With the rapid development of animal husbandry in China, it is an indisputable fact that the phenomenon of human and animal food competition and the shortage of protein raw materials. Because pigs have low utilization of nitrogen in the diet, nitrogen that is not digested and absorbed is excreted into the feces, and nitrogen that is not used for digestion and absorption is mainly excreted in the form of urinary nitrogen, which is not only for protein raw materials. Waste also increases the pressure on the ecological environment. 70% of pigs' nitrogen intake is discharged into the environment through feces, of which about 20% is excreted in the feces and 50% is excreted in the urine. Therefore, the nitrogen content and digestibility of the feed determine the yield of fecal nitrogen, and the amount of nitrogen absorbed and its utilization determine the content of urinary nitrogen. The addition of synthetic amino acids to pig diets and reduction of feed crude protein inputs have become important technologies for reducing aquaculture costs and reducing environmental pollution caused by excessive nitrogen excretion. 1. History and current status of feed amino acid industry development Amino acids are a general term for organic compounds containing amino and carboxyl groups, of which 20 are protein amino acids and 450 are non-protein amino acids. Amino acids are the basic unit of protein and an important part of biological organisms. As a human nutrition additive, flavoring agent, feed nutritive additive, medicine, etc., amino acid has been widely used in food industry, agriculture, animal husbandry and human health care, among which feed additives account for 30% - 40%. From 1866, the German chemical family Howson used sulphuric acid to hydrolyze wheat gluten to get glutamic acid. In more than 140 years, the amino acid industry has developed rapidly. In 2011, the global feed-only amino acid industry reached 2.89 million tons, with a market value of more than 9.5 billion US dollars. Among them, China's feed amino acid production reached 910,000 tons, and the top four were lysine, methionine, threonine and tryptophan. 2. The role of adding amino acids in the feed The addition of amino acids to feeds has been widely adopted. Generally, the content of lysine and methionine in cereals and soybean meal-based plant feeds is low, which can not meet the needs of animals. Adding amino acids can greatly reduce protein dosage, reduce feeding cost and improve animal production efficiency. Adding amino acids can improve the quality of pork. Adding lysine to the hog diet can improve meat quality and increase lean meat. Experiments have shown that the addition of lysine to animal diets can increase the efficiency of protein synthesis in animals, relatively reduce carcass fat content, increase lean meat ratio, and improve meat quality [4]. Adding amino acids can improve the digestive function of animals and reduce environmental pollution. The crude protein content in the diet of intensively reared piglets is high, and it is prone to digestive diseases such as diarrhea. This not only causes waste of feed, but also affects animal growth. By reducing the protein level of the animal diet, additional methods such as methionine, lysine and glutamic acid can effectively improve the digestive function of the animal, reduce the disease, enhance the disease resistance of the animal, and reduce the fecal nitrogen. And the discharge of urinary nitrogen, effectively improve the small environment of the pig house and reduce the pressure of manure treatment. Adding amino acids can reduce the stress in pigs. Stress is caused by changes in external environmental conditions, causing unacceptable phenomena in animals, which will lead to a sharp decline in feed intake and affect the yield and quality of livestock products. Tests have shown that the addition of tryptophan can prevent stress such as tail biting caused by weaning and excessive density of pigs. 3. Pigs on the difference between the use of synthetic amino acids and protein The amino acids added to the feed are mainly in the form of amino acid salts, and the amino acids in the animal and plant raw materials are mainly in the form of proteins or peptides, and there are some differences in the process of using the two. First, the amino acids in the animal and plant raw materials need to be degraded into amino acids by the digestive enzymes secreted by the animal or the added exogenous digestive enzymes. Synthetic amino acids are often added directly to the feed in the form of a salt, without the involvement of digestive enzymes. Second, animals use different efficiencies. The utilization efficiency of animal-derived amino acids is limited by their own compositional characteristics and animal digestive enzymes, and the digestibility is very different. Although synthetic amino acids are not restricted by this, the synthetic amino acids are mostly racemic, ie, L-configuration and A 50% mixture of each of the D configurations. Animal organisms can only use L-form amino acids, and D-form amino acids must be converted to L-form amino acids in order to be utilized. Due to the different deamination strengths of various D-type amino acids, the body has different utilization rates for D-type amino acids. 