Rice field crab technology
Rice-field crabs offer a sustainable and mutually beneficial system where rice, fish, and crabs can be raised together in harmony. To ensure a successful harvest, it's essential to focus on several key aspects.
First, selecting the right field and setting up an effective breeding system is crucial. The chosen area should have good water quality, sufficient water supply, easy access, and good water retention. It should also be a low-lying, contiguous area with shallow sections. The breeding system includes digging trenches, field channels, storage ponds, and raised dikes around the field. These structures must be interconnected, with trenches leading to ponds. The width of crab farming trenches is typically 3 to 6 meters, about 1 meter deep, and the spacing between trenches should be around 15 to 30 meters. Before use, the trenches need to be disinfected, and two layers of anti-escape nets should be installed at the inlet and outlet. An escape prevention wall should be built around the area, with rounded corners. The wall should extend at least 20 cm above the ground, and the inner surface should be smooth to prevent crabs from climbing out.
Second, choosing the right plantlets and crab species is important. In submerged areas, plants like Vallisneria and Potamogeton edulis should be planted. In open ponds, floating or emergent plants such as water lilies are suitable. For crabs, "yellow crabs" with uniform size and strong vitality are ideal. Stocking density depends on the planned yield, pond size, feeding conditions, and management level. To achieve larger sizes in one season, stocking should be slightly reduced, aiming for a weight gain of 5 to 7 times. A small number of fish, such as tilapia, silver carp, or grass carp, can also be added to improve water quality and utilize space more effectively. For example, in every 100 square meters, you might stock 4 to 5 kg of crabs (120–160 per kg) and 6 to 8 kg of fish. By the end of the year, both crabs and fish can reach about 25 kg each, significantly increasing overall productivity. If crabs are to be stocked, they should first be grown in holding ponds before being moved to the main field.
For rice cultivation, short-season, disease-resistant varieties that are fertilizer-tolerant are recommended. Seedlings should be large and well-established, with wide spacing to allow room for crabs and fish. Crabs and fish should only be introduced after the rice has taken root.
Third, feeding and management play a vital role. The feeding strategy should vary according to the growth stage of the crabs, combining fine, green, and coarse feeds with animal and plant-based sources. The amount of food should increase gradually as the crabs grow and temperatures rise. Feeding should be done in fixed locations, with specific quantities and timing. About 30% of the daily feed is given in the morning, and 70% in the evening, leaving a small portion uneaten the next day. When the temperature drops below 18°C, feed about 5% of the crab’s body weight, while above 18°C, it increases to about 11%. Early-stage crabs mainly rely on green materials, while later stages require more protein for fattening. Controlling premature maturation is important to promote larger individuals. Efforts should be made to find cost-effective feed sources to reduce expenses.
Regular monitoring of water quality is necessary, with water changes every 2 to 3 days or every 7 to 15 days initially. Each time, replace about 1/4 of the water or add 20–30 cm. During patrols, observe crab behavior, flush water when needed, and maintain clean conditions. Preventing escapes, controlling pests like rats, and managing diseases are also critical. Rats not only eat crabs but also create burrows that help them escape. Optimizing the environment reduces burrowing and promotes better growth.
For rice management, the goal is to ensure healthy crop growth. Use heavy base fertilizers, light top-dressing, avoid excessive flooding, and minimize pesticide use. If pesticides are unavoidable, choose low-toxicity, high-efficiency options. Apply them in small amounts, in portions, and target the leaves. After pesticide use, increase water exchange to reduce chemical residues. Urea can be used for nitrogen top-dressing.
Based on successful experiences, rice-field crab farming has minimal impact on rice yield and greatly reduces the need for pesticides. Thanks to the crabs' natural feeding, the use of chemical fertilizers is also reduced. As a result, farmers can save significantly on input costs while maintaining or even improving productivity.
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