the economic return by conserving less rainwater in situ and the remaining ex situ for using it to grow two crops (rice and fish) could exceed the value of rainwater if all of it is conserved by using a high weir height to grow only rice. The purpose of this study was therefore, to determine the optimum weir height to optimize the quantity of rainwater to be conserved in situ for rice production and ex situ for production of fish and prawns. The research effort was intended to develop and standardize a simple, cost effective, location specific two-stage rainwater conservation technique for successful rice–fish farming.

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2. Material and methods 2.1. Water balance model for diked rice field A lumped water balance model for diked rice field was developed to estimate its irrigation and drainage requirements. The water balance of a diked rice field is expressed in terms of water depth and a time unit of a day of 24 h considered suitable for simulation. The mass balance equation is expressed as: WLe = WLb − ET − DP + RF + IR − DR

(1)

where WLe = water level at the end of the day (cm), WLb = water level at the beginning of the day (cm), ET = actual evapotranspiration during the day (cm), DP = deep percolation and seepage loss in a day (cm), RF = total rainfall during the day including possible inflow from other areas (cm), IR = irrigation water required/supplied during the day (cm), and DR = runoff water from the paddy field during the day (cm). In order to perform the daily water balance computation, the boundary conditions and assumptions are specified. WLmax and WLmin are defined as the maximum possible and minimum required water levels in the field. For WL > WLmax , runoff (DR = WL−WLmax ) will occur and for WL < WLmin , irrigation (IR = IAD − WL) will take place, where WL is the water level at a particular time and IAD the irrigation application depth. The model computes the depth of standing water in the rice field, the volume of water that spills out and irrigation water to be applied daily, if any, for the crop growth period for diked rice fields with various weir heights. The crop growth period (transplanting to harvesting) for the purpose of the water balance study was considered to be from 15 July to 15 November. The weir height of the diked rice fields for simulations was kept as 6–30 cm at an interval of 4 cm. These weir heights were considered as WLmax values for simulation. It was decided to irrigate the rice field just a day after the complete disappearance of standing water from the land surface. Hence, for the model study, WLmin was considered at the land surface or zero depth. Daily water balance simulation for the crop growth period was performed for 32 years taking into account the daily rainfall data of Bhubaneswar for the period 1960–1991. Daily ET rate of rice crop was estimated for the same period by the modified Penman method (ASCE, 1993) using data on the daily maximum temperature, minimum temperature, relative humidity, wind velocity and sunshine hours. The daily water loss due to deep percolation and seepage was calculated using the relationship obtained from experimental field observation (Mishra et al., 1998) which is as follows: DP = −0.164 + 0.079D,

R2 = 0.865

(2)

where D is the average depth of water stored in the rice field (cm). 2.2. Estimation of runoff and refuge size at different weir heights The modeling results revealed that there is drastic reduction in runoff values after the first half of August for 15 cm weir height plots. Thus, for the study area, a weir height of more than 15 cm does not provide sufficient runoff for the aquaculture refuge. Keeping the upper

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Table 1 Fortnightly runoff in mm (mean of 32 years) from rice plots with various weir heights through water balance model simulation Weir height (cm)

