Research Progress of Acidifier and Plant Essential Oil in Weaned Piglets

Research Progress of Acidifier and Plant Essential Oil in Weaned Piglets

Li Zhizhong^1,2

1 DadHank (Chengdu) Biotech Corp., Chengdu, 611130

2 Sichuan animal feeding regulation engineering technology research center, Chengdu 611130

Abstract: Since July 1, 2020, China has officially entered the era of "Forbidding to add antibiotic to feed, reducing antibiotic used in food animals and antibiotic-free in animal product", but the feeding problems such as piglet diarrhea after weaning are still grim. In this paper, the application of acidifier and plant essential oil in weaning stage of piglets was reviewed, hoping to provide some references for feed and feeding enterprises to choose functional additive products.

 

Since Fleming, an Englishman, discovered penicillin in 1928, human beings have used antibiotics for nearly a century, and antibiotics have been used in animal husbandry for decades. According to statistics, in 2013, the amount of antibiotics used for meat animals in the world exceeded 50% of the total amount of antibiotics used, and about 90% of them were used for the purpose of promoting growth and improving feed conversion rate ^[1]. Among the countries that use antibiotics the most in the world（in the global livestock industry）, China accounts for the largest proportion, up to 23%, followed by the United States with 13%, Brazil with 9%, India with 3% and Germany with 3% ^[2]. With the great increase of antibiotics use in animal husbandry, the problems caused by antibiotic abuse are also obvious. The first problem is drug resistance. Drug resistance of bacterial is a defensive strategy produced by bacteria in an environment that is not conducive to their survival. Because the effect of antibiotics in animal husbandry production is very obvious, the problem of bacterial resistance has become very serious, and more and more super bacteria can tolerate multiple antibiotics ^[3]. The second problem is drug residues. In a study in（at） Fudan University in 2015, the morning urine of 1,000 children in Jiangsu, Zhejiang and Shanghai was tested, and 58.3% of them detected at least one antibiotic in their morning urine. It can be said that the problem of antibiotic residues has seriously endangered human food safety, thus threatening human health. The final problem is environmental protection and the abuse of antibiotics has also caused great harm to the environment.

It is precisely because of the above hazards that "Antibiotic-free" has become the consensus of all countries. First of all, Sweden banned the use of antibiotic additives in feed, and then in 2000, Denmark began to ban the addition of antibiotic additives in livestock and poultry feed. Since 2006, the European Union has completely banned the use of antibiotic additives in feed. In China, from 2016, the Ministry of Agriculture and Rural Affairs issued Announcement No.2428, prohibiting the addition of colistin sulfate to feed to start the "Antibiotic-free" step. By 2019, the Ministry of Agriculture and Rural Affairs issued Announcement No.194, stipulating that feed production enterprises must stop producing commercial feed containing antibiotic growth promoting feed additives (except Chinese herbal medicines) since July 1, 2020. In June 2020, Announcement No.307 was issued, which stipulated the use of additives in feed prepared by farmers themselves, and no substances outside the feed catalogue were allowed. It can be said that since August 1, 2020, the era of China's  "forbidding to add antibiotic to feed, reducing antibiotic used in food animals and antibiotic-free in animal product " is coming.

Although the antibiotic-free in feed is the only way in the development of animal husbandry, it is undeniable that the related problems solved by using antibiotics before have not disappeared with the prohibition of feed antibiotics. Taking pig production as an example, the problems caused in（by）weaning stage of piglets are even more difficult because of prohibition of antibiotics. With the prohibition of antibiotics in feed, related so-called "antibiotic substitutes" feed additive products are also emerging one after another. Of course, after prohibiting adding antibiotic additives in feed, the health of animals, the improvement of feeding management, the choice of feed ingredients and the nutritional balance of feed formula are all more important. Feed additives are only a part of feed nutrition, and we can't expect one or more kinds of feed additives to solve all the problems in feed and feeding. However, it is undeniable that selecting feed additives scientifically and reasonably is undoubtedly of positive significance in promoting animal health, achieving healthy feeding, and thus improving the end benefit of feeding. According to the statistics of WATT International Media, 74.2% of the previous studies on functional additives of "Antibiotic substitutes" were proved to be effective, which was the highest among all such functional additives. In the study of plant essential oil, 60.3% was proved to be effective. Therefore, according to the previous research, acidifier and essential oil have positive effects on "Altering antibiotics" in weaning stage of piglets. Therefore, it is of great significance for feed and feeding enterprises to choose functional additives after "Prohibiting antibiotics" by understanding the principle of adding them separately, the mechanism of action and whether they can achieve synergistic effect.

