Rice-prawn and rice-shrimp culture in coastal areas of Viet Nam
Tidal flats in coastal areas are flooded periodically during high tides. During the dry season, salinity is usually higher than 5 ppt (per thousand); thus, most paddy fields are in fallow. In the rainy months, salinity declines so rice cultivation is possible. Farmers in these coastal areas of southern Viet Nam have living standards lower than their counterparts in freshwater regions. Integrating freshwater prawn culture with rice during the rainy season, as well as marine shrimp monoculture in the dry season, is one way of increasing their incomes. Both systems are described here.
- The field should be close to a river or channel.
- Choose a low and flat place so it is easy to get water during high tide.
- Avoid high acid sulphate soils.
Dike and trench construction
- Surface area of field: 1 000-3 000 mē
- Trench is 2-3 m wide and 0.8-1.0 m deep with trench-to-ricefield rate of 10-20 percent.
- Peripheral dikes should be at least 20 cm higher than the annual flooded level.
- Install 2-3 inlet and outlet pipes (at least 20 cm diameter) made of coconut trunk or wood. The inlet pipe should be installed so as to let water into the paddy field at high tide; the outlet pipe should allow water to drain from the trench when opened.
- Inlet and outlet pipes should be screened to prevent the intrusion of predators.
- Cover the trench surface with tree branches or plant water hyacinths, etc., along the trench to discourage poaching.
- Stock juveniles of giant freshwater prawn (Macrobrachium rosenbergii) at a density of 1.2/mē (at least 4-5 g each).
- Stock 10-15 days after transplanting.
- Criteria for juveniles: vigorous, strong and uniform in size
Note: If stocking density is higher than 1/mē, supplementary feeding should be done and trench-to-ricefield rate should be higher than 10 percent. If water exchange is poor, do not stock higher than 1/mē.
- Prawn can subsist on natural food in the paddy field, especially if it is loaded with manure.
- The following supplementary feed can be given: rice bran, rice grain, copra, oil cake, cassava root, broken maize, fiddler crab (Uca spp.), shrimp or prawn head wastes and trash fish.
- Feeds can be given daily at 5 percent of the prawn's body weight (if no manure loading) or 2-3 percent (with manure loading). Mix ingredients thoroughly, form them into balls and put them in feeding trays. The use of feeding trays controls consumption of feeds and prevents wastage.
- Feed twice a day: one-third of the quantity in the morning and the rest in the afternoon.
- Check feed consumption daily to adjust the feeding regime as necessary. Below is a recommended formula for prawn in rice paddies.
50 percent - rice bran, broken rice or rice grain
20-30 percent - cassava root or broken maize
20-30 percent - trash fish, shrimp or prawn head wastes or oil cake
Predators include sea bass, tilapia, snakehead and other wild fish that compete with the prawn for feeds. Predation can result in very low prawn yields.
Before stocking prawn, use any of the following measures:
- Drain ricefields and apply lime at the rate of 10 kg/100 mē (15-20 kg for acid sulphate soils).
- Apply Derris root (Derris elliptica), 1-1.5 kg soaked in 10 litres water/1 000 mē.
- Release ducks into the ricefields for several days.
Within the culture time, put gill nets in the trenches to catch the predators going to the ricefields.
Care and maintenance
- Water exchange is essential to supply oxygen to the prawn and to remove detrimental substances in the water. This should be done at least twice a month. The more frequent the water is changed, the more suitable it is for the prawn's growth and development.
- Water exchange also improves the pH value in the fields especially in sulphate acid soils.
- Dikes should be repaired yearly.
- Cover crab holes along the dikes to prevent leakage.
- Check daily the screen mesh on the outlet and inlet pipes.
- Harvest prawn 5-6 months after rice harvest.
- Open the outlet pipe at low tide and drain the field and trench.
- Hand-collect prawn in the ricefield and use a net to harvest in the trench.
- Harvest only the big (more than 15 g) prawn. The small ones are reserved for the next culture season.
Note: Transfer small prawn immediately to a hapa (net cage) to keep them alive for the next culture. Bring harvested prawn as soon as possible to the dealer or keep them in ice so that they stay fresh.
Land preparation and transplanting for rice
- Local varieties are recommended. Transplanting should be done when the salinity is lower than 5 ppt.
- Plough and harrow thoroughly before transplanting.
- Transplant 3-40 days after seeding.
- Apply 50 kg diammonium phosphate and 5 t manure/ha before ploughing.
- Use 50 kg urea/ha for top-dressing.
- No pesticide or herbicide is applied in integrated prawn-rice culture.
- Use brown planthopper-resistant varieties of rice.
- Release one-month old ducks into ricefield to feed on insects, especially hoppers.
Note: In case the above measures cannot control pests, pesticide application can be an alternative. Before applying the pesticide, drain water in the field to let prawn take refuge in the trench for 3-5 days.
Monoculture of prawn or shrimp in fields during dry season
- Nipa and coconut trees are indicators of salinity lower than 10 ppt. Rhizophora (a mangrove species) is an indicator of salinity higher than 10 ppt.
- During the dry season, when salinity level is high and therefore not suitable for rice growing, the fields can be used for shrimp monoculture.
- Freshwater prawn (M. rosenbergii) can be grown if the salinity is not higher than 10 ppt. The procedure is similar to that applied in the rainy season. When the salinity is higher than 10 ppt, freshwater prawn become stunted.
- Tiger shrimp (Penaeus monodon) and banana shrimp (P. merguiensis) can be cultured in ricefields when the salinity is higher than 10 ppt in the dry season.
Stocking density - 1/mē
Stocked juveniles - 2 g/head
Feeding rate - 2-3 percent of body weight
Feed formula - 50 percent rice bran (broken rice), 50 percent trash fish (fiddler crab, oil cake)
Culture time - 5-6 months
Other procedures are similar to freshwater prawn culture.
Estimated cost and return (in VN$) of rice-prawn culture in 1 ha coastal areas in South Viet Nam
Issues for further consideration
The situation described here is that before 1992 when the major shrimp disease outbreak in Asia occurred. Since then, disease in shrimp farming has become a major risk that has totally altered the economics of the system and has to be considered according to local prevalence. It is a major constraint to farmers, even for very extensive operations. Existing and ongoing epidemiological studies by Stirling University is applying epidemiological approaches to understand risk factors for farmers in these systems.
Resource constraints to these systems should be given attention. All the feeds given represent an investment in time or cash and this should be considered. Collection of snails and fish is a major activity that is not typically shared equally by all household members.
There are large areas of semi-saline zones around Asia (e.g. Bangladesh and India) and elsewhere, and this case study should be of relevance there. These tend to be marginal areas where poorer people live.
In other contexts, after shrimp production became established, outside interests have become influential and extract much of the value from shrimp culture. In Viet Nam, government policy prioritizes increased rice production areas. Large areas are being protected from saline intrusion, and converted to triple rice crop production, from previous shrimp culture, although in the transition, farmers are trying to grow shrimp as long as possible (e.g. grown at salinities less than 4 g/litres) due to much higher returns.