July second half

August first half

August second half

September first half

September second half

October first half

October second half

November first half

10 12.5 15

45.54 32.07 20.95

46.76 31.50 18.62

40.40 30.36 3.86

32.63 23.29 8.75

21.32 15.16 5.42

14.98 8.38 0.77

15.96 10.89 1.72

6.19 4.42 1.69

limit of weir height for dual production system (rice + fish) as 15 cm, two other weir heights were chosen i.e., 10 and 12.5 cm for experimental study. Table 1 shows fortnightly runoff, which is expected to spill out from weir heights of 10, 12.5 and 15 cm. The predicted runoff values from the model were found to be in close agreement with the observed experimental values (Mishra, 1999). Further, it was decided to maintain a depth of 1.75–2.00 m in the refuges for fish culture. Keeping this depth of storage in mind, the refuge area for different weir heights was calculated. It was found that rice fields with weir heights of 10, 12.5 and 15 cm would need about 12, 9 and 4% of the rice field area for construction of refuge, respectively. 2.3. Experimental layout The first phase of experiment was conducted for three consecutive years (1993–1995) at WTCER research farm, Bhubaneswar (20◦ 30 0 N latitude and 87◦ 48 10 E longitude). This experiment was aimed at determining the optimum weir height for in situ conservation of rainwater, soil and nutrients in rainfed rice lands (Mishra et al., 1998). After completion of the first phase experiment, it was then conceptualized to conserve a portion of rainwater in the rice field and the remaining part in a refuge (primarily meant for fish culture), having a depth of 1.75–2.00 m, constructed at the down stream end of the rice field for multiple uses of conserved rainwater. For this purpose, the second phase of experiment was carried out at the same location for three consecutive years (1999–2001), the results of which are presented in this paper. As obtained from the water balance model simulation study, three weir heights, i.e. 10, 12.5 and 15 cm were considered as treatments with three replications each. Rice plots of 30 m × 10 m size surrounded by 45 cm dikes were laid out. The plots were leveled by planking. Polythene sheets of 200 ␮m gauge were embedded within the dike vertically to a depth of 60 cm below the ground level as lining material for preventing seepage from one field to another. Each rice plot was provided with a brick masonry broad-crested rectangular weir at the partition dike between the refuge and rice field. By doing so, a portion of the rainwater was allowed to store in the rice field up to the weir crest level (weir height). The excess rainwater above the crest level was allowed to spillover the weir for further conservation in the refuge. Fig. 1 presents the schematic diagram of an experimental rice plot using the two-stage rainwater conservation technique. The excess rainwater spilling over the weir was harvested in the refuge for fish culture and supplemental irrigation. Two inlet pipes were

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Fig. 1. Schematic diagram of experimental plot using two-stage rainwater conservation technique.

provided with each refuge at the bottom surface of the trench. These inlets remain opened most of the time. Fish and prawns stocked in the refuge were allowed to move into the rice field through the refuge inlets. Only when the water level in the rice field receded (post-monsoon period), the fish and prawns come to the refuge from the rice field through the inlets. Rice (var. Swarna) was grown in each plot planted at a spacing of 20 cm × 10 cm. Transplanting was carried out during second week of July. The seed and fertilizer application rate was 50 kg ha−1 and 80:40:40 (N:P2 O5 :K2 O) kg ha−1 , respectively. Fifty percent of N and all of P2 O5 and K2 O were given as basal dose at the time of transplanting. The rest of the nitrogen was applied in two equal split doses during tillering and panicle initiation stages. Crop growth and yield parameters were recorded at regular intervals. No pesticide was applied in the experimental plots to prevent fish mortality. Refuge preparation included application of lime at 2000 kg ha−1 , raw cattle dung at 5000 kg ha−1 and fertilizers (urea containing 46% N: single super phosphate containing 16% P2 O5 , 1:1) at 3 ppm was carried out prior to stocking. Seven days after refuge

