Role of acidifiers in livestock nutrition and health: A review
Abstract
Ever since the European ban on use of in-feed antibiotics in food animals, the search for alternate antibiotic-free growth promoter is undertaken worldwide. There are few alternatives such as probiotics, pre-biotics, phytochemicals, enzymes and organic acids. Among these alternatives, the organic acids or simply acidifiers play an important role in gut health in animals. The acidifiers could be used to favourably manipulate the intestinal microbial populations and improve the immune response, hence perform an activity similar to antibiotics in food animals in countering pathogenic bacteria. Acidifiers also improve the digestibility of nutrients and increase the absorption of minerals. The incorporation of organic acids also leads to thinning of the intestinal lining which facilitates better absorption of nutrients and its efficient utilization. However, their effect will not be similar among all types of organic acids as their mechanism of activity is based on its pKa value. Moreover, there are claims about the neutralization of acids by the secretion of bicarbonates in the initial part of intestine, reactivity with metallic items in feed mills and reduced palatability due its bitter taste demands non-reactive and targeted delivery for better performance. Currently, coated salts of acidifiers are available commercially for use in food animals especially pigs and poultry. The present review highlights the role of different acidifiers in livestock nutrition with their potent applications in improving nutrient digestibility, mineral utilization, meat quality, enhancing immunity, antimicrobial effects in countering pathogenic bacteria, boosting performance and production, and thus safeguarding health of livestock animals and poultry.
1 INTRODUCTION
The heightened public concern over the emergence of antibiotic-resistant strains prompted the exploitation of alternate growth promoters to antibiotics (Dhama et al., 2014; Yadav et al., 2016). Among such alternatives, one such striking preference includes acidifiers, especially in swine and poultry sectors. The potential of acidifiers in livestock feed industry is known for decades for their preservative and nutritional qualities (Partanen & Morz, 1999; Spratt, 1985). Organic acids, commonly employed for feed acidification form the natural constituent in several feedstuffs, are generated during metabolism in animals. Being known for their defensive effects against bacteria, fungus or mould, they have been applied as an in-feed prophylactic measure to counter such pathogens in feed industry (Frank, 1994). Performance and health-promoting impacts have been elucidated for a number of organic acids, such as fumaric, formic, lactic and citric acids and their salts (Yang, Xin, Yang, & Yang, 2018). Besides hygiene effects and the resulting diminution of pathogen intake, their role in feed digestion, nutrient digestibility, eubiosis in gut flora, health and performance promotion effects have also been elucidated in numerous investigations (Mosenthin, Sauer, Ahrens, De Lange, & Bornholdt, 1992; Ndelekwute, Unah, & Udoh, 2016; Overland, Bikker, & Fledderus, 2009; Russell, 1992). Thus, acidifiers in livestock nutrition are a cost-effective performance-enhancing option, exerting their effects through feed, intestine and metabolism of animals (Kirchgessner & Roth, 1988; Roth et al., 2017). With the above advantages, there still persist few concerns about their palatability, site of action and neutralization, which all together forces the scientists to devise alternate ways to utilize these as a feed additive for antibiotics free growth promotion. The preparations of coated salts of acids are performing better when compared to the uncoated/naïve acids both in terms of performance and palatability. Hence, the present review highlights the acidifier chemistry, classification, selection and use of acidifiers in livestock nutrition and their beneficial effects on performance, production, nutrient digestibility, mineral utilization, immunity, meat quality and countering pathogenic bacteria.
2 CHEMISTRY OF ACIDIFIERS
Broadly, the acidic specificities of acidifiers as feed additives are attributed to the carboxyl functional group, -COOH of the organic acids, including the fatty and amino acids. They include either simple mono-carboxylic acids (formic, acetic, propionic and butyric acids) or carboxylic acids with the hydroxyl group (lactic, malic, tartaric and citric acids) or short-chain carboxylic acids containing double bonds (fumaric and sorbic acids) (Shahidi, Maziar, & Delaram, 2014). The short-chain organic acids (C1–C7) have specific antimicrobial activity; however, their effect in pH reduction and antimicrobial activity varies with their dissociation status depending on the specific pKa of each acid. Hence, lower the pKa value, stronger the acid which describe its ability to lower the pH of environment. Most acids used as feed additives have their pKa value between 3 and 5 (Kirchgessner & Roth, 1991). Many acids are also used as salts of sodium, potassium or calcium, the benefits being less odour, easy handling during feed manufacture, less corrosive and more soluble in water than the free acids (Huyghebaert, Richard, & van Immerseel, 2011). The inorganic acidifiers under use, particularly hydrochloric, sulphuric and phosphoric acids are cheaper than organic acids but in pure form they are very corrosive and hazardous liquids (Kim, Kil, Oh, & Han, 2005). The chemical properties of some acids and salts are presented in Table 1.
