STUDIES ON THE CULTURE OF THE FRESHWATER PRAWN MACROBRACHIUM ROSENBERGII AT VARIOUS STOCKING SIZE AND DENSITY IN RICEFIELDS
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MATERIALS AND METHODS
OBSERVATIONS AND RESULTS
- Watersupply, waterlevel, fencing
- Land utilization
- Hydrological and soil analyses
- Sampling, harvesting method
- Survival rate
- Population structure
- Survival rate and production in relation to abiotic and biotic factors
- Rice production
- Cost analyses
Staff of the Network of Aquaculture Centres in Asia, Farming System Research Institute and Sakonnakorn Fisheries Station, respectively
Experiments were conducted to investigate the possibility of prawn culture in ricefields. The work was undertaken in cooperation with the Farming System Research Institute in Bangkok and the Sakolnakorn Fisheries Station in Sakolnakorn in the North-east Thailand. The experiments were conducted on farmers' fields. Eight farmers participated in the research. They carried out day to day management of the 16 plots.
The plots were prepared for the experiments by digging a trench around the ricefield. Paddy was planted and tillered before the prawn were stocked. Prawn were of two sizes, just metamorphosed postlarvae (PL 1) measuring 1–1.5 cm in length, others as PL 60 measuring 4.5 – 4.8 cm in length. The latter ones were first nursed for 60 days in ponds before stocking in the ricefield. The PL 1 were stocked at the rate of 1.25/m2 meter and PL 60 at 0.31, 0.63 and 1.25/m2. Thus the rate of stocking varied from 3100 to 12500/hectare. The size of each of the ricefields in this experiment was about 1600 square meter.
After a culture period of 90 to 100 days the prawn were handpicked after draining or pumping the water out of the field. Six plots belonging to three farmers were infested with large predatory fish following heavy rain that flooded the fields and damaged the screening material of 50 cm height put on the dikes surrounding the plots. Flooding and infestation with fish were perhaps the main reasons for very low survival rates of 1 to 9% in these plots. Other fields gave survival rates of 10 to 89 %.
The PL 1 in ricefields grew to 5.4 grammes after a rearing period of 90 days. However 34 % of the population had an average weight of 17.1 grammes indicating clearly that the stocked PL grew extremely well in a ricefield ecosystem.
The PL 60 grew to an average weight of 16.8 grammes in 91 – 100 days in ricefields. But those stocked at a rate of 0.63/m2 grew up to 45 grammes with feed and contributed to 53% of the production in this field. The difference in growth was not considerable between the three stocking densities. However the production was higher with higher stocking density. Thus it appears that a stocking density of 1.25 prawn/m2 seems appropriate for a production potential of 13.0 g/m2. Survival rate as high as 89% can be expected. The water level in the ricefield needs to be maintained above 30 cm especially after the tillering of rice.
Fencing material on the dikes and screening material at the outlet and inlet need to be strong to ward off predatory fishes.
The results show that a high rice plant density has adverse effect on the survival rate of the prawn. Thus the present investigation shows that the ricefield can be used as nursery ground for prawn PL 1. Also the PL 60 can be raised in ricefields in concurrent cultivation with rice about half of them reaching marketable size within 3 months time, adding to the farmers' income. Thus the integrated farming system of rice and prawn has potential to increase farmers' income.
The cultivation of rice provides an excellent environment for a number of fish to breed and live in. The methods of rice fish culture and the innovations made in the region have been described (Raman 1968, Greg Chapman 1987, Taylor 1987 Naravut 1987, and others). Similary trapping and raising of prawn in ricefields and experimental work on rice cum prawn culture have been described (Pillay 1957 and Guerrero 1981).
This study investigated the possibility of raising prawn in the paddy fields in Thailand, where the paddy price has gone down and the farmers need to diversify their crop to add to their income (Anon 1984).
The study was undertaken in collaboration with the Farming Systems Research Institute in Bangkok and the Sakolnakorn Fisheries Station in Sakolnakorn. Eight farmers in the Lam Nam Oon irrigation project located in the North Eastern Part of Thailand near Sakolnakorn provided fields for the experiments (Figure 1).
Studies were undertaken under three different rates of stocking with different sizes of prawn, with and without feeding. The details of the experiments are shown in Table 1.
Each farmer provided two plots of about the same size for the experiment. These plots were stocked with the same stocking density and size. The prawn in one plot got supplementary feeding, the ones in the other plot had none. The plots had access to the irrigation system, so that they were kept under water throughout the experimental period.