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preparation, fry of Catla catla, Labeo rohita, Cirrhinus mrigala, Cyprinus carpio and post larvae (PL3–5 ) of freshwater prawn Macrobrachium rosenbergii were stocked in the refuge with a species composition of 30:30:15:15:10, respectively. Stocking density of fry was kept at 15,000, 25,000 and 35,000 ha−1 (refuge area only) during first, second and third year of experimental study, respectively and rearing was continued for 120 days. Supplemental feed (rice bran: groundnut oil cake, 1:1) at 10, 8, 6 and 4% of mean body weight of fish (MBW) was provided twice a day, during first, second, third and fourth month to harvesting, respectively. Periodic manuring at 500 kg ha−1 and liming at 200 kg ha−1 were carried out at every 15 days interval to maintain plankton population in the eco-system. Weekly measurement of the following water quality parameters were made: water temperature (using thermometer graduated in ◦ C); water transparency using Secchi disc method (Biswas, 1993); pH (using digital pH meter, model Checker–1, HANNA, USA, with accuracy of +0.01); total alkalinity (by titration against 0.2N H2 SO4 using phenolphthalein and methyl orange indicator); dissolved oxygen (using dissolved oxygen meter, model YSI-55, USA); ammonia (using spectrophotometer at 635 nm, model Chemito-2600, India); nitrate-N by phenoldisulphonic acid method and inorganic phosphate by Denige’s cerulomolybdate method (Biswas, 1993). Water samples were collected from all the treatments and their replications between 07.00 and 08.00 h. Similarly, monthly observation on soil pH (using soil pH meter, model DM-13, Japan), organic carbon by rapid titration method, available P by using spectrophotometer at 690 nm, and available N by alkaline permanganate extraction method (Biswas, 1993) was carried out. Plankton samples were collected at fortnightly intervals by filtering 50 l of refuge water between 08.00 and 09.00 h through a 45 cm conical plankton net (25 cm diameter) of bolting silk no. (0.64 mm mesh size). The plankton samples were fixed in 4% formalin. The quantitative counts were made using a microscope and Sedgwick–Rafter counting chamber. Fish growth parameter and feed conversion ratio was recorded using the following formulae: (a) Mean body weight (g) = sample weight (g)/sample size (n). (b) Survival rate (%) = (total number of fish harvested/number of initial stocking) × 100. (c) Average daily growth (g per day) = (final weight (g) − initial weight (g))/rearing duration (days). (d) Biomass of fish (kg) = (total feed required per day (kg)/percentage of feed required according to the mean body weight of fish) × 100. (e) Condition factor (Kn) = (weight of fish (g)/length of fish (cm)3 ) × 100. (f) Feed conversion ratio (FCR) = cumulative total feed (kg)/final biomass or yield (kg). Statistical analysis with regard to growth performance and yield amongst treatments was carried out through one-way ANOVA and means were compared using Duncan’s multiple range test (DMRT) to find out the difference at 5% (P < 0.05) level (Duncan, 1955). To compare the effect of fish and prawn on rice yield, three plots of equal size and different weir heights of 10, 12.5 and 15 cm were taken as control, where only Swarna variety of rice was grown without integration of fish and prawn.

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3. Results and discussion 3.1. Soil and water quality of refuges Fish and prawn are limited in their ability to tolerate changes in water quality due to daily and seasonal climatic fluctuation (Mohanty et al., 2002). In this experiment, various hydro-biological parameters did not show any distinct trend between the treatments except in the cases of total suspended solid, dissolved oxygen and total alkalinity. The recorded minimum and maximum values of various water and soil quality parameters were: water temperature 27.5–30.2 ◦ C; water pH 6.9–8.8; dissolved oxygen 3.9–8.1 ppm; total alkalinity 69–129 ppm; dissolved organic matter 0.6–4.7 ppm; nitrite-N 0.006–0.071 ppm; nitrate-N 0.06–0.52 ppm; ammonia 0.01–0.21 ppm; total suspended solid 89–319 ppm; phosphate-P 0.06–0.34 ppm; refuge water depth 89–175 cm; soil pH 6.6–7.0; available N in soil 7.9–10.7 mg per 100 g; available P in soil 0.29–0.67 mg per 100 g; organic carbon in soil 0.16–0.53% and total plankton count 2.4 × 102 − 9.1 × 103 numbers/l. Most of these fluctuations were within the optimum range (Banerjea, 1967; Boyd and Pillai, 1985; Pillay, 1992). The dissolved oxygen content showed a decreasing trend with time in response to the gradual increase in fish biomass that resulted in higher oxygen consumption. Water level and concentration of total suspended solid in the refuges increased with decreasing weir height, which could be due to increased runoff from the rice field along with sediment and other nutrients. The availability of CO2 for phytoplankton growth is related to total alkalinity. Water having 20–150 ppm total alkalinity produces sufficient CO2 to permit plankton production (Boyd and Pillai, 1985). The recorded minimum and maximum range of total alkalinity during the experimental period was 69 and 129 ppm, respectively, which was maintained due to periodic liming. Average primary production of refuge water in the first month of rearing ranged between 93 and 117 mg C m−3 h−1 , which improved further (514.5+89.4 mg C m−3 h−1 ) with the advancement of rearing period. Low primary production in the initial phase of rearing was probably due to low availability of N, P and K, fixation of nutrient ions by suspended soil/clay particles and organic matter (Mohanty et al., 2002). 3.2. Growth and yield of rice Table 2 presents the biometric parameters and yield of rice (variety Swarna) in different treatments. The highest number of panicles/m2 (235) was obtained for a weir height of 15 cm followed by weir height of 12.5 cm (233). A similar trend was observed for number of filled grains/panicle. However, the highest test weight (weight of 1000 grains) was recorded for the weir height of 12.5 cm. The test weight was not significantly different amongst treatments. The highest grain yield was obtained for the weir height of 15 cm (3629 kg ha−1 ), which was significantly superior to that for the weir height of 10 cm (2988 kg ha−1 ). However, its superiority over weir height of 12.5 cm was not statistically significant. The similar trend was also followed in case of straw yield. The highest grain yield obtained with a weir height of 15 cm was probably the result of a higher number of panicles/m2 (235) and number of filled grains/panicle (122). Table 3 shows the grain yield of rice in rice–fish integrated plots and control plots. Perusal of the table reveals that in all the treatments yield of rice in rice–fish integrated plots was

