Rice-prawn culture in the Mekong Delta of Viet Nam
Rice-prawn farming, a traditional practice in freshwater areas of the Mekong Delta, has become more popular for the past few years. This paper is based on two rice-prawn farming systems tried in one of the key sites (Phung Hiep district, Han Giang) of the Mekong Delta Farming Systems Research and Development Centre as part of an OXFAM-funded project. In 1990, the following farming systems were tried in the site:
- System 1 - prawn integrated in dry season modern variety rice-wet season, modern variety rice (DS MVR-WS, MVR).
- System 2 - prawn integrated in wet season modern variety rice-wet season transplanted rice (WS MVR-WS TPR).
The following are the eight main steps in rice-prawn culture:
1. Field selection
- The field should be close to a water source and should not have salinity or acidity problems. Water depth can range from 20 to 30 cm.
- Flat fields are often preferred with an average of 1-2 ha. Peripheral dikes should be high enough to prevent flooding in the wet season and should be compacted so that water does not leak and percolate in the dry months.
2. Land preparation
- Dig trenches inside the dikes on all four sides of the field, 3-4 m wide and 1-1.2 m deep. Make supplementary trenches outside the dikes to store prawn juveniles or adult prawn as needed.
- Make at least three sluice gates with 0.3-0.4 m diameter. Two are installed at 0.2 m above field level to retain water and one at the lowest level of the trench for draining during harvest time. Put a net or woven strips of bamboo on the sluice gates to prevent fish and prawn from escaping.
- Prepare the field thoroughly before stocking. Deposit materials and mud should be removed.
- Apply powdery lime (100 kg/ 1 000 mē) or Derris elliptica roots (1-1.5 kg soaked in 10-15 litres water/1 000 mē) to help get rid of wild fish and other carnivorous animals (e.g. crabs, snakes, frogs, etc.)
- Sun-dry trench bottom for 3 days so that it becomes solid. This prevents mud from penetrating into the prawn's filament chamber and feeds from sinking into the mud.
- Cover 8-10 percent of the water surface in the trench with plant branches to discourage poaching.
3. Selection of prawn juveniles and rice seed varieties
- Gather healthy juveniles of freshwater giant prawn (Macrobrachium rosenbergii) from the river. They should be confined in supplementary trenches or stocked immediately if the trenches are already prepared.
- Short-duration modern varieties can be used, such as IR66, MTL85, MTL86, MTL88 (100-110 days) or medium maturity modern rice or high-yielding local varieties such as Mot bui lun, Lua thom, Trang tep, Tai nguyen (which flower on 20-30 November). Use seed stock with more than 80 percent germination rate.
4. Rice crop establishment and prawn stocking
- Stock with juveniles (size of 100-250/kg) at the rate of 7-8 kg/ha.
- Put the juveniles in a big bamboo basket and slowly submerge this into the water. Stocking should be done after land preparation or rice broadcasting or transplanting.
- If two modern rice crops are grown: Broadcasting or transplanting time for dry season crop is in November- December. Prawn fry are stocked in December at 10 days after broadcasting or 5 days after transplanting. Prawn will be stored in the trenches during harvesting or dry season crop or during land preparation for wet season crop.
After broadcasting/transplanting wet season rice crop in March-April, allow prawn into the ricefield at 10 days after broadcasting or 5 days after transplanting.
- In wet-season modern rice-transplanted local ricefields: Prawn juveniles are also stocked in December in supplementary trenches for storage and these can be released into the field in March-April (10 days after broadcasting or 5 days after transplanting of wet-season crop) and in July-August (10 days after broadcasting or 5 days after transplanting of transplanted local rice). They can be placed in supplementary trenches during harvesting of wet-season crop (June-July) or land preparation for trans-planted local rice crop (July-August).
5. Feeding, weeding and fertilizing
- Cassava, sweet potato, broken rice, milled rice (soaked or cooked), rice bran
- Crabs, snails, trash fish
- Combined feeds: 50 percent rice bran, 10-20 percent cooked broken rice, 20-30 percent trash fish, 10 percent oil cake
- Feeding ratio: 3-5 percent of prawn weight
- Feeding times: at least twice a day (1/3 at 5-6 a.m. and the remaining 2/3 at 5-6 p.m.)
Put the feeds in feeding trays and place them anywhere along the bottom of the trench.
