In modern pig production, in order to improve the performance of sows, most of them are weaned at 3 to 5 weeks old, but the digestive system and immune organs of early weaned pigs are not well developed, and the secretion of enzymes and gastric acid in the digestive tract is insufficient. The normal intestinal tract Micro-ecological systems have not yet been established (Yu Xuhua et al., 2002). Weaned piglets suddenly changed from sucking breast milk to eating solids and diet. Due to insufficient secretion in the stomach, the acidity in the stomach decreased significantly, which led to a decrease in feed digestibility, which led to low feed intake and weak disease resistance. Weaning syndrome, which is characterized by diarrhea and growth retardation (Tzipori et al., 1980; Barnett et al., 1989), seriously affects the growth performance of weaned piglets and is one of the major problems in pig production. Because acidifiers have the effect of lowering gastrointestinal pH, activating pepsin, improving the gastrointestinal microflora and improving the performance of early weaned piglets (Sciopioni et al., 1978; Kidder and Manners, 1978; Johnson, 1991; Gabert and Sauer) , 1994), at the same time, it has the advantages that antibiotics and other drugs do not have, and it has received people's attention. However, some scholars (Burnell et al., 1988; Risley et al., 1992) reported different results, which may be many factors affecting the action of acidifiers. At present, the addition of acidifiers in the feed of early weaned piglets has been widely used, but the effects of these acidifiers have been reported differently. The effects of different types of acidifiers are also different. The mechanism of action of various acidifiers is not the same. There are many factors affecting the action of acidifiers, so the mechanism of action needs further research. This paper reviews the mechanism of action of acidifiers and the influencing factors.
1 Mechanism of acidifier action
The application of acidifiers has been around for more than 30 years, but its mechanism of action is still not very clear. The mechanisms currently widely accepted are as follows:
1.1 Acidification effect
The acidifying agent can reduce the pH value of the young animal's stomach, provide a reasonable animal digestion environment, activate the pepsin and maintain high activity, help the softening of the feed, the decomposition of nutrients, and prevent the pathogenic microorganisms from entering the body through the digestive tract from the external environment. . Stimulates the duodenum to secrete more trypsin, allowing the protein to be completely absorbed.
1.2 Participate in the metabolism of the body, anti-stress
Some organic acids are directly involved in the body's energy, structure and enzymatic reactions, and can provide the energy needed for animal stress. For example, fumaric acid and citric acid are indispensable components in the tricarboxylic acid cycle, and are easy to use energy (Wang Yanyan, 2002) ). In particular, fumaric acid is an essential component of the cyclic aerobic process of tricarboxylic acid. The molecular energy is consistent with glucose, but its energy generation pathway is shorter than glucose. It can be used for emergency generation of ATP under stress and in livestock production. Stress agent.
1.3 Promote the absorption of nutrients
The addition of KDF (potassium diformate) to the feed increases the digestibility and retention of some nutrients (including dry matter, energy, fat, nitrogen, some essential amino acids and phosphorus) (Roth et al., 1998a, b). Organic acids can form readily chelating chelate with some mineral elements. For example, fumaric acid and citric acid can form a bioavailable chelate with Ca, Cu, Zn, Fe and Mg to promote their absorption and retention. (Shi Baoming, 1999). Many scholars have confirmed that the addition effect of high copper and acidifier has an additive effect, that is, fumaric acid, citric acid or phosphoric acid forms a bioavailable complex with copper, which promotes the absorption and retention of copper and reduces it. Oxidation catalytic activity of copper. Some organic acids such as fumaric acid have antioxidant effects, and citric acid is a synergist for antioxidants. The addition of fumaric acid to the premix was preserved for 6 months, and the stability of VA and VC was improved compared to the absence of fumaric acid. At the same time, the acidic environment of the small intestine is beneficial to the absorption of VA and VD.
1.4 Impact on microflora
Lin Yingcai et al (2001) added complex acidifier to early weaned piglets at 14 days of age. The results showed that the addition of complex acidifier 1 500g/t increased the number of lactobacilli, E. coli and total bacteria in the small intestine of piglets, and the total number of bacteria in the small intestine. The proportion of lactobacilli in the middle increased, the proportion of E. coli decreased, and the ratio of the number of lactobacilli to the number of E. coli also increased. This indicates that the complex acidifier not only directly affects the growth of bacteria in the intestinal tract of piglets, but also affects the balance of microflora in the intestinal tract.
1.5 Antibacterial effect
Cherring et al (1991) and Russell (1992) summarize the different views on the mechanism of antibacterial action of organic acids. The antibacterial activity of organic acids is related to the decrease in pH, and the antibacterial activity increases with the decrease of pH. Non-dissociated organic acids are fat-soluble and thus able to enter microbial cells, but the carrier mechanism also appears to be involved in membrane transport. Once in the cell, the acid releases protons in a relatively alkaline environment, resulting in a decrease in intracellular pH. The microbial mechanism inhibits the action of important enzymes of the microbe and forces the bacterial cells to dissipate protons, causing the accumulation of anions in the cells, which is dependent on the pH gradient of the membrane. Acid anions are important for the antibacterial action of organic acids and their salts. Nuria Canibre et al (2002) reported that some studies have shown that organic acids have a strong bactericidal effect without significantly reducing the pH of the gastrointestinal tract. In general, lactic acid bacteria can survive at lower pH values, which means that they are more tolerant to organic acids than other bacteria such as E. coli. One explanation is that the concentration of potassium in the cells of Gram-positive bacteria is higher and neutralizes the anion of the acid (Russell & Diez-Gonzalez, 1998).
1.6 Other functions
The acidifying agent lowers the pH value of the stomach contents, and the acidic chyme enters the small intestine to chemically and mechanically stimulate the small intestine wall, so that it secretes enterostatin, reflexively inhibits gastric peristalsis, slows the gastric emptying speed, and makes the protein more Time digests in the stomach, reducing the burden on the small intestine (Kirchgessner and Roth, 1982; Donald et al, 1991). The acidifying agent can also enhance immunity, improve the palatability of the diet, and reduce the metabolism of intestinal microbes such as ammonia and polyamines, and serve as a mold preservation preservative for the preservation of the feed.
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