Nutritional requirements and production experience of compound feed for cultured river crab

River crab is a unique aquatic product in China, which is famous at home and abroad for its high nutritional value and unique flavor, and has become an important product for China's export to earn foreign exchange. Therefore, from lakes, reservoirs to ponds and rice fields are carrying out river crab culture on a large scale, and the rapid development of river crab breeding requires that the formula feed industry should keep up with it. However, there are relatively few studies on the nutrition of compound feed of river crab in our country. In this paper, the nutritional requirements of river crab and the development of formula feed are briefly described.

1 nutritional requirements of river crab

1.1 proteins and amino acids

The crude protein content of the formula feed of river crab from scratch larval stage to mature stage is 45%-28%. Using white fish meal, cake meal, gluten powder, shrimp bran powder and egg yolk powder as the main raw materials, the formula feed for the larval stage of river crab with 45% crude protein was fed on the Z1~Z5 stage larvae with fineness of 80% 250 mesh. the larvae had high fecal rate, neat metamorphosis, short molting time and high survival rate. After the crab seedlings came out of the pool, there was little residual bait at the bottom of the pool. The big-eyed larvae to V-stage young crabs were fed with a compound feed (opening material) with 42% crude protein. After about one month of feeding, the young crabs grew fast, lively and healthy, and the survival rate was about 50%. The feed coefficient is about 0.8. The crude protein content of the formula feed during the river crab cultivation period is 28% to 36%. Under this condition, the individual river crab fed is larger, the disease is less, and most farmers can make a profit. According to the physiological characteristics of river crab, the temperature is suitable in the early stage of development, and the compound feed of 36% crude protein can be fed. In the middle stage, the water temperature is higher and the growth of river crab is relatively slow. It can be fed with 28% crude protein compound feed, combined with some aquatic plants and miscellaneous grains. In the later stage, the water temperature is more suitable for the growth of river crabs. At this time, river crabs eat a lot of food and prepare nutrients for overwintering, and 33% of the compound feed can be fed. Therefore, the river crabs caught in late autumn and early winter are crab fat cream yellow. This feeding mode is also in line with the feeding principle of "fine at both ends and thick in the middle" in river crab breeding.

The essence of the crab's need for protein is the need for amino acids. The protein obtained from the feed is digested into peptides, amino acids and other small molecular compounds before it can be finally transformed into the crab tissue. The essential amino acids needed by river crabs are basically similar to shrimp, namely, threonine, valine, leucine, isoleucine, tryptophan, methionine, phenylalanine, histidine, lysine and arginine. The suitable requirement of river crab for all kinds of essential amino acids has not been reported, and shrimp is the main reference in the design formula, but the growth effect of river crab on the added free amino acids is not significant. In order to make the amino acids in the formula feed complement each other and meet the needs of the growth and development of river crab to the maximum extent, under the premise of ensuring the quality of feed raw materials, we should use as many kinds of raw materials as possible, such as fish meal, all kinds of cakes, yeast, bran and so on. However, attention should be paid to the use of cakes: although peanut meal can be widely used in river crab feed, it is easy to produce aflatoxin, and attention should be paid to storage and quality; cotton meal contains gossypol, rapeseed meal, sinaponin and other toxic substances, and tastes bitter, so it is not palatable for river crab, so the addition amount in feed should not exceed 7%.

1.2 lipids

Lipids are essential energy substances in the growth and development of river crabs. They can not only provide nutrients such as fatty acids, cholesterol and phospholipids for the growth of river crabs, but also help to absorb fat-soluble vitamins. Among the fatty acids of river crabs, unsaturated fatty acids account for a large proportion of the total fatty acids, about 70% to 83%. Among them, polyunsaturated fatty acids, especially eicosapentaenoic acid [20 ∶ 5 (NMel 3), EPA] and 22 carbon hexaenoic acid [22 ∶ 6 (NMAE 3), DHA] account for a high proportion. In addition, the content of saturated fatty acid in juvenile crab was significantly higher than that in adult crab, while the content of monoenoic acid in juvenile crab was significantly lower than that in adult crab. The content of 22 carbon tetraenoic acid in juvenile crab was more than 2 times higher than that in adult crab. Therefore, the crude fat content from larva to juvenile crab is designed to be 6%-8%, and that for adult crab is 3%-6%. Fish oil contains a variety of highly unsaturated fatty acids and some unknown growth factors, which is a suitable oil raw material for river crab feed.

Adding a certain amount of phospholipids to the feed can improve the survival rate and accelerate the molting of young crabs, and the appropriate amount of phospholipids in the feed is 1% 1.5%.

1.3 crude fiber

Crude fiber can not be used by river crabs, but it is necessary to maintain the health of river crabs. An appropriate amount of crude fiber can stimulate the secretion of digestive enzymes and promote the peristalsis of the digestive tract and the digestion and absorption of nutrients such as protein. The suitable content of crude fiber is 4%-5% from big-eyed larva to young crab, and 6%-7% in mature stage.