Newly metamorphosed prawn larvae (PL) used for the experiments were airfreighted from Bangkok. This is the normal procedure followed by the large prawn farmers since it is cheap and convenient to get seed from Bangkok.
Post larvae (PL) stocked directly into the rice fields are referred to in this report as PL 1. Those PL 1 nursed for 60 days in ponds before stocking in the ricefield are referred to as PL 60. Two nursery ponds were stocked at a rate of 42.5 and 65 PL1/m2 respectively.
The supplementary feed was a commercial prawn feed commonly used in Thailand containing about 35% protein.
Ricefield preparation consisted of phoughing, leveling, rice planting and tillering.
The farmers dug a trench covering about 10% of the plot measuring 1 meter wide and 50 cm deep. Screens were placed on the dikes to prevent fish, frogs, tadpoles and others animals from climbing into the plots. Inlet and outlet pipes were screened to prevent the entry of the fishes and other animals through the water.
The soil and water were analysed, using routine analyses, to indicate fertility and watercondition. Data on growth of fish and routine water analyses like oxygen, pH, conductivity, and others were observed. The rice planted were three photosensitive varieties RD6 (rice department no 6), NSPT (Neow San Pa Tong) and KDML 105 (Kao Dok Mali 105). To study rice production, standard methods were used such as measuring plant length, counting number of riceplants (hills) per square meter based on the sampling of 4 two square meter plots, counting number of stems (tiller) per hill and weighing rice from the sampled area.
All plots had access to the water provided by the irrigation system of the Lam Nam Oon project during the culture period from the beginning of August up to first of November. During the last two weeks of the culture no water was available to keep the water level at an appropiate level. Some farmers tried to pump water from their fish ponds.
The water level in the trench and on the platform varied from plot to plot (Table 2). Levels were recorded from 28 to 150 cm in the trench and from 5 to 40 cm in the field. However parts of the ricefield dried up when the water distribution system did not irrigate certain areas for some time. The trench was constructed on all the plots. The width varied from 50 to 200 cm and depth from 28 to 150 cm.
Fencing material on the dikes was generally effective in preventing predatory fishes like catfish, snakehead and perch to climb into the ricefields. Inlet and outlet pipes were screened using bamboo and/or nylon mesh screens. When the screen was accidently lifted for short periods of time small snakehead, climbing perch, Tilapia and carp entered through the small meshes. In one plot a large snakehead weighing 0.5 kg was caught. The survival rate of the prawn in this plot was 0.6%.
Land utilization for rice cultivation and for the trench at the 16 experimental plots is shown in Table 3. The area planted to rice varied from 72 to 94%, whereas the trench occupied about 6 to 28% of the total ricefield area. In 5 plots the trench was dug at four sides, in 6 at 3 sides, in 3 at 2 sides and in 2 at one side only.
After 58 to 61 days of nursing PL 60 were harvested from the two nursing ponds. Prawn grew from 1–1.5 cm to 4.3 –4.8 cm TL (approx. 0.47 – 0.49 grammes). The survival rates in the two ponds were 44 and 57%.
Temperature of the water in the trench and in the field did not differ more than 2 degrees from the air temperature. The water temperature ranged from 26 to 32 degrees celsius. Temperatures measured at the bottom of the trench and the ricefield also varied but it was only by one degree Celsius.
Water pH varied from 6 to 9.8 in all the plots while soil pH varied from 4.4 to 5.8 (table 4).
Oxygen levels varied from 2.4 pm to 7 ppm in the afternoon (Figure 2). Alkalinity ranged from 30 to 110 mg/L. Visibility was, low and varied from 20 to 35 cm. Total ammonia-N ranged from 1.2 to 2.0 mg/l.
During the culture period of 90 to 100 days, the prawn were sampled once a month to monitor their growth. Prawn was collected from the trench only. Details of the growth rate is shown in table 5.
The water level in the field was reduced by draining or by pumping the water overnight, sometimes pumping continued in the morning. However some water is left in the field to keep the prawn alive. Harvesting prawn from a ricefield is somewhat more difficult than harvesting fish. Fish normally take refuge in the trench as soon as the water level gets low in the ricefield and those left in the ricefield are easily spotted and caught. But the prawn remain in the ricefield even when the waterlevel is 4–5 cm and they need to be handpicked. Thus harvesting in the 1600m2 plot usual require 6 to 8 people for 2 to 4 hours.
