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A rice–fish system was investigated for 3 years to determine its effect on the growth and harvest of rice and the income to farmers. A ridge–ditch cropping system was used.

Method

An experimental ricefield of average fertility was plowed and levelled. A ridge–ditch system was used. The ditches were 20 cm wide and 30 cm long and the ridges were 30 m long and 50 cm wide. The ditch was 25 cm deep (from the surface of the ridge to the bottom of ditch). Rice was planted on the ridges and fish were stocked in the ditches. Rice plants were spaced at 17 cm x 13 cm, with 4–5 plant per clump and three line of rice plants per ridge. The ridges and ditches were estimated to cover 84% and 16% of the field area, respectively. There were three replicates for each of three different treatments. Treatments were randomly arranged. Nine small (0.02 ha) experimental areas were established. The total experimental field was 0.18 ha, and there was a 0.04-ha protective area around the field. The different treatments were separated by a low bank that was covered with a 50-cm plastic membrane that prevented fish escape and leakage of fertilizer.

Carp (7500 fry/ha) and grass carp (450 fry/ha) were released immediately after the rice seedlings were transplanted. Supplemental feed (375-390 kg/ha) was given until the rice plants bloomed. Fertilization and management techniques were the same as used for ordinary ricefields.

Results and Discussion

Fertilization of Ricefield

When the ricefield was stocked with fish, the nitrogen, phosphorus, and potassium (NPK) contents of the soil and water were increased significantly. Total nitrogen was particularly high. Weeds and plankton in the ricefield normally compete with rice for fertilizer. However, they were eaten by the fish and converted to a fertilizer that could be used by the rice. The physical and chemical properties of the soil also became more suitable for growth and development of the rice.

Oxidation and Reduction Potential of the Soil

A rice–fish ecosystem benefits both crops. Fish movements in the shallow water break the surface membrane formed by the microorganisms covering the soil. This increases the dissolved oxygen level in the soil and elevates its oxidation and reduction potential during the period of rice growth. These changes improve the oxygen content and effectively increase the utilization rate of soil nutrients. The ridge–ditch system allows water to be drawn into the soil in the ridge without having a negative impact on the fish. The sun can also increase the temperature of cultivation layer, which helps increase rice yields, especially of late rice. The ridge–ditch system can allows for the use of direct seedling, ratooning, and zero-cultivation method of rice planting.

NPK Content of Rice Plants

The NPK contents of the leaves and culm of rice plants grown with fish were higher than in the control. These differences were correlated to the differences in NPK levels in the soils in the two plots.

Chlorophyll Content of Plants

The chlorophyll content of rice plants at every developmental stages were significantly higher in the experimental ricefield. The high chlorophyll content indicates that the process of photosynthesis was more efficiency, which would lead to the accumulation of more carbohydrates.

Surface Area of Leaves

The surface area of leaves has higher in the early developmental stages in the experimental ricefield. In the booting and mature stages the factors were 6.9 and 2.5, respectively. In the control fields, the corresponding figures were 5.6 and 1.4. The larger surface area of the leaves and the higher content of chlorophyll will increase the efficiency of photosynthesis, and therefore increase the number of effective ears, the number of grains per ear, and the weight of the grains.

Activity of the Root System

The activity of the root system is expressed by the volume of water that flows through a wounded stem per unit of time. Strong activity means that the root system can absorb more nutrients from the soil. The root systems of rice plants grown in the experimental field always had stronger activity than the roots of plants in the control field at all developmental stages.

Accumulation of Dry Matter

The NPK content, surface area of the leaves, chlorophyll content, and activity of the root system were all higher in the rice–fish system. These differences are also expressed in the accumulation of dry matter. The total dry weight of the whole rice plant in experimental ricefield was 17.1% higher than in the control field. This is a fundamental condition for an increase in rice production.

Effect on Tillering

Tillering of the rice plant during the early stages of development is crucial stage to the production of effective ears. The number of ears and the time of earing are closely related to fertilizer level. The rice plants grown in the experimental field had a greater rate of tillering per day and more effective ears per plant. Although both fields originally received the same amount of fertilizer, the fish in the experimental field promoted more efficient use and distribution of NPK. The fish reduced the loss of fertilizer and increased soil fertility.

Weed Growth

Carp are omnivorous and grass carp are herbivorous. However, grass carp fingerlings also eat aquatic insects. When these two species of fish are stocked together, weeds are can be controlled in the ricefield. In the experimental field, there were significantly fewer weeds throughout the growing period.

Economic Benefits

The rice–fish system creates a mutually beneficial ecosystem. In the ridge–ditch system, the production of fish can reach 642 kg/ha. At the same time, the fish add fertilizer and eliminate pests and weeds from the ricefield. Rice yields were increased by 14.4%. It has been estimated that the ridge–ditch system can double total earnings.

Document(s) 1 of 41