Application Methods of Phytase in Pig and Poultry Diets

Phytic acid or phytate is commonly present in plant feedstuffs, and most of the phosphorus in them exists in the form of phytate phosphorus. Phytic acid can form stable complexes called phytates with metal ions such as calcium, iron, zinc, and copper ions, and can also bind with proteins and starch, reducing the activity of endogenous amylase, protease, and lipase, and significantly lowering the digestibility of various nutrients. Phytic acid is not absorbed by monogastric animals and is mostly excreted in feces, which not only greatly reduces phosphorus utilization but also causes environmental pollution, especially water pollution.

In recent years, phytase, as a new type of feed additive, has attracted increasing attention from experts and technicians in the feed industry. It is the general term for a class of enzymes that catalyze the hydrolysis of phytic acid (inositol hexaphosphate) and phytates into inositol and phosphoric acid (or phosphate), mainly derived from plants and microorganisms. Monogastric animals have very low utilization of phosphorus in phosphates. Numerous studies have confirmed that adding phytase to pig and poultry diets can effectively improve the utilization of phytate phosphorus, reduce the amount of inorganic phosphorus supplementation, and lower the phosphorus content in excreta. In addition, adding phytase to diets can release the complexing effect of phytic acid on iron, zinc, calcium, copper, and other elements, thereby improving the utilization of these mineral elements.

After joining the WTO, with the improvement of people's living standards, the strengthening of environmental awareness, and the rapid development of large-scale enterprise farming, people are paying more and more attention to solving the problems of phosphorus utilization and phosphorus pollution in feed, and the best way to improve phosphorus utilization and solve its pollution problem should be to add phytase to diets.

1. Application of Phytase in Pig and Poultry Diets

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

Experiments by Simons et al. (1993) and Vah et al. (1994) showed that during the entire laying period of laying hens, phytase could replace the addition of inorganic phosphorus and improved calcium utilization. Jeroch et al. (1994) added 500U/kg phytase to laying hen diets with total phosphorus (TP) of 0.33%, which increased eggshell strength by 11.3%. Song Jincai et al. (2000) added 175U/kg or 300U/kg phytase to diets with AP 0.183%, and eggshell strength was higher than that of the group without enzyme. When 300FTU/kg phytase is added to laying hen diets, the phosphorus content (whether TP or AP) in the diet should be reduced by 0.1% (Gary, 1996; Boilng, 2000). In laying hen diets, phytase can completely replace the addition of calcium hydrogen phosphate, and the laying rate and feed-to-egg ratio of laying hens are improved to a certain extent (Zhang Haitang, 2000b; Zhang Ruohan et al., 1996; Simons, 1996). Han et al. (1995) reported that 1000U/kg phytase could increase the nitrogen utilization of broiler chickens by 12.60%-12.98%. Adding phytase to broiler diets can increase phosphorus utilization to 60% (P<0.01), while also improving feed conversion and promoting bone growth (Simons et al., 1990). Adding phytase to chicken diets can significantly promote the growth and development of chickens, increasing 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 adding phytase (600U/kg) to low-phosphorus corn-soybean meal diets could increase the retention of Ca, Cu, and Zn in three-week-old male broiler chickens by 12.2%, 19.3%, and 62.3% respectively. Sebastion et al. (1996), Officer et al. (1993), and Yi et al. (1996) reported that adding phytase to chicks and growing pigs could significantly increase the apparent digestibility of amino acids and nitrogen. Fallauf (1992) reported that adding phytase to pig diets could increase the apparent digestibility of Mg, Zn, Cu, and Fe by 13%, 13%, 7%, and 9% respectively. Adding 800U/kg phytase to broiler diets (containing Zn 27mg/kg) reduced the Zn content in excreta by 12% and lowered the incidence of leg disease (Thiel et al., 1992). Under the action of phytase, the phytic acid-protein bond is broken, releasing protein, improving protein utilization, which is beneficial for increasing the protein-to-energy ratio of the diet and improving protein retention and synthesis efficiency (Kyriazikis and Emmans, 1992; Wei Shiping, 1995). (Poulson et al., 1994) reported that adding phytase to growing pig diets could significantly improve the utilization of calcium, phosphorus, and protein, and could improve ileal nitrogen digestibility and nitrogen retention in pigs. Mros et al. (1991) and Kemme et al. (1993) reported that wetting diets with water (with added microbial phytase) could improve Ca and P digestibility, so this wet-feeding technique for pigs can not only moisten the feed but also promote the catalytic action of enzymes and improve the utilization of dietary nutrients.

Here, a brief explanation of the dosage of phytase added to diets: The amount of phytase added to diets depends on the phytic acid content in the feed, animal species, age, and other factors. For example, in laying hen feed, when the substrate (phytic acid) concentration is about 0.18%-0.20%, the phosphorus equivalent is 300U, and phytase is equivalent to 0.08%-0.10% available phosphorus, meaning 50% of phytic acid is hydrolyzed, while for broilers and pigs, 500U of phytase is needed. The recommended addition levels for broiler chickens, pigs (piglets and growing pigs), and laying hens are generally 500-700, 400-500, and 300-500U/kg of diet.