Survival rate varied considerably within and among the treatments (Table 6A). The rate of survival was for PL 1 47.5 ± 24.7 (n=2) and for PL 60 is 54.87 ± 22.58 (n=8). Appearently, stocking of large and old postlarvae does not necessarily lead to higher survival rate as well. The survival rates in 6 plots were not considered because the fields suffered from flooding and entry of large predatory fish.
Table 6 shows the details of production. As it is shown plots stocked with PL 1 gave a production rate of 5.33 kg/rai. Small prawn (average 7.4 cm and 7.1 grammes) contributed 66% (3.7 kg) and medium size prawn (average 10.9 cm and 15.1 grammes) 34% (1.6 kg) to the total production. Feeding of prawn during the grow out period in the rice field did not give a better survival rate (28 i.s.o 67 %), nor a higher production rate.
In plots stocked with PL 60, production varied more or less according to the stocking density. Prawn stocked at a rate of 500 /1600 m2 gave a production of 7.17 kg/1600 m 2, while those stocked at 1000/m2 the production was 22.10 kg/1600 m2 with the stocking rate of 2000/m2 the production was 21.33 kg/1600 m2.
The population structure of the prawn is based on the 3 groups selected at harvest : small (smaller than 9 cm), medium (between 9 and 11 cm) and large (above 11 cm). The percentage of each of these groups to the total production is shown in Table 7. prawn stocked as postlarvae grow well but the culture period of 3 months is too short to get more than 35% of the prawn bigger than 9 cm total length. Prawn stocked as PL 60 and at different densities show a size distribution in which it is evident that the lower stocking density results in larger size prawn (above 11 cm total length).
Table 8 shows abiotic and biotic factors. Low water depth in the field and in the trench below 30 cm and 50 cm, respectively may be related to low survival and consequently low production as had been observed in plot 2B and 3B. The application of feed is only leading to a higher survival rate in 1 of the 5 replicates not faced with calamities.
In fields where rice is planted at 10–18 plants/m2 and PL 1 stocked, prawn grew up to 10.4 g with a survival rate of 46%. At a rice plant density of 20–26/m2. PL 1 grew only up to 4.7 g with a survival rate of 2%.
PL 60 grew in fields where rice was planted at 10–18/m2 to an average weight of 20.03 g with a survival rate of 46%, while at a density of 20 – 26 plants/m2. PL 60 reached 13.0 g with 29% survival. So rice planted at low density (below 18 per square meter) may be an indication to expect good growth and survival rates.
In 7 plots the fish catch was recorded which constituted of carp, Tilapia and snakehead. One big snakehead was found (0.5 kg) in a plot with very low survival rate of 0.6%. However no clear relationship can be found in extraneous fishes quantity harvested and prawn production. A survival rate of 89% of prawn is possible besides 9.4 kg of fish (101 fishes). A low survival rate of 4.1% and 10.6 kg of fish (72 fishes) was observed as well, but can be explained by lifting of a screen during heavy rain. Fishcatch cannot be related to feed supplied into the field. No feed can give higher fish catch than plots where feed was given (2000 PL 60/1600 m2).
Table 9 and 10 show the rice production. Farmers planted 3 kinds of rice RD6 (Rice Department no 6), NSPT (Neow San Pa Tong) and KDML 105 (Kao Dok Mali 105). The first two are glutinous varieties, while the latter is a non-glutinous one giving generally a lower production than the glutinous ones. Rice was grown for 136 to 151 days. Standard methods were used to assess the rice production of the field (Natavudh pers. comm.). Rice production of the 16 experimental plots based on the standard method ranged from 340 to 642 kg/1600 m2. Within the same treatment with and without feed at the same farmers site, the difference in rice production was 12 to 66 kg/rai. However at one site the two plots (2000 PL 60/rai, no 4A -5A) gave a production difference of 136 kg. A possible reason for this is that the plot is located at the lowest point thus getting all the nutrient-rich water from other plots.
The rice production of the experimental plots indicates that feed supplied to the prawn (17 kg for plots with 500 PL 60/rai, 35 kg for 1000 PL 60/rai and 70 kg for 2000 PL60 /rai) contributed to some extend to a higher rice production perhaps through the fertilizing effect of the left over feed.
Considering the rate of production of a rice field 6 to 26 % used as a trench could have produced 29 to 128 kg of rice resulting in 58 to 256 Baht. Prawn production of the same area (based on 138 kg/1600 m2) would be 8 to 36 kg resulting in an income of 1200 to 5400 Baht.