 

1. Challenges Faced by Piglets in Weaning Stage

1.1 Environmental change challenges

Before weaning, piglets are raised with suckling sows in the delivery house, but after weaning, they are transferred from the delivery house to the nursery house, from living with sows to without sows, and in addition （at the same time,）, they have to go through mixing flock and re-establishing the order. The environmental temperature and humidity in the nursery house are quite different from those in the delivery house, and many environmental changes will also cause piglets to easily produce stress reactions.

 

2.2 Function and mechanism of acidifier

2.2.1 Adjusting pH value of diet and stomach contents

According to its name, one of its main functions is to adjust the pH value of feed and stomach contents. For pigs, the digestion of protein begins with the action of pepsin in the stomach, and the gastric mucosa secretes pepsinogen, which can be quickly converted into pepsin at pH 2, so it can be said that the activity of pepsin is the highest at pH 2. However, when （at）pH>3.5, the activity of pepsin will be significantly reduced, and when （at）pH>6, pepsin will be basically inactivated ^[5]. Because the ability to secrete gastric acid in weaned piglets is very poor, the main source of gastric acid in weaned piglets is lactic acid produced by bacterial fermentation of lactose in breast milk. However, a series of unfavorable factors, such as poor gastric acid secretion, lack of lactose substrate, unstable food intake and so on, will lead to the increase of pH value in stomach, which will lead to more undigested protein entering the hindgut to be fermented, thus increasing the probability of diarrhea. Therefore, by adding acidifier to reduce the pH of the diet, the pH value of the gastrointestinal tract of piglets can be adjusted, and the activity of pepsin can be improved, which is helpful to improve the diet utilization of piglets, reduce diarrhea and promote the health of piglets ^[6].

 

2.2.2 Inhibition of pathogenic bacteria

The bacteriostatic effect of acidifier can be described in two aspects. The first point is also based on the effect that acidifiers can adjust the pH value. A study showed that there were more than 400 kinds of bacteria in the digestive tract of piglets, of which only 30-50 kinds can promote the digestion of diet ^[5]. In addition, other pathogenic bacteria (such as Escherichia coli) are more suitable to live in neutral and alkaline external environment, so the addition of acidifier can adjust pH value of gastrointestinal tract of piglets, so that the pH value of gastrointestinal tract of piglets is in an acidic environment, creating an environment that is not suitable for the survival of harmful bacteria, but beneficial to the survival environment of beneficial bacteria (such as Lactobacillus) ^[7]. The second point is based on the strong bactericidal ability of organic acids. The undissociated organic acid is lipophilic and can penetrate the cell membrane of Gram-negative bacteria (such as Escherichia coli). When it enters the cell, due to its high pH, it will dissociate and release hydrogen ions to lower the pH value. At this time, the enzyme activity in the cell will be inhibited, and the bacteria will be forced to use energy to expel protons from the cell membrane through the H⁺-ATP pump for pH balance. After a period of time, the bacteria will be killed. Other beneficial bacteria, such as lactobacillus, which are not very sensitive to pH value, will not be affected.

 

2.2.4 Energy sources

Some organic acids, such as fumaric acid and citric acid, are intermediate products of tricarboxylic acid cycle, which can be used as an energy source. A Study has shown that fumaric acid is an easily available energy source, which may have a local nutritional effect on the small intestinal mucosa of pigs, and it can also increase the surface area and ability of small intestine absorption due to the rapid recovery of gastrointestinal epithelial cells in weaned pigs. In addition, because fumaric acid has a shorter pathway to generate energy than glucose, ATP can be synthesized urgently to cope with stress under some special situations ^[9].

 

2.2.5 Other functions

Because acidifier has sour taste, it can mask the bad smell in some ingredients and stimulate saliva secretion, thus improving the palatability of feed ^[10]. Additionally( At the same time), acidifier can inhibit some molds in feed and prolong the shelf life of feed ^[11]. In addition, a study has shown that feed acidifier can also promote the absorption of mineral elements ^[12], promote the growth of intestinal epithelial cells, maintain intestinal morphology and play many other positive roles.