| Acid/Salt | pKa | Molecular weight (g) | Gross energy (KJ/g) | Physical state | Solubility in water |
|---|---|---|---|---|---|
| Formic acid | 3.75 | 48.0 | 5.8 | Liquid | Very good |
| Acetic acid | 4.75 | 60.1 | 14.8 | Liquid | Very good |
| Propionic acid | 4.87 | 74.1 | 20.8 | Liquid | Very good |
| Lactic acid | 3.08 | 90.1 | 15.1 | Liquid | Good |
| Fumaric acid | 3.03/4.44 | 116.1 | 11.5 | Solid | Low |
| Citric acid | 3.14/5.95/6.39 | 210.1 | 10.3 | Solid | Good |
| Ca-formate | – | 130.1 | 3.9 | Solid | Low |
| Na-formate | – | 68.0 | 3.9 | Solid | Very good |
| Ca-propionate | – | 16.6 | 16.6 | Solid | Good |
| Ca-lactate | – | 10.2 | 10.2 | Solid | Low |
2.1 GASTROINTESTINAL TRACT AND REDUCTION OF pH
The gut health plays a crucial role in influencing growth rate and feed efficiency of poultry. Gracia, Catala-Gregori, Hernandez, Megias, and Madrid (2007) stated that supplementation of formic acid in broiler feed had significantly increased the villus height with 1,273 and 1,250 µm at 0.5 and 1.0% level, respectively, than the control value with 1,088 µm. Also, the jejuna crypt depth was significantly higher (266 µm) than the antibiotic fed birds (186 µm). Inclusion of 3% butyric acid in chicken diet by Adil, Banday, Bhat, Mir, and Rehman (2010) showed a significant increment in the height of intestinal villi in duodenum (1,410.38 vs. 1,166.88 µm), jejunum (1,124.72 vs. 984.05 µm) and ileum (876.32 vs. 676.13 µm) than the respective control groups.
Acidification enables lowering of pH in feed and digestive tract of animals. While antimicrobial action is the key activity of acidifiers in poultry, it is the reduction of pH in case of swine (Desai, Patwardhan, & Ranade, 2007). While the young animals have limited capacity of hydrochloric acid production in stomach which advances with age, incorporation of acidifiers in their diet helps to maintain the optimum pH in stomach for enzymatic actions and ensuring proper protein digestion in the intestine. Moreover, pepsinogen, the inactive enzyme precursor of pepsin, has its active conversion catalysed by the acidic environment (Luckstadt & Mellor, 2011).
The main source of acidity in suckling animals is the conversion of lactose from dam's milk to lactic acid by the Lactobacillus bacteria. Though high level of lactate in stomach inhibits the secretion of hydrochloric acid, the intake of solid or creep feed stimulates its secretion. In case of weaned pigs, factors like low acid secretion, lack of lactose substrate, high buffering capacity of feed ingredients and variability in feed consumption all tend to increase the stomach pH over 5. This can lead to reduced digestion and pathogen colonization in the hind gut causing diarrhoea. Acidified diets with 1% citric acid and 0.7% fumaric acid showed drop in stomach pH from 4.6 to 3.5 and 4.6 to 4.2, respectively, in weaned piglets (Sciopioni, Zaghini, & Biavati, 1978). The acidic environment also creates a barrier to the entry of bacteria and their colonization in the intestine (Easter, 1988). Though inorganic acids like hydrochloric or phosphoric are highly gut persistent, reduce pH and buffering capacity of feed in stomach, they showed no betterment in growth rate and feed conversion of pigs (Metzler & Mosenthin, 2007). However, organic acids showed better results in combination with phosphoric acid than alone (Schoenherr, 1994).
