Application of phytase in diets of pigs and chickens

Phytic acid or phytate is common in plant feed, and most of the phosphorus exists in the form of phytate phosphorus. Phytic acid can form a stable complex with metal ions such as calcium ion, iron ion, zinc ion and copper ion, and can also combine with protein and starch, reducing the activities of endogenous amylase, protease and lipase. The digestibility of nutrients decreased significantly. Phytic acid is not absorbed by monogastric animals, and most of it is excreted in the form of feces, which not only greatly reduces the utilization rate of phosphorus, but also causes environmental pollution, especially water pollution.

In recent years, as a new type of feed additive, phytase has been paid more and more attention by experts and technicians in the feed industry. it is the general name of enzymes that catalyze the hydrolysis of phytic acid (inositol hexaphosphate) and plant salt water to inositol and phosphate (or phosphate). The main sources are plants and microorganisms. Monogastric animals have a low utilization rate of phosphorus in phosphate. A large number of studies have confirmed that the addition of phytase to the diet of pigs and chickens can effectively improve the utilization rate of phytate phosphorus, reduce the amount of inorganic phosphorus supplement and reduce the content of phosphorus in excreta. In addition, the addition of phytase to the diet can relieve the complexing effect of phytic acid on iron, zinc, calcium, copper and other elements, thus improving the utilization of these mineral elements.

After China's entry into WTO, with the improvement of people's living standards, the strengthening of environmental protection consciousness and the rapid development of large-scale aquaculture in enterprises, people pay more and more attention to solving the problems of phosphorus utilization and phosphorus pollution in feed, and the best way to improve the utilization rate of phosphorus and solve the pollution problem is to add phytase to the diet.

1. Application of phytase in diets of pigs and chickens.

The application of phytase in pig and chicken diets has been confirmed by many domestic and foreign experts and technicians in experiments and practice. the following is a simple summary of its previous research results.

The experiments of Simons et al. (1993) and Vah et al. (1994) showed that phytase could replace the addition of inorganic phosphorus and improve the utilization of calcium during the whole laying period of laying hens. Jeroch et al. (1994) adding 500U/kg phytase to layers with a total phosphorus (TP) of 0.33% could increase eggshell strength by 11.3%. Song Jincai et al (2000) added 175U/kg or 300U/kg phytase to the AP0.183% diet, the eggshell strength was higher than that of the non-enzyme group. When 300FTU/kg phytase was added to the diet of laying hens, the content of phosphorus (whether TP or AP) in the diet should be reduced by 0.1% (Gary,1996;Boilng,2000). Phytase could completely replace the addition of calcium hydrogen phosphate in the diet of laying hens, and the laying rate and feed-egg ratio of laying hens were improved to a certain extent (Zhang Haitang, 2000b; Zhang Ruohan et al., 1996). Han et al. (1995) reported that 1000U/kg phytase could increase the nitrogen use efficiency of broilers by 12.60% Mel 12.98%. The addition of phytase to the diet of broilers could increase the utilization rate of phosphorus to 60% (P < 1990), increase the feed conversion rate and promote the growth of bone (Simons et al., 1990). The addition of phytase to chicken diet could significantly promote the growth and development of chickens, and increase the feed conversion rate of laying hens and chicks by 11.6% and 38.8%, respectively (Jongblod et al., 1991, Dierick et al., 1994). Sebastian et al. (1996) reported that the addition of phytase (600U/kg) to low-phosphorus corn-soybean meal diet could increase the retention of Ca, Cu and Zn in three-week-old male broilers by 12.2%, 19.3% and 62.3%, respectively. Sebastion et al. (1996), Officer et al. (1993) and Yi et al. (1996) reported that the addition of phytase to chickens and growing pigs significantly increased the apparent digestibility of amino acids and nitrogen. Fallauf (1992) reported that the addition of phytase to pig diet increased the apparent digestibility of Mg, Zn, Cu and Fe by 13%, 13%, 7% and 9%, respectively. When phytase of 800U/kg was added to the diet of broilers (including Zn27mg/kg), the content of Zn in excreta decreased by 12%, and the incidence of leg disease decreased (Thiel et al., 1992). Under the action of phytase, phytic acid protein bond is destroyed, protein is released, and protein utilization is improved, which is beneficial to increase dietary protein energy ratio, improve protein storage rate and synthesis efficiency (Kyriazikis and Emmans,1992; Wei Shiping, 1995). (Poulson et al., 1994) reported that the addition of phytase to the diet of growing pigs could significantly increase the utilization rate of calcium, phosphorus and protein, and improve the digestibility and retention of nitrogen in the ileum of pigs. Mros et al. (1991) and Kemme et al. (1993) reported that soaking diets with water (adding microbial phytase) could increase the digestibility of Ca and P, so this wet feeding technique could not only humidify feed, but also promote enzyme catalysis and improve dietary nutrient utilization.

Here, a simple explanation is made on the amount of phytase additives in the diet: the amount of phytase in the diet depends on the content of phytic acid in the feed, animal breed, age and other factors. For example, in layer feed, when the concentration of substrate (phytic acid) is about 0.18% 0.20%, the phosphorus equivalent is 300U, and phytase is equivalent to 0.08% 0.10% available phosphorus, that is, 50% phytic acid is hydrolyzed, while 500U phytase is needed for broilers and pigs. The recommended diet for broilers, pigs (piglets, growing pigs) and laying hens is generally 500 700400 500300 500U/kg.