 

2.3 Application of Acidifier in Weaned Piglets

In the weaning stage of piglets, due to the above-mentioned challenges and changes, piglets are most prone to diarrhea and other problems in this stage, and it is precisely because of the above-mentioned many positive effects that acidifiers are widely used in feed. Many studies have shown that adding acidifier to diet has a positive effect on the health and growth of piglets. Studies by Zhou Zhiping et al. ^[13] and Liu Qiang et al. ^[14] found that adding compound acidifier to piglet diet can significantly increase the daily gain of piglets, reduce the diarrhea rate of piglets in weaning stage, and improve the feed-gain ratio in that stage. Scipioni et al. ^[15] found that adding citric acid to diet of weaned piglets can significantly reduce the number of Escherichia coli in stomach, colon and rectum by 38.1%, 62.5% and 66.7% respectively. Feng Shanglian et al. ^[16] found that adding lactic acid to piglet diet can significantly improve the activity of digestive enzymes in piglets, thus improving the feed utilization of piglets and promoting piglet growth. Lawlor P G and other studies ^[17] found that adding 2% fumaric acid to diet of weaned piglets can significantly increase the feed intake, average daily gain and improve the feed-gain ratio of piglets. In addition to the above research, there are many studies that show that adding single acidifier or different compound acidifiers to diet has positive effects on production and health of livestock and poultry, which won't be listed here.

 

3.2 Characteristics and action mechanism of plant essential oil

3.2.1 Bactericidal and bacteriostatic effects

Studies have shown that many plant essential oils, especially spicy plant essential oils such as thymol, cinnamaldehyde and carvacrol etc., have strong antibacterial and bactericidal abilities. In livestock and poultry feeding, they have good bacteriostatic ability against some main pathogenic bacteria, such as Salmonella, Escherichia coli and Staphylococcus aureus ^[18]. In addition(t the same time), some studies have shown that cinnamaldehyde and thymol have inhibitory effects on harmful bacteria such as Escherichia coli, but they have beneficial effects on beneficial bacteria, such as Lactobacillus ^[19]. Therefore, there is no need to worry too much about the negative impact on beneficial bacteria because of the broad-spectrum antibacterial properties of plant essential oils. Because of the complexity of the components of plant essential oil, its antibacterial mechanism is not caused by a single mechanism. At present, it is thought that the antibacterial characteristics of plant essential oil are realized by acting on cell membrane and entering the cell. The antibacterial mechanism of phenolic compounds in plant essential oil is as follows: phenolic hydroxyl groups with fat-soluble and hydrophobic properties act on the surface of cell membrane, thus changing its cell membrane structure, leading to the outflow of intracellular substances of bacteria and eventually leading to bacterial death ^[20]; The antibacterial mechanism of aldehydes is generally believed to be that aldehydes can react with bacterial cell membrane proteins and enter bacterial cells. The carbonyl group of aldehydes can combine with protein to inhibit the activity of amino decarboxylase, and affect the activity of protein and enzyme in bacterial cells, thus achieving antibacterial effect ^[21].

 

4 Study on Synergistic Effect of Acidifier and Plant Essential Oil

Because acidifiers and plant essential oils have the above positive effects on livestock and poultry, they are often added to diet at some special stages, such as weaned piglets, to promote animal health and improve animal production performance. However, a study has found that the synergistic effect of "1+1>2" often occured when acidifier and plant essential oil were added at the same time. In in vitro bacteriostatic tests, studies have shown that the compound of citric acid and plant essential oil had obvious bacteriostatic effect on Salmonella, Escherichia coli and Staphylococcus aureus, and the effect was better than that of adding plant essential oil or citric acid alone, indicating that plant essential oil and organic acids had synergistic effect ^[26]. And thymol and carvacrol combined with citric acid have stronger bacteriostatic ability against Gram-negative bacteria than using plant essential oil or acidifier alone. In livestock and poultry feeding, Akyurek found that the levels of yeast and Lactobacillus in broiler ileum were significantly improved after adding acidifier and plant essential oil to diet, and the effect was better than that of adding plant essential oil alone. However, when 0.15% acidifier and plant essential oil were added to diet of weaned piglets, the level of Escherichia coli in the intestines of weaned piglets decreased significantly, while the level of beneficial bacteria such as Lactobacillus, increased significantly, and the improvement effect was better than that of adding plant essential oil or acidifier alone ^[27].

As for the mechanism of synergistic effect produced by the combination of acidifier and plant essential oil, on one hand, because essential oil can have better effect in acidic environment, the hydrophobicity of essential oil will be enhanced at low pH, which can make it pass through bacterial cell membrane more easily; on the other hand, because plant essential oil can destroy the integrity of bacterial cell membrane, undissociated organic acids can enter bacterial cells more easily to achieve antibacterial and bactericidal effects. From the current research, it is also confirmed that there is a synergistic effect between them, and they are often added together, which can achieve better results with smaller adding amount.