Nutritional Requirements of River Crab Farming and Experience in Compound Feed Production

The river crab is a unique aquatic delicacy of China, renowned at home and abroad for its high nutritional value and distinctive flavor, and has become an important export product for earning foreign exchange. Therefore, from lakes and reservoirs to ponds and rice fields, river crab farming is being promoted on a large scale. The rapid development of river crab farming requires the compound feed industry to keep pace accordingly. However, research on the nutrition of compound feed for river crabs in China is still relatively limited. Here is a brief overview of the nutritional requirements of river crabs and the development of compound feed.

1 Nutritional Requirements of River Crabs

1.1 Protein and Amino Acids

The crude protein content of compound feed for river crabs from the zoea larval stage to the grow-out stage is 45%-28%. Compound feed for the zoea larval stage of river crabs with a crude protein content of 45%, made mainly from white fishmeal, oilseed meals, wheat gluten meal, shrimp bran meal, egg yolk powder, etc., has a product fineness of 80-250 mesh. When fed separately to Z1-Z5 stage larvae, the larvae show a high fecal excretion rate after feeding, uniform metamorphosis, short molting time, and high survival rate. After the crab seedlings leave the pond, there is very little residual feed on the pond bottom. Using compound feed with 42% crude protein (starter feed) to raise megalopa to V-stage juvenile crabs, after about 1 month of feeding, the juvenile crabs grow quickly and are lively and healthy, with a survival rate of about 50% and a feed conversion ratio of about 0.8. The crude protein content of compound feed during the grow-out stage of river crabs is 28%-36%. Under these conditions, the river crabs raised are relatively large, with fewer diseases, and the vast majority of farmers can profit. According to the physiological characteristics of river crabs, during the early grow-out period when temperatures are suitable, compound feed with 36% crude protein can be fed; during the mid-period when water temperatures are higher and river crabs grow relatively slowly, compound feed with 28% crude protein can be fed, combined with some aquatic plants and miscellaneous grains; during the late period when water temperatures are more suitable for river crab growth, river crabs consume large amounts of feed to store nutrients for overwintering, so compound feed with 33% protein can be fed. Therefore, river crabs caught in late autumn and early winter are fat and full of roe. This feeding pattern also conforms to the river crab farming principle of "refined feed at both ends and coarse feed in the middle."

The protein requirement of river crabs is essentially a requirement for amino acids. River crabs can only ultimately transfer protein obtained from feed into crab body tissues after digesting it into small molecular compounds such as peptides and amino acids. The essential amino acids required by river crabs are basically similar to those of shrimp, namely 10 types: threonine, valine, leucine, isoleucine, tryptophan, methionine, phenylalanine, histidine, lysine, and arginine. The appropriate requirement levels of various essential amino acids for river crabs have not yet been reported. When designing formulas, reference is mainly made to shrimp. The growth effect of supplemental free amino acids on river crabs is not significant. In order to make amino acids in compound feed complementary and maximize the satisfaction of river crab growth and development needs, under the premise of ensuring the quality of feed ingredients, when using feed ingredients, as many varieties of ingredients as possible should be used, such as fishmeal, various oilseed meals, yeast, bran, etc. However, when using oilseed meals, attention should be paid to the following: although peanut meal can be used in relatively large amounts in river crab feed, it is prone to producing aflatoxin, so storage and quality must be carefully managed; cottonseed meal contains gossypol, rapeseed meal contains toxic substances such as glucosinolates, and they are bitter, with poor palatability for river crabs. Therefore, the amount added to feed should not exceed 7%.

1.2 Lipids

Lipids are essential energy substances in the growth and development of river crabs. They can provide fatty acids, cholesterol, phospholipids and other nutrients needed for river crab growth, and can also help the absorption of fat-soluble vitamins. Among the fatty acids of river crabs, unsaturated fatty acids account for a large proportion of total fatty acids, about 70%-83%, and among them, polyenoic fatty acids, especially eicosapentaenoic acid [20:5(n-3), EPA] and docosahexaenoic acid [22:6(n-3), DHA], account for a very high proportion. In addition, the saturated fatty acid content of juvenile crabs is significantly higher than that of adult crabs, while the monounsaturated fatty acid content is significantly lower than that of adult crabs. The docosatetraenoic acid content of juvenile crabs is more than 2 times higher than that of adult crabs. Therefore, the crude fat content is designed at 6%-8% for the larval to juvenile crab stage and 3%-6% for the adult crab stage. Fish oil contains various highly unsaturated fatty acids and some unknown growth factors, making it a suitable oil ingredient for addition to river crab feed.

Adding a certain amount of phospholipids to feed can improve the survival rate of juvenile crabs and accelerate molting. The suitable addition amount in feed is 1%-1.5%.

1.3 Crude Fiber

Crude fiber generally cannot be utilized by river crabs, but it is necessary for maintaining river crab health. An appropriate amount of crude fiber can stimulate the secretion of digestive enzymes, promote gastrointestinal peristalsis, and enhance the digestion and absorption of proteins and other nutrients. The suitable crude fiber content is: 4%-5% for the megalopa to juvenile crab stage, and 6%-7% for the grow-out stage.