2. Existing problems

2.1PH value

In general, the optimum pH value of phytase in plants is 4.06.0. when the pH value is 3.0, the activity of phytase decreases significantly, even completely inactivated (EclhoulandDePaepe,1994), so it is difficult for phytase to play its role in the strong acid environment of monogastric animal stomach (PH2.5-3.0). After entering the small intestine, the environmental PH increases to 6.07.0. Therefore, the absorption of phosphorus in the digestive tract of monogastric animals mainly occurs in the proximal small intestine, while phosphorus in feed can be absorbed to the maximum only when digested in the stomach, which is contradictory to the strong acid environment of the stomach.

Microbial phytase has two optimum PH values of 2.5 and 5.5 at 40 ℃, and its activity is very low at pH 7.0, which can partially satisfy the strong acid environment of stomach, but the cost of microbial phytase is higher than that of plant phytase, which is also a contradiction.

2.2 High temperature and heat resistance

Phytase is a kind of protein, so it is easy to be inactivated at high temperature. Although phytase has high heat resistance, in terms of technology, the high temperature resistance of the tertiary structure of enzyme protein has not been well solved. this has become one of the key issues whether phytase can be widely used in the modern feed processing industry of high-temperature granulation and high-temperature sterilization. By adding enzyme after granulation (that is, after the feed particles are cooled, the liquid enzyme is sprayed on the surface of the particles), the activity can be retained as much as possible. It is generally believed that the optimum temperature range of phytase activity is between 60 and 70 ℃.

2.3 add level

For different enzyme activity of phytase products, different feeding objects, different feed varieties, the optimal addition level of phytase is different. If phytase plays its role by acting on the substrate (phytic acid), different feeds contain different phytic acid, then the amount of phytase is different, adding more or less will cause the waste of phytase. The proportion of phytase added to the diet should be determined according to its activity and action characteristics. It is generally required that phytic acid phosphorus in the feed is more than 0.2% before it is considered to be added. Khan et al. (1995) reported that phytase at the level of 800-1000U/kg had the best effect (for the highest apparent digestibility of phosphorus), but this level would increase the cost of 40-50 yuan per ton of feed, which greatly exceeded the economic benefits of replacing inorganic phosphorus and was unacceptable to producers and farmers. Considering the comprehensive benefit, the addition amount of 300-500U/kg is generally recommended, without pursuing the highest digestibility of phosphorus.

In addition, there are still some problems, such as product quality standards and inspection method standards, as well as the contradiction between product quality, variety, promotion, production capacity and market demand, which will affect the expansion of phytase production and promotion.

3. Methods and countermeasures

3.1 Technical aspects

When the phytase gene with good characteristics was transferred into the high yield Aspergillus strain, a recombinant enzyme was obtained. when phytic acid was used as substrate, the activity of this enzyme had three peaks at PH 2.8,4.5,6.5, respectively. With this wide range of PH, phytase has the potential to adapt to different acid environments in the digestive tract of monogastric animals. Adding enzyme after granulation mentioned above is a better way to solve its thermal stability. In addition, in the process of phytase storage, high temperature and high humidity should be avoided, the storage time should be shortened as much as possible, and should be fed as soon as possible after processing. For different feeds and animals, the ratio of calcium hydrogen phosphate should be controlled and added according to the corresponding recommended amount (this aspect is not perfect and needs further study). Compared with microbial phytase, the biggest advantage of plant phytase is its low cost, because it is a component of feed, generally does not need additional cost, through appropriate treatment, such as germination, can greatly increase the activity of the enzyme. And in the process of germination, the contents of other kinds of enzymes and microorganisms also increased significantly, and the nutritional value was very high. Directly adding germinated feed or extracting phytase from it may be an effective way to reduce the production cost of phytase (germinated feed is more suitable for traditional farming). In view of the above advantages of plant phytase, great efforts should be made to develop phytase in order to reduce the cost of phytase and then popularize its use.

3.2 Policy and subjective aspects

The state should issue policies and laws conducive to the promotion of the use of phytase, so that the content of phosphorus caused by the use of phytase does not reach the national standard, so that many companies are not willing to "risk" large-scale production and promotion of feed.

In recent years, our government attaches great importance to strengthening environmental protection while feed production and animal feeding, so the state should give policy support to the promotion and application of phytase, such as reducing (or exempting) its import and value-added tax. It is precisely because the application of phytase is a new technology, and in view of the present situation of the technical force of modern feed production in our country, we must strengthen the training of the technical team, and at the same time vigorously publicize the characteristics and advantages of phytase, train the technical force of production enterprises, and guide the technical points in the process of production, storage and application, so as to accelerate the promotion of the new technology of phytase application.

4. Final words

Although there are many difficulties in popularizing the use of phytase, which need to be further solved by industry experts, phytase will become one of the most important additives in modern feed industry because of its advantages in improving the utilization rate of phosphorus and other nutrients, saving phosphorus resources and reducing environmental pollution, especially in reducing water pollution.