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Table 2 Yield attributes and yield of rice in different weir height plots in rice–fish integration system (averages for 1999–2001) Treatment (weir height) (cm)

Number of panicles/m2

Number of filled grain/panicle

Test weight (g)

Grain yield (kg ha−1 )

Straw yield (kg ha−1 )

10 12.5 15

204 233 235

110 118 122

19.5 19.9 19.3

2988 3595 3629

3572 4303 4420

15

7

NS

157

160

Critical difference (0.05) NS: non-significant.

higher than that of control plots. On average, the increase was 8.3%. This increase is possibly due to the frequent locomotary movement and stirring activity of fish and prawns that increases the dissolved oxygen levels. Further, addition fish fecal matter improves the soil organic matter/nutrient status. Introduction of fish and prawns also controls plankton population/macro and micro aquatic insects/bacteria/organic detritus that compete with rice for material and energy resulting in enhanced rice yield. Similar findings were reported by Hora and Pillay (1962) who attributed an increase in rice yield of 15% in the Indo-Pacific countries to better aeration of water, greater tillering effect and additional supply of fertilizer in form of leftover feed and fish excreta. Taking into account the average value of control yield, test of significance at 5% level was carried out for rice–fish system yield. The yield at weir height of 10 cm was found non-significant while the yields at remaining two weir heights were significant (Table 3). 3.3. Growth, survival and yield of fish and prawn Table 4 presents the average daily growth and survival rate of cultured species at different stocking densities and weir heights. Faster average daily growth rate was recorded for C. catla followed by C. carpio, C. mrigala, L. rohita and M. rosenbergii during 120 days of culture at all the stocking densities. Bottom feeders (C. carpio and C. mrigala) performed better than L. rohita, probably due to the fact that being surface and column dweller, L. rohita is more sensitive to oxygen depletion, while bottom dwellers C. carpio and C. mrigala are more tolerant to fluctuations in oxygen concentration (Vijayan and Verghese, 1986). Moreover, the faster growth rate of C. catla and bottom dwellers could Table 3 Rice yield enhancement due to introduction of fish and prawn in the rice field (3 years average) Weir height (cm)

10 12.5 15

Grain yield of rice (kg ha−1 )

Percent increase (%)

Rice–fish integration system

Only rice (control)

2988 (NS) 3595 (S) 3629 (S)

2756 3309 3362

8.4 8.6 7.9

Grain yield of rice (rice–fish system’s yield vs. control yield) at 5% level = 194.35; NS: non-significant; S: significant.