1.4 Minerals and Vitamins

There are very few reports on the mineral and vitamin requirements of river crabs. In production practice, the calcium and phosphorus contents in compound feed are: before the juvenile crab stage, calcium is 1.5%-2.0%, phosphorus is 1.8%-2.5%, and the calcium-to-phosphorus ratio is about 1:1.2; during the adult crab stage, calcium is 1.2%-2.0%, phosphorus is 1.0%-1.8%, and the calcium-to-phosphorus ratio is about 1:0.9.

Due to the special requirements of molting physiology and growth, river crabs have relatively high requirements for vitamins, choline, and inositol. Vitamin C can participate in the synthesis of chitin in the river crab body, enabling normal hardening of the shell, improving molting survival rate and growth speed, and enhancing disease resistance of river crabs; vitamin D3 plays an important role in promoting the absorption of calcium and phosphorus in the intestine and their deposition in the bone matrix. Vitamins are easily destroyed by high temperatures during the pelletizing process. Units with the capability can use a spraying method.

2 Several Key Points in the Production of Compound Feed for River Crabs

2.1 Ingredient Selection

In livestock and poultry feed formulas, corn is one of the ingredients used in the largest amounts, so livestock and poultry feed is usually called energy feed. However, river crabs have a low ability to metabolize sugar and a relatively high protein requirement. The dietary protein level and the ratio of animal to plant protein both directly affect crab feed intake, growth rate, and feed utilization rate. In addition, river crabs, like fish, are poikilothermic animals, with body temperature changing with water temperature. The waste product of protein metabolism is ammonia nitrogen. This metabolic form consumes much less energy than livestock and poultry metabolic wastes, which are mainly urea and uric acid. Therefore, river crabs basically utilize protein, and crab feed is also called protein feed, which differs greatly from livestock and poultry feed.

2.2 Ingredient Grinding Particle Size

River crabs have simple and short digestive organs, and the ratio of digestive tract length to body length is much smaller than that of livestock and poultry. The digestive glands are not well developed, various digestive enzymes have low activity due to low body temperature, and the types and quantities of bacteria with digestive functions in the intestine are relatively few. The time food stays in the digestive tract is significantly shorter than in livestock and poultry. Therefore, in feed processing, crab feed ingredients require a finer grinding particle size. Usually, the ingredient fineness requirement for ordinary fish feed is that all passes through 40 mesh, while crab feed requires more than 95% to pass through 80 mesh. A finer grinding particle size is conducive to improving the digestion and absorption of nutrients by river crabs, and also helps improve feed mixing uniformity and pellet forming rate, and improves the stability of feed pellets in water.

2.3 Feeding Method and Feed Disintegration Time in Water

The feeding method of livestock and poultry is chewing or pecking, with no special requirements for feed shape; fish swallow, and as long as the feed particle size matches the fish mouth size, the feed can be ingested by fish after entering water. There are also not many special requirements for feed disintegration time; normally, the disintegration time in water is required to be no less than 0.5 h. However, river crabs hold their food, using their two large chelae to send compound feed into the chewing apparatus to grind it, and only then does it enter the mouth. Therefore, the disintegration time of river crab compound feed in water must be more than 12 h. To achieve suitable stability in water, the following measures can be taken: (1) increase the amount of starch-rich ingredients such as wheat middlings and wheat, so that starch gelatinization during pelletizing produces viscosity, thereby increasing the water stability of the feed; (2) add non-nutritive special binders. At present, the most commonly used are high-polymer binders such as polyurea formaldehyde, but this binder is toxic when used in large amounts, so it must be added according to the specified amount; (3) adopt a reasonable processing technique. River crab feed requires a relatively high degree of gelatinization and water stability. For this reason, first, increase the compression ratio of the ring die, which can be above 20, so that the extrusion strength of the ingredients in the ring die increases and the pressing time is longer, which can improve the stability of pellets in water; second, use a 3-stage conditioner, with a conditioning temperature above 90 degrees C. Sufficient conditioning causes starch gelatinization and protein denaturation, improving feed digestibility and water resistance. In addition, a post-ripening process can be used, allowing freshly extruded hot pellets to remain under high temperature and high humidity for a period of time so that starch is further gelatinized. When pellets are kept in the post-ripening device for 12 min, the starch gelatinization rate can increase from 33.3% to 53%. Post-ripened pellets can improve water resistance.

2.4 Additives

In addition to using binders to improve water stability, river crab feed also requires attractants to stimulate animal appetite, increase feed intake, and promote growth; antioxidants are also needed to prevent oil oxidation. River crabs do not consume as much feed as fish, and the amount of feed per batch is relatively small, so the feed needs to be stored for a longer time. To prevent mold, in addition to adding mold inhibitors during hot and rainy seasons, the feed moisture content also needs to be controlled within 11%.