A general survey of ricefields near the experimental plots indicated that the production of rice (table 10 column VII) is less than the production in paddy-cum-prawn culture (column VI). The factors contributing to the higher production in rice- cum prawn field need to be investigated.
An estimation is made of variable costs for the fields stocked with prawn (table 11). The variable costs consist of costs for rice seed, PL 1 and PL 60, field preparation (netting material based on 3 years depreciation) and labour required for rice and prawn culture activities (labour for making the trench and placing the netting material based on 3 years depreciation). These costs are for one rai (1600 m2) field with prawn Baht 1170 to 1650 ($47 to $66) and rice Baht 980 ($40). Costs to prepare a rice field for rice cum prawn culture require little investment.
Prawn marketable after the growout period in the rice field were sold by some farmers. Based on their sales (Baht 180/kg for 30 count) sale potential of marketable size prawn as harvested during the experiments is calculated for PL 1 and PL 60 for the three stocking densities. Below a summary of table 11 is presented.
|PL 1||PL 60||PL 60||PL 60||Rice|
|Stocking rate 1600 m2||2000||2000||1000||500|
|Sales (after 100 days of culture)||---||1104||2142||1044||1800|
The table shows that prawn sales after the culture period are covering the variable costs for PL 60 stocked at rate of 1000/1600 m2. For the other two densities of PL 60 prawn require more time to reach marketable size than the 100 days used in this experiment.
The rice area is reduced by 10% and used for the trench. This reduction of land area results in 50 kg less rice and consequently Baht 100 less income. This amount is covered by the sales of the prawn.
The pH in the paddy field and in the trench varied from 6 to 9.8 which is suitable for prawn to thrive in. This was also the case for temperature, which fluctuated from 26 to 32 degrees Celsius.
Oxygen levels in the experimental plots were sufficiently high since the lowest concentration recorded at 5 am was 2.4 ppm. Green (1977) describes the massive killing of prawn when an oxygen level is recorded as low as 0.5 ppm at 6.30 am. The time of sampling is important. Boyd (1987) stresses that proper judgement of pH, temperature and oxygen data in shrimp ponds is strongly related to the time the sample is collected. It is therefore advisable to collect data at least early morning and not at random times as was mostly done during the experiment.
The rice fields need to be prepared properly with fencing the dikes, screening the inlet and outlet pipes and digging a trench around the rice culture area. However proper leveling of the field appeared to be highly essential to facilitate harvesting of prawn.
The rate of survival and growth of PL 1 and PL 60 in the present investigation is comparable to those observations in ponds. Ra'anan (1983) stocked M. rosenbergii 0.57 grammes, 72 days old, at a density of 25 per square meter in ponds. After 54 days the juveniles weighed an average of 6.4 grammes (variance 10.90) with a survival rate of 48.9%. Green (1977) stocked the giant freshwater prawn with a mean length of 2.39 cm in a pond at a rate of 3.8/m2 with a mean length of 2.39 cm. In 55 days prawn grew upto 9.39 cm and 8.07 grammes. In the experimental period of 100 days PL 1 grew from a total length of 1.5 cm to 8 cm and a weight of 10.4 grammes. In the same period PL 60 grew from 4.5 cm and 0.5 gramme to an average of 11.1 cm and 18.7 gm. The results clearly show that prawn grew well in the ricefield. Guerrero (1981) concluded from his experiments with Macrobrachium lanchesteri in rice fields that rice plants are beneficial for prawn growth and survival rate by providing additional shelter and increasing feeding surface area for the prawn. Chien (1980) stocked crayfish (Procambus clarkii) in ricefields and ponds. Crayfish production was significantly higher in ricefields than in control ponds.
The experiments further show that prawn density of 0.3–0.6 and 1.25/m2 is sufficiently low and not leading to growth depression. Ra'anan (1983) compared the growth in two stocking densities: one and 10 per liter. She observed that growth of postlarvae stocked at 10 per liter is lower than that of postlarvae stocked at one per liter.
Surrounding ricefields not stocked with prawn gave generally a rice production lower than fields stocked with prawn (Table 11). The higher rice production in the experimental fields may be explained by prawn disturbing the soil so that nutrients are freed, by proper water management and by daily attention to the experimental plots. Factors contributing to higher rice yields need in depth analysis.