1.4 minerals and vitamins

There are few reports on the demand for minerals and vitamins in river crabs. In production practice, the content of calcium and phosphorus in formula feed: before the crab stage, the calcium is 1.5%-2.0%, the phosphorus is 1.8%-2.5%, the ratio of calcium to phosphorus is about 1 ∶ 1.2, the adult stage calcium is 1.2%-2.0%, the phosphorus is 1.0%-1.8%, and the ratio of calcium to phosphorus is about 1 ∶ 0.9.

Due to the special requirements of molting physiology and growth, the river crab has a high demand for vitamins, choline and inositol. Vitamin C can participate in the synthesis of chitin in the river crab, make the carapace hardening normally, improve the molting survival rate and growth rate, and improve the disease resistance of the river crab. Vitamin D3 plays an important role in promoting the absorption of calcium and phosphorus in the intestinal tract and the deposition in the bone base. Vitamins are easy to be destroyed by high temperature in the process of granulation. Conditional units can use spraying method.

(2) several key points in the preparation of compound feed for river crab

2.1 Raw material selection

Corn is one of the most widely used raw materials in livestock and poultry feed formula, so livestock and poultry feed is usually called energy feed. On the other hand, the ability of sugar metabolism of river crab is low, and the requirement of protein is high. The level of feed protein and the ratio of animal protein to plant protein directly affect the feed intake, growth rate and feed utilization rate of crabs. In addition, river crab and fish are variable temperature animals, and their body temperature changes with the change of water temperature. The waste product of protein metabolism is ammonia nitrogen, which consumes much less energy than livestock and poultry metabolic waste mainly urea and uric acid. Therefore, the river crab basically uses protein, which is also called protein feed, which is quite different from livestock and poultry feed.

2.2 crushing particle size of raw materials

The digestive organs of river crabs are simple and short, and the ratio of digestive tract length to body length is much smaller than that of livestock and poultry. The digestive glands are underdeveloped, the activities of various digestive enzymes are not high because of low body temperature, the species and number of digestive bacteria in the intestinal tract are less, and the stay time of food in the digestive tract is obviously shorter than that of livestock and poultry. Therefore, in feed processing, crab feed raw materials are required to have a finer crushing particle size. in general, the fineness of ordinary fish feed is required to pass 40 mesh, while crab feed requires more than 95% to pass 80 mesh. Fine crushing particle size can not only improve the digestion and absorption of nutrients, but also improve the mixing uniformity and pellet forming rate of feed, and improve the stability of feed particles in water.

2.3 feeding mode and feed dissolving time

The feeding mode of livestock and poultry is chewing or pecking, and there are no special requirements for the feed shape; for fish to swallow, as long as the feed size is in line with the caliber of the fish, the feed can be eaten by the fish after entering the water, and they are not very particular about the dissolving time of the feed. normally, the dissolving time in water is not less than 0.5 hours. On the other hand, the river crab holds food and uses two large claws to grind the formula feed in the chewing device before entering the mouth. therefore, it is required that the formula feed of river crab must dissolve in water for more than 12 hours. In order to achieve suitable stability in water, the following measures can be taken: ① increases the amount of starch-rich raw materials such as secondary flour and wheat, which produces stickiness after starch gelatinization in the granulation process, thus increasing the stability of feed in water; ② adds non-nutritive special adhesives, at present, high polymer adhesives, such as polyurea formaldehyde, are most used, but this adhesive is toxic after too much use, so it should be added according to the prescribed amount. ③ adopts reasonable processing technology. River crab feed needs a high degree of gelatinization and stability in water, for this reason, one is to increase the compression ratio of the ring mold, which can be more than 20, so that the extrusion strength of the raw material in the ring mold increases, and the agent pressure time is longer, which can improve the stability of the particles in water; the second is to use a three-stage conditioner, the quenching and tempering temperature is above 90 ℃, full conditioning makes starch gelatinization, protein denaturation, improving the digestibility and water resistance of feed. In addition, the post-ripening process can be used to allow the hot particles just out of the mold to last under high temperature and high humidity for a period of time, so that the starch can be further gelatinized, and the pellet feed can keep 12min in the post-ripening machine, and the starch gelatinization rate can be increased from 33.3% to 53%. After ripening, the pellet feed can improve the water resistance.

2.4 Additives

In addition to using adhesives to improve stability in water, river crab feed also needs to stimulate animal appetite, increase food intake, promote growth and use attractants; antioxidants are also needed to prevent oil oxidation. The food intake of river crab is not as large as that of fish, the amount of feeding in batches is relatively small, and the feed needs to be preserved for a long time. In order to prevent mildew, in addition to adding anti-mildew agent in the high temperature and rainy season, the feed moisture should be controlled within 11%.