In ruminants, acidified milk replacers, either organic or phosphoric based acidifiers caused less digestive disorders than the regular ones, especially when fed ad libitum (Fallon & Harte, 1986; Hand, Hunt, & Phillips, 1985). In calves, they showed increased feed conversion efficiency and live weight gain. The reduction in the pH of digestive tract contributes to increased rennin secretion that favours clot formation and improved digestibility. Inclusion of phosphoric acid-based acidifiers at required levels present economical benefits like lowering the cost of acidification in the starter diets or milk replacers, less space in formulation than organic acids, provides totally available phosphorus and enhanced growth. In poultry, supplementation of different organic acids or its salts has expressed no significant change in pH of different gut segments, reason being strong buffering action in the digestive tract of poultry (Hernández et al., 2006). In broiler, Ndelekwute, Assam, and Assam (2018) studied the influence of some organic acids (0.25% acetic acid, formic acid, citric acid and butyric acid) in drinking water on faecal moisture, gut pH and digesta viscosity of broiler chickens. The authors stated that organic acids reduced gut pH, faecal moisture and duodenum digesta viscosity (p < .05). Also, Ndelekwute, Unah, and Udoh (2019) studied the impact of feeds acidified with different organic acids (0.25% of acetic, citric, butyric and formic acids) on digesta pH, digesta viscosity, faecal moisture and apparent nutrient digestibility of broilers. Feeding of butyric and acetic acids resulted to significant reduction of digesta pH and digesta viscosity in the duodenum (Ndelekwute et al., 2019).
3 ANTIMICROBIAL PROPERTY
Acidifiers in feed inhibit the growth of pathogenic bacteria and curtail the microbial competition for host nutrients by influencing the pH. The proliferation of most pH sensitive bacteria (E. coli, Salmonella and Clostridium perfringens) is minimized below pH 5 while acid-tolerant ones survive. The undissociated form of acid being more lipophilic, penetrate freely across the semi-permeable membrane of the bacterial cell into the cytoplasm of neutral pH, after which will dissociate and release protons (H+), resulting in pH reduction inside the cell. Consequently, the enzymatic reactions of glycolysis signal transductions and nutrient transport of the microbes are impeded, causing energy deprivation on its effort to balance the pH to normal (Mroz et al., 2006). The trapped anions of the acid also turn toxic to the cell metabolites and disrupt the bacterial membranes (Freese, Sheu, & Galliers, 1973; Russell, 1992). On the contrary, acid-tolerant bacteria such as Lactobacillus sp. and Bifidobacterium sp. can endure the imbalance between the external and internal pH, wherein the acids can depart the bacteria by returning to its undissociated form at the lower internal pH. In Gram-positive bacteria, the higher level of intracellular potassium may neutralize the acid anions as well (Russell & Diez-Gonzalez, 1998).
Several studies evidenced decline in the number of bacteria in stomach and duodenum (Hellweg, Tats, Manner, Vahjen, & Zentek, 2006; Kirchgessner & Roth, 1991). Knarreborg, Miquel, Granli, and Jensen, (2002) reported that in pigs, organic acids selectively remove the target species like coliforms but not Lactobacillus, creating eubiosis. The modified gut microflora benefits the host metabolism via reduction in ammonia, amines and toxins (Dibner & Buttin, 2002a, 2002b). The acids also expressed a specific minimum inhibitory concentration (Strauss & Hayler, 2001). While the Gram-positive bacteria are susceptible to long-chained organic acids, the Gram-negative bacteria cannot resist acids with fewer than eight carbons (Partanen et al., 2001). The efficacy of acids against the coliform bacteria follows the order: benzoic > fumaric>lactic > butyric>formic > propionic acid. Bearson, Wilson, and Foster (1998) found resistant strains of Salmonella sp, when exposed to lower pH for a long time. Therefore, as microbes can develop tolerance to survive the acidic environment, proper attention is essential when specific acids are used for an extended period. Lately, feeding the mixture of organic acids and medium-chain fatty acids in pigs reduced the pathogen activity with improved performance on account of down regulated expression of proinflammatory cytokines along with propagation of Lactobacillus sp. (Kuang et al., 2015).