Investment costs for farmers to integrate fresh water prawn into the culture of rice is low ($47–66/1600 m2). After a growout period of 100 days a profit was made of $ 32 for one of the three stocking densities. The stocking density having the best potential lies somewhere between 0.6 and 1.25/m2. The experiments indicate that profit can be expected for the farmer when he integrates prawn with rice.
To increase prawn production in rice fields the nursing period in ponds could be 75 days and the grow out period in the rice field 120 days. In table 12 the time schedule for this is worked out.
These preliminary experiments on prawn in ricefields have clearly shown that the system has potential. Confirmation and refining the techniques is necessary.
Dr. Kitjar Jaiyen, Director of the National Inland Fisheries Institute and Mr. Chen Foo Yan, Coordinator of the Network of Aquaculture Centres in Asia (NACA) provided assistance and support. The experiments described in this report could not be possible without the continuous support of my counterparts Mr. Natavudh Bhasayavan, Agronomist of the Farm Systems Research Institute and Mr. Chumnarn Pongsri, Biologist of the Sakolnakorn Fisheries Station.
Special thanks are rendered to Dr. V.R.P. Sinha, Senior Aquaculturist of NACA for his suggestions and guidance in finalizing this report.
Anon. 1984. Report on the socio-economic survey of the intensive and extensive land consolidation area of the Lam Nam Oon integrated rural developement project. Division of Economic Project and Program Evaluation, Office of Agriculture and Coorperatives, Bangkok.
Boyd C.E., 1987. Water quality management for brackishwater ponds with emphasis on shrimp farming in Thailand. Report for the Asian Developing Bank.
Chapman G. et al, 1987. The role of cultured carp (Cyprinus carpio), nile Tilapia (Oreochromis niloticus) and Thai silver barb Puntius gonionotus) as agents of rice insect pest and disease control, and an analysis of stomach content, in low land paddies of North-East Thailand. Report Farm Systems Research Institute Sakon Nakorn, Thailand.
Chien Y. and Avault J.W., 1980. Production of crayfish in ricefields. The progressive fish-culturist, vol. 42, no 2, April.
Green P.J. et al, 1977. A massive kill of pond reared Macrobrachium rosenbergii. Aquaculture 11:263–272.
Guerrero L.A., Circa A.V. and Guerrero III R.D., 1981. A preliminary study on the culture of Macrobrachium lanchesteri (de Man) in paddy fields with and without rice. In Giant Prawn Farming, Elsevier Scientific Publishing Company, Amsterdam.
Naravut J., 1987. Ricefield fish culture in Surin. In Abstracts of Seminars on Fisheries, DOF, Thailand.
Pillay T.V.R. and Bose B., 1957. Observations on the culture of brackishwater fishes in paddy fields. In West Bengal (India). Proc. Indo-Pacific. Fish.Coun., 7(II–III), 187–192.
Ra'anan Z., 1983. The effect of size ranking on the moulting cycle of juvenile stages of the freshwater prawn Macrobrachium rosenbergii (de Man) when reared individually and in pairs (Decadopa, Caridae). Crustaceana 45 (2), 131–138.
Raman K., 1968. An experiment in prawn-cum-Tilapia culture in paddy field. Indian Journal of Fisheries 15 (1/2): 175–179.
Taylor. S. et al, 1987. Guidelines for highland pond and rice-fish culture. Report Farm Systems Research Institute Chiangmai unit Thailand.
Figure 1. Location of the project sites in Lam Nam Oon irrigation area in the North East of Thailand
Figure 2. Oxygen levels in the trench and on the platform during 3 consecutive days in November 1987.
Table 1. Set up of experiments.
|Stocking material||Plot no.||Stocking density per rai*||Stocking density per sq.m||No feed||Feed|
PL = postlarvae.
PL 60 = postlarvae nursed during 60 days before stocking in the ricefield. N.B. All treatments were done in duplicate.
Rai - 1600 square meter.
Table 2. Water depths as measured at the trench and platform of the different experimental plots.
|Plot number||Waterdepth on platform in cm||Waterdepth in trench in cm||Width of the trench (cm)|
Table 3. Different parameters for land, trench(t) and rice area(r).
|% trench landarea|
trench*- trench around the ricefield at 1,
2, 3 or 4 sodes.
m* square meter.
Table 4. Soil analyses in 9 experimental plots.
|plot number||pH||lime requirement kg/rai||electric conductivity*||texture|
|9B||5.3||200||0.012||sandy clay loam|
* - electric conductivity (1:5) in millimhos at 25 degrees Celcius
Table 5. Length and weight increasement based on the samples collected during four different days.
|No./rai||Feed||Plot No.||Length (cm) on day||Weight (gm) day|
PL = postlarvae,
+ feed supplied,
- not recorded,
ns = no shrimp collected in the sample.