4. Countermeasures and precautions for rational use of synthetic amino acids The widely used feed amino acids are mainly lysine, methionine, threonine and tryptophan, and other amino acids are used less. Pay attention to the following points when adding synthetic amino acids: 4.1 Balance between amino acids The importance of dietary amino acid balance has long been noticed. Harper found that amino acid imbalances can cause antagonism or poisoning in animals. Since the 1950s, the theory of amino acid balance has gradually taken shape. The ideal protein concept was first proposed by Howard, when it was called complete protein. Cole, Fuller, etc. have all proposed their own ideal proteins. Although many scholars express different concepts of ideal protein, they have similar meanings. An ideal protein is a protein that has an optimal balance between essential amino acids and its non-essential amino acids. Therefore, the essence of an ideal protein is the balance between amino acids. With the rapid development of amino acid analysis, fermentation, extraction and chemical synthesis technology, the research on the nutritional role of amino acids has been carried out from the simple study of the requirements of various essential amino acids to the comprehensive consideration of the equilibrium mode of various amino acids. The ideal amino acid pattern (ideal amino acid pattern) refers to the optimal balance of various amino acids in the diet, which cannot be improved by increasing or decreasing or replacing any amino acid. 4.2 lysine and energy balance NRC (1998) pointed out that there is an optimal ratio of lysine to energy for growing pigs. The study of lysine energy ratio has been studied from the study of lysine digestion energy ratio and lysine metabolism energy ratio to the lysinetonetenergy ratio (Lys:NE) in diet. Because net energy best reflects the energy needs that are actually used for animal maintenance and production purposes. Therefore, based on low protein diets, the addition of synthetic amino acids, combined with the net energy system to formulate pig diets can save protein and energy more effectively, while at the same time better play the role of the added synthetic amino acids. 4.3 Different amino acid absorption synchronization The transport process of the amino acid in the body determines its composition to the final utilization site. The amino acids absorbed by pigs into the body cannot be stored as free molecules. They must enter the anabolic pathway to form peptides, proteins, hormones and other biologically active molecules, or enter the catabolic pathway to decompose into urea. It is hoped that pigs will absorb more amino acids into the anabolic pathway than the catabolic pathway. However, the anabolic pathway requires the simultaneous presence of multiple amino acids, which requires that the amino acid digestion and absorption process in the diet be as synchronized as possible, and complete synchronization in the actual production process is impossible. Studies have shown that feeding less than 3 times a day can significantly reduce the effect of synthetic amino acids added to the diet, a result that can be improved by eating fewer meals or eating ad libitum. Dietary Electrolyte Balance Electrolyte balance has different representations, the most common of which is electrolyte balance (dEB). Dietary electrolyte equilibrium value dEB = Na + + K + - Cl - . The amino acids added to pig feed are mainly lysine, methionine, threonine and tryptophan, and the amount of lysine added is the largest. The addition of lysine to the feed is generally added in the form of L-lysine hydrochloride or L-lysine sulfate, and Cl- or SO 4 2 - is added at the same time as the addition of lysine. When the other conditions are the same, the addition of L-lysine hydrochloride reduces dEB. It is generally believed that in diets with low K or low dEB levels, the mechanism of action to increase dEB levels or increase nutrient digestibility is unclear. When the dietary protein level is too low or lysine is deficient and tryptophan is low, adding a metabolic salt of K or Na can increase the growth rate of the pig. This phenomenon is called the lysine saving effect of K or Na salt. (Lysinesparingeffect). Leibholz et al. (1996) first discovered this effect, later Calvert et al. (1981), Froseth et al. (1982), Austic et al. (1983), Miya-da et al. (1983), Wahlstrom et al. (1983), and Kephart et al. (1990). It also confirms the existence of this effect. This effect may be related to the absorption of lysine by the metabolic salts of K or Na. It is worth noting that if a fixed salt of K or Na (such as a sulfate) is added, growth may be inhibited. This theory has been applied to actual production abroad, and KHCO3 is added to the diets of piglets, growing pigs and finishing pigs to reduce the amount of lysine added. Adding KHCO 3 1.82kg/t to the fattening pig compound feed can save 0.09% of lysine, which is equivalent to 1kg L-lysine hydrochloride. In summary, the addition of synthetic amino acids to pig feed can be carried out according to the following steps and methods: First, determine the energy level of the diet. When designing the energy level of the diet, the energy concentration of the raw materials and the feed intake of pigs at specific stages should be fully considered. Second, design the ratio of lysine energy; third, design the ratio of various synthetic amino acids to non-synthetic amino acids, calculate the amount of addition; fourth, calculate the formula, check the content of non-essential amino acids, to ensure sufficient, balanced; Fifth, check and adjust the electrolyte balance.
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