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Table 4 Growth and survival performance of fish and prawn in rice–fish integration system at different stocking densities Refuge of different treatments

Species reared

15000 ha−1

25000 ha−1

35000 ha−1

ADG (g)

SR (%)

ADG (g)

SR (%)

ADG (g)

SR (%)

Refuge 1 (10 cm weir height)

C. catla L. rohita C. mrigala C. carpio M. rosenbergii C. catla

1.02 0.73 0.76 0.78 0.21 1.00

79 84 70 60 67 73

0.85 0.63 0.73 0.74 0.20 0.81

59 62 59 59 60 58

0.71 0.44 0.47 0.47 0.18 0.75

52 62 67 54 50 55

Refuge 2 (12.5 cm weir height)

L. rohita C. mrigala C. carpio M. rosenbergii C. catla

0.65 0.69 0.71 0.20 0.94

80 88 62 78 71

0.63 0.72 0.74 0.19 0.74

61 61 46 50 50

0.45 0.46 0.47 0.69 0.69

61 67 61 44 44

Refuge 3 (15 cm weir height)

L. rohita C. mrigala C. carpio M. rosenbergii

0.60 0.64 0.65 0.20

71 76 50 67

0.56 0.61 0.62 0.17

67 50 50 33

0.38 0.42 0.43 0.17

50 62 50 40

ADG: average daily growth; SR: survival rate.

be due to effective utilization of ecological niches and the rich detrital food web (Sinha, 1998) that was maintained through periodic manuring, liming and fertilization. Further, average daily growth rate of all species decreased with increase in stocking density from 15,000 to 35,000 ha−1 , which may be due to mutual competition for food and space causing physiological stress (Wedemeyor, 1976) and degraded water quality (Smart, 1981) because of increased biomass (Trzebiatowski et al., 1981). Sinha and Ramachandran (1985) also reported that, with crowded condition at higher stocking densities, fish suffer stress due to aggressive feeding interaction resulting in lower feeding rates and slower growth. Higher survival rate was recorded at stocking density of 15,000 ha−1 . C. catla and L. rohita showed a distinct declining trend in survival rate with increase in stocking density from 15,000 to 35,000 ha−1 while, no such trend was marked in case of C. mrigala, C. carpio and M. rosenbergii. This might be due to mutual competition for food among the bottom dwellers at the lower level of food web (Mohanty et al., 2002). Productivity of fish and prawn (kg ha−1 per 120 days) was higher in refuges with weir height of 10 cm, irrespective of stocking density, while overall yield performance was good at stocking density of 25,000 ha−1 (Table 5). By increasing stocking density from 25,000 to 35,000 ha−1 , the yield of fish and prawn increased by 2.0 and 126.0 kg ha−1 per 120 days in refuges with weir heights of 10 and 12.5 cm, respectively. However, the yield decreased by 46 kg ha−1 per 120 days (4.5%) in refuges with weir height of 15 cm. We conclude that increasing the stocking density beyond an optimum level, does not result in increased yield. In India, although fresh water fish production in rice–fish integration system has been recorded as high as 1100 kg ha−1 , the highest recorded production of Indian major carps was 800 kg ha−1 (Lightfoot et al., 1992). An average productivity of 907–1282 kg ha−1 per

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Table 5 Effect of stocking density on fish and prawn yield at different weir heights Stocking density (numbers ha−1 )

Fish and prawn yield (kg ha−1 )

15000 25000 35000

10.0 cm

12.5 cm

15.0 cm

1027 b 1243 a 1245 a

963 c 1156 b 1282 a

907 c 1031 a 985 b

Values are means of three replications. Means having different superscripts in a column by Duncan’s multiple range test (DMRT) differed significantly (P < 0.05).