Table 6A. Stocking and harvest data of freshwater prawn (Macrobrachium rosenbergii) cultured in ricefields
|Farmer name||Stocking rate||Harvesting data||Culture days||Plot No.||Stocked per rai||Actual||Number||Survival rate|
PL - postlarvae,
PL 60 - postlarvae nursed during 60 days before stocking in the ricefields,
* - commercial prawn feed supplied at 5% (PL were given initially 15%),
o - 918 PL,
oo - 1400 PL,
1 rai - 0.16 ha
Table 6B. Continuation at Table
|Farmer name||Plot No.||Total Length(cm)||Average weight(gm)||Sample||Average||Production(kg)||Production|
Table 7. The production of the experimental plots based on percentage wise contribution of three size groups
|Prawn stocked per rai||large||medium||small||total||number of plots|
small - below 9 cm total length-,
medium - between 9 and 11 cm total length- and
large - above 11 cm total length)
Table 8A. Various parameters in relation to the survival the prawn in the 16 experimental plots.
|trench in % of trench +rice culture area.||6||7||7||8||25||26|
|width of the trench (cm)||150||150||100||100||150||15|
|depth of the trench (cm)||28–60||35–62||30–50||40–50||120–150||65|
|waterlevel on the platform (cm)||17–36||15–30||15–25||0–25||15–30||0|
|rice plant density per square meter||13–16||10–14||20–22||22–23||22–24||20|
|stocking density per square meter||1.25||1.25||1.25||1.25||1.25||1.25|
nr - not recorded.
* - including a 0.5 kg snakehead.
Table 8B. Various parameters in relation to the survival the prawn in the 16 experimental plots.
|trench in % of trench + rice culture area.||9||19||13||14||23||26|
|width of the trench (cm)||100||100||50–150||50–150||200–400||200–400|
|depth of the trench (cm)||60||60||30–35||35–40||150||150|
|waterlevel on the platform (cm)||20–36||25–40||5–25||10–25||50||50|
|rice plant density per square meter||18–20||16–22||23–26||19–22||20–22||21–22|
|stocking density per square meter||1.25||1.25||0.62||0.62||0.62||0.62|
Table 8C. Various parameters in relation to the survival the prawn in the 16 experimental plots.
|SURVIVAL % 63||63||61||1||9|
|trench in % of landarea.||12||6||9||7|
|width of the trench (cm)||150||150||120||110|
|depth of the trench (cm)||42–60||40–50||50||46–50|
|water level on the platform (cm)||10–30||15–40||20–30||25–30|
|stocking size (cm)||4.5||4.5||4.5||4.5|
|rice plant density meter/m2||10–16||11–18||10–16||12–15|
|stocking density per/m2||0.31||0.31||0.31||0.31|
*- plot flooded.
Table 9. Rice production at the experimental plots.
|Plot no.||rice-variety||date of transplanting||date of harvest||culture days||plant height (cm)|
RD6 - rice department no 6 (glutineous rice variety),
NSPT - Neow San Pa Tong (glutineous rice),
KDLM105 - Kao Dok Mali (non glutineous rice).
Table 10. Continuation of table 9.
|Plot no.||hill no.*||tiller no/hill||production kg/8 sq.m.||production kg/rai based on column IV||production kg per rai including trench based on col. IV||actual production kg/rai of fields without prawn|
* sample collected from 8
nr - not recorded,
na - no rice field available for control.
Table 11. Variation costs (in Bath) of experimental plots with and without feed and a trench covering 10% of the field area. The calculation is made for an one rai (160 sq.m) field.
|No feed Stocking rate||Rice|
|Labour for prawn:|
|Rice yield (kg)||450|
|Prawn yield (kg)|
after 100 days
after 100 days
|Sale of rice||-||1800|
* - costs based on 50% survival after nursing including feed and
# - based on 3 years depreciation.
** - costs for one man day Bt 30.
@ - based on the actual sales of prawn by two farmers (Bath 180/kg for 30 count).
@@- due to draught in 1987 the price of rice is high, 4 Baht/kg.
Table 12. Suggested work schedule for rice culture integrated with fresh water prawn.
|prawn nurser||75 days|
|rice integrated with prawn|