Hassan, Mohamed, Youssef, and Hassan (2010) reported a decline in intestinal E. coli and Salmonella spp. in the intestinal microflora of broilers by incorporating a combination of organic acids or salts. Chaveerach, Keuzenkamp, Urlings, Lipman, and Knapen (2002) observed a significant reduction in campylobacter colonies by supplementing formic, acetic and propionic acids in the ratio of 1:2:3 and 1:2:5 through feed/water of chicken. Mohyla, Bilgili, Oyarzabal, Warf, and Kemp (2007) evidenced reduced load of salmonella only in the upper gastrointestinal tract of broilers when 600 ppm of acidified sodium chlorite was administered in the drinking water of broilers. Mikkelsen et al. (2009) demonstrated reduced mortality of birds due to necrotic enteritis caused by Clostridium perfringens by feeding 0.45% of potassium diformate. However, in broilers, Jozefiak, Kaczmarek, and Rutkowski (2010) stated that the beneficial role of acidifiers in the lower alimentary tract is declined. A few other authors suggested that the short-chain fatty acids in the feed or water get metabolized and absorbed in the upper digestive tract (Hamed & Hassan, 2013; Thompson & Hinton, 1997). This led to microencapsulation of acids in a protective lipid shell which will ensure the slow release of acids throughout the digestive tract (Fernndez-Rubio et al., 2009). Inclusion of 0.2% protected acids in feed found to decline Salmonella sp., E. coli and Clostridium perfringens without hindering Lactobacillus spp. in the gut contents of broiler chicks (Gheisari, Heidari, Kermanshahi, Togani, & Saraeian, 2007). Breeders fed with formic acid revealed reduced contamination of tray liners and hatchery waste with Salmonella Enteritidis (Humphrey & Lanning, 1988). When sprayed over the litter material, organic acids suppress microbes that assist uric acid breakdown, restricting ammonia release (Hajati, 2018). Also, the probiotics, acidifiers and antibiotic reduced the number of pathogenic microbes especially coliforms, total aerobes and E. coli (Youssef, Mostafa, & Abdel-Wahab, 2017). Moreover, acidifiers have been observed to possess antibacterial effects on pathogens (Khan & Iqbal, 2016; Kim, Kim, & Kil, 2015). Furthermore, probiotics and acidifiers improved the count of Lactobacilli, since the beneficial effect of these additives could be attributed to their effects in suppressing the pathogenic bacteria and improving the growth of beneficial one (Getachew, 2016; Mirza, Rehman, & Mukhtar, 2016). In pigs, dietary supplementation of increasing levels of protected organic acids linearly improved the counts of faecal Lactobacillus, while the Salmonella and E. coli counts, faecal ammonia, diarrhoea score and acetic acid emissions were linearly decreased (p < .05) (Yang, Lee, & Kim, 2019). Broiler fed rations supplemented with organic acids showed a decrease (linear, p = .002) in E. coli populations and increase (p = .042) in Lactobacillus populations (Nguyen, Lee, Mohammadigheisar, & Kim, 2018). Use of some organic acids such as tartaric, citric and acetic acids decreased S. typhimurium at doses of 0.312%, 0.512% and 0.625% for the three levels of strain: 10, 100 and 103 CFU/ml respectively (El Baaboua et al., 2018). The potent role of acidifiers in countering important poultry pathogens including of food-borne bacterial pathogens and others having public health concerns need to be exploited further for safeguarding poultry health and combating zoonosis (Dhama et al., 2013; Ramees et al., 2017). (Figure 1).
4 ROLE OF ACIDIFIERS IN NUTRITION
4.1 Effect of organic acids on nutrient digestibility and mineral utilization
Organic acids are accepted to be an appealing alternative for improving the nutrient digestibility in swine and poultry industry. The multifunctional role of organic acids including the reduction of gastric pH, increased gastric retention time, stimulation of pancreatic secretions, influence on mucosal morphology and serving as substrate in intermediary metabolism all lead to improved digestion and absorption (Partanen & Morz, 1999). Blank, Mosenthin, Sauer, and Huang (1999) exhibited increased digestibility