120 days of fish and prawn was recorded in this experiment, which is much higher than the reported productivity in a season in India. Overall growth, survival rate and yield of cultured species increased with the decrease in weir height of rice fields. This is possibly due to increased runoff water resulting in higher nutrient input into the refuges, which increased natural food availability. 3.4. System’s rice equivalent yield To assess the return from the system in a single unit, rice equivalent yield (REY) was calculated by considering the base price of rice as Rs. 4.00 kg−1 and fish as Rs. 40.00 kg−1 (Table 6). The highest REY was recorded in weir height of 12.5 cm with 35,000 stocking density of fish and prawn. Further, in all 3 years, the highest rice equivalent yield was recorded in weir height of 12.5 cm. The REY at weir height of 12.5 cm (4.22–4.55 t ha−1 ) was significantly superior to that of weir height of 15 cm (3.88–3.95 t ha−1 ). REY between weir heights of 12.5 and 10.0 cm was found statistically insignificant. REY was comparatively low (3.88–4.22 t ha−1 ) at stocking density of 15,000 ha−1 and improved to (3.95–4.43 t ha−1 ) when stocking density increased from 15,000 to 25,000 ha−1 . HowTable 6 System’s rice equivalent yield (REY) at different weir heights and stocking densities Productivity (t ha−1 )

Stocking density (numbers ha−1 )

Weir height (cm)

Area (m2 ) Rice field

Refuge

Rice

Fish

15000

10.0 12.5 15.0

300 300 300

45 35 15

2.99 3.59 3.63

1.03 0.96 0.91

3.93 4.22 3.88

25000

10.0 12.5 15.0

300 300 300

45 35 15

2.99 3.59 363

1.24 1.16 1.04

4.21 4.43 3.95

35000

10.0 12.5 15.0

300 300 300

45 35 15

2.99 3.59 3.63

1.24 1.28 0.98

4.22 4.55 3.92

REY (t ha−1 )

REY = rice equivalent yield, selling price of rice at Rs. 4.00 kg−1 , selling price of fish and prawn at Rs. 40.00 kg−1 . US$ 1 = Rs. 48.00 (approximately).

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Table 7 Cost of cultivation and return from rice–fish farming system Weir height (cm)

Stocking density (numbers ha−1 )

Cost of cultivationa (Rs. ha−1 )

Yield (t ha−1 )

Rice

Fish

Rice–fish

Rice

Straw

Fish

Gross return

Net return

10.0 12.5 15.0

15000 15000 15000

8674 8674 8674

4650 4650 4650

8149 8253 8482

2.99 3.59 3.63

3.57 4.30 4.42

1.03 0.96 0.91

16987 18468 17234

8837 10214 8751

10.0 12.5 15.0

25000 25000 25000

8674 8674 8674

6717 6717 6717

8419 8469 8581

2.99 3.59 3.63

3.57 4.30 4.42

1.24 1.16 1.04

18119 19251 17481

9701 10781 8901

10.0 12.5 15.0

35000 35000 35000

8674 8674 8674

8990 8990 8990

8715 8707 8689

2.99 3.59 3.63

3.57 4.30 4.42

1.24 1.28 0.98

18130 19777 17382

9415 11070 8693

a b

Return from rice–fish systemb (Rs. ha−1 )

Cost of cultivation includes cost of seed + fertilizer + lime + feed + labour + interest on working capital. Selling price of paddy at Rs. 4.00 kg−1 ; straw at Rs. 40.00 per quintal, and fish + prawn at Rs. 40.00 kg−1 .

ever, no significant increase in REY (3.92–4.55 t ha−1 ) was obtained at higher stocking densities. 3.5. Economics of rice–fish farming In the rice–fish farming system, the rice yield was 7.9–8.6% (Table 3) greater than that of the control. Further, the area devoted for refuge also adds to the profit of the system due to the higher selling price of fish and prawn over paddy per unit weight (10 times higher). Thus, to assess the profitability of rice–fish farming system over rice cultivation alone in a cropping season, the net return from both the systems were calculated (Tables 7 and 8). Table 7 presents the cost of cultivation and return of the rice–fish farming system. The cost of rice cultivation includes the cost of seed (Rs. 400.00 ha−1 ), fertilizer (Rs. 1275.00 ha−1 ), labor (Rs. 6720.00 ha−1 ) and interest on working capital at 10%. Similarly the cost of fish culture includes the cost of fish seed (Rs. 200.00 per 1000 fry), labor (Rs. 720.00 ha−1 per 120 days), lime + fertilizer (Rs. 700.00 ha−1 per 120 days), feed at Rs. 380.00, 580.00 and 980.00 ha−1 per 120 days for stocking density of 15,000, 25,000 and 35,000 ha−1 , respectively and interest on working capital at 10%. The selling price of rice Table 8 Cost of cultivation and return from control plots (only rice crop) Weir height (cm)