2. Existing Problems

2.1 pH Value

The optimal pH value of general plant phytase is 4.0-6.0, and at pH 3.0, the activity decreases significantly or even completely deactivates (Eclhoul and DePaepe, 1994). Therefore, this enzyme is difficult to function in the strongly acidic environment of the monogastric animal stomach (pH 2.5-3.0), and after entering the small intestine, the environmental pH rises to 6.0-7.0. Therefore, the absorption of phosphorus in the digestive tract of monogastric animals mainly occurs in the proximal small intestine, while phosphorus in feed can only be maximally absorbed when digested in the stomach, which is a contradiction with the strongly acidic environment of the stomach.

Microbial phytase has two optimal pH values of 2.5 and 5.5 at 40°C, and its activity is very low at pH 7.0. It can partially satisfy the strongly acidic environment of the 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 Tolerance

Phytase is a protein, so it also has the characteristic of being easily deactivated at high temperatures. Although phytase has relatively high heat tolerance, in terms of process technology, the high-temperature resistance of the tertiary structure of enzyme proteins has not been well resolved, which has become one of the key issues determining whether phytase can be widely applied in the modern feed processing industry involving high-temperature pelleting and high-temperature sterilization and disinfection. Adding enzyme after pelleting (i.e., after the feed pellets cool, spraying liquid enzyme onto the pellet surface) can achieve the purpose of retaining its activity to the greatest extent. It is generally believed that the optimal temperature range for phytase activity is between 60-70°C.

2.3 Addition Level

For phytase products with different enzyme activities, different feeding targets, and different feed varieties, the optimal addition level of phytase varies. For example, phytase functions by acting on the substrate (phytic acid), and different feeds contain different amounts of phytic acid, so the required amount of enzyme differs. Adding too much or too little will cause waste of phytase. The proportion of phytase added to diets should be determined based on its activity and mode of action. It is generally required that the phytate phosphorus in feed be above 0.2% before addition is considered. Khan et al. (1995) reported that the best effect (in terms of the highest apparent digestibility of phosphorus) was achieved when the phytase addition level was 800-1000U/kg, but this level would increase the cost by 40-50 yuan per ton of feed, greatly exceeding the economic benefit of replacing inorganic phosphorus, and is unacceptable to producers and farmers. Considering comprehensive benefits, the recommended addition level is generally 300-500U/kg, without pursuing the highest phosphorus digestibility.

In addition, there are still problems such as product quality standards and inspection method standards, as well as contradictions between product quality, variety, promotion, production capacity, and market demand, all of which will affect the expansion of phytase production and the promotion of its use in feed.

3. Methods and Countermeasures

3.1 Technical Aspects

Transferring the phytase gene with good characteristics into high-yield Aspergillus strains can produce a recombinant enzyme. When this enzyme uses phytic acid as a substrate, it has three activity peaks at pH 2.8, 4.5, and 6.5. Such a wide pH range gives phytase the potential to adapt to different acid environments in the digestive tract of monogastric animals. The aforementioned post-pelleting enzyme addition is a good method to solve its thermal stability. In addition, during the storage of phytase, high temperature and high humidity should be avoided, storage time should be shortened as much as possible, and it should be fed as soon as possible after processing. For different feeds and animals, the ratio with calcium hydrogen phosphate should be controlled, and it should be added according to the corresponding recommended amount (this aspect is not yet perfect and needs further research). Compared with microbial phytase, the greatest advantage of plant phytase is its low cost, because it is itself a component of feed and generally does not require additional cost. Through appropriate treatment, such as germination, the enzyme activity can be greatly increased. Moreover, during the germination process, the content of other types of enzymes and microorganisms also increases significantly, and the nutritional value is very high. Directly adding germinated feed to 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). Given the above advantages of plant phytase, it should be vigorously developed to reduce the cost of phytase and thereby promote its use.

3.2 Policy and Subjective Aspects

The state should introduce policies and laws conducive to the promotion and use of phytase, so as to prevent many companies from being unwilling to "take the risk" of large-scale production and promotion of its use in feed due to the phosphorus content failing to meet national standards as a result of using phytase.

In recent years, our government has attached great importance to strengthening environmental protection work alongside feed production and animal raising, so for the promotion and application of phytase, the state should give policy support, such as reducing (or exempting) value-added tax on its imports and various business links. Precisely because the application of phytase is a new technology, and given the current state of technical strength in China's modern feed production, it is necessary to strengthen the training of technical teams. At the same time, it is also necessary to vigorously publicize the characteristics and advantages of phytase, train the technical strength of production enterprises, and guide them on the technical key points in the process of production, storage, and application, so that the new technology of phytase application can be promoted more rapidly.

4. Concluding Remarks

Although there are many difficulties in promoting the use of phytase that still need to be further resolved by industry experts, its advantages in improving the utilization of phosphorus and other nutrients, saving phosphorus resources, and reducing environmental pollution, especially reducing water pollution, determine that phytase will inevitably become one of the very important additives in the modern feed industry.