- Weeding. Hand-weeding is recommended at 15 and 35 days after broadcasting or 15 and 30 days after transplanting. Herbicides should only be applied if necessary.
- Fertilizing. Both organic and inorganic fertilizers are used. Organic fertilizers and phosphorus can be basally incorporated in the soil. Nitrogen should be split (divided between two applications). Complete fertilizers are applied at 10 days after broadcasting or split for transplanted rice. Potassium can be basally applied and top-dressed.
Fertilizer formula for modern rice per hectare:
- 200 kg monosuperphosphate + 200 kg urea + 50 kg potash
- or 100 kg 18-46-0 days after planting + 100 kg urea + 50 kg potash
And for transplanted local rice per hectare:
- 200 kg monosuperphosphate + 100 kg urea + 50 kg potash
- or 100 kg days after planting + 50 kg urea + 50 kg potash
Material flow in rice-prawn integrated farming systems of Mekong Delta, Viet Nam
6. Management of water, sluice gates and dikes
- Change water every day with the tidal regime. Water level in the field is kept at more than 20 cm and at 100 cm in the trenches.
- Check water quality every day. If it is acidic or hot, change the water or add more water into the field.
- The water surface of trenches should be exposed to air; only 15-20 percent of surface may be covered by floating weeds or vegetables like water spinach, etc.
- Check surrounding dikes regularly for leaks. Put fish nets or bamboo fences on sluice gates to prevent prawn from escaping.
7. Other management practices
- An advanced management practice is to record growth rate of prawn monthly by weighing them. Average growth rate is about 5-6 g/prawn/month. If growth rate falls below 3 g, growing conditions should be improved or more feeds should be added.
- Prawn molt about twice a month. After each molting, prawn weight will increase from 3 to 5 g. Molting often occurs in the early morning or at night time during low tide.
- If water is deficient in oxygen, prawn often appear at the water surface in the early morning. When deficiency is more serious, most of the prawn may die. Maintain oxygen levels by keeping recommended water depth and feed regularly to prevent water from getting polluted.
- Some minor diseases can hamper the prawn's growth rate. Improving growing conditions (e.g. using powdery lime before prawn stocking, keeping water clean during the growing season), using good fry and controlling parasites normally suffice to prevent diseases.
- Control carnivorous fishes and other animals.
- Use rice varieties resistant to major insects and diseases to minimize use of chemicals. If chemical application cannot be avoided, partially drain the field so that the prawn can take refuge in the trenches. Water should be changed completely 3-4 days after pesticide application.
8. Harvesting of rice and prawn
- Prawn are harvested in November/December before harvesting of transplanted local rice crop and before land preparation for dry-season rice crop. They can also be partially harvested after 4-5 months of growing. Only the larger ones will be taken and the rest are restocked with an additional amount of the same-size prawn.
- Rice can be harvested when 80 percent of the crop is mature. Delay in harvesting can result in serious grain shattering. After threshing, rice should be sun-dried and stored.
Average prawn and rice yields in the 1990 crop season
DSMVR: dry season modern variety rice
WSMVR: wet season modern variety rice
WSTPR: wet season transplanted rice
2MV: dry season modern rice followed by wet season modern rice
MVTPR: wet season modern rice followed by wet season transplanted rice
Partial budget (in thousand VND/ha) for two dominant rice-prawn systems
1992: US$1 = 7 000 VND
This table shows partial budget of rice monoculture and of prawn integrated in the two
dominant rice cropping systems. Net return from prawn in the two systems seems to be the
same while net return from WSMV/TPR is higher than that of 2MV system. Prawn contributed
significantly to net income of rice farmers in the site.
Issues for further consideration
The system described is characteristic for the Mekong Delta and other areas where wild or hatchery-reared Macrobrachium juveniles are available and rice production is of moderate intensity. The existing rice varieties and culture systems (e.g. wet season, modern varieties, transplanting, etc.) and options for modifying these will influence the design of an adoption.
With a harvest of 13-24 kg/ha, questions arise as to the continued viability of these systems. Different options exist to improve these systems. Associated risks to the farmers will need to be considered.
In assessing economic viability in given local situations, additional rice fertilization due to more frequent water exchanges, feed costs for prawns, costs of other off-farm materials such as trash fish, have to be included.