Control (only rice cultivation) Rice yield (t ha−1 )

Straw yield (t ha−1 )

Gross return (Rs. ha−1 )

Cost of cultivation (Rs. ha−1 )

Net return (Rs. ha−1 )

10.0 12.5 15.0

2.76 3.31 3.36

3.29 3.96 4.11

12338 14820 15092

8674 8674 8674

3664 6146 6418

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at Rs. 4.00 kg−1 , straw at Rs. 40.00 per quintal and fish and prawn at Rs. 40.00 kg−1 was taken into account for calculating the gross return. The percentage of area devoted for fish culture and rice culture was also taken into consideration (Table 7). Fish stocking density of 25,000 ha−1 resulted in the highest net return at weir heights of 10 and 15 cm. In case of weir height of 12.5 cm, the highest net return was recorded at a stocking density of 35,000 ha−1 . However, the net return is not significantly greater than that obtained for a stocking density of 25,000 ha−1 at weir height of 12.5 cm. The increase was only Rs. 299.00 ha−1 with an increase in stocking density of 10,000 ha−1 . Thus, stocking density of 25,000 ha−1 may be considered to have resulted in the best net return. This strengthens our earlier conclusion that weir height of 12.5 cm along with fish stocking density of 25,000 ha−1 may be considered optimum for the study region. Further, in the rice–fish integration system net profit of Rs. 9701, 10,781, 8901 ha−1 were obtained for 25,000 stocking density at weir heights of 10, 12.5 and 15 cm, respectively (Table 7). Similarly, the net profit of Rs. 3664, 6146 and 6418 ha−1 were obtained for control plots at weir heights of 10, 12.5 and 15 cm, respectively (Table 8). At the recommended weir height of 12.5 cm and fish stocking density of 25,000 ha−1 , there is an overall increase in net profit by Rs. 4635 ha−1 in the rice–fish farming over control (rice farming alone). An initial investment of Rs. 60,000–65,000 ha−1 (fixed cost) is expected to incur for construction of refuges, peripheral trenches, dikes and weirs (earthworks and masonary works). With a cropping intensity of 100%, the farmer will start getting net return of Rs. 10,781 ha−1 after a gestation period of almost 6 year. However, situation will improve if a second crop is undertaken during winter season utilizing the harvested water from the refuge.

4. Conclusions Weir height is a function of soil type, land gradient, rainfall, crop characteristics, etc. Weir height of 12.5 cm, provision of peripheral trench on three sides of rice fields (0.5 m width and 0.3 m depth) and a refuge occupying 9% of field area are found suitable for two-stage rainwater conservation along with pisciculture in rainfed rice lands. Using this technology short-duration fish and prawn rearing (about 120 days) with a stocking density of 25,000 ha−1 along with rice crop produced a rice equivalent yield of about 4.4 t ha−1 without any pesticide application. This has resulted in a net profit of Rs. 10,781.00 ha−1 . Higher weir height (20–25 cm) though helps in conserving substantial amount of rainwater in situ, weir of relatively lower height (about 12.5 cm) for two-stage rainwater conservation resulted in better economic return by integrating pisciculture component. Further, this system generates employment, increases income of farmers, provides nutritional security and minimizes the risk of damage to rice crop brought about by natural calamities.

References ASCE, 1993. In: Jensen, M.E., Burman, R.D., Allen, R.G. (Eds.), Evapotranspiration and Irrigation Water Requirements. Manuals and Reports on Engineering Practice No. 70. American Society of Civil Engineers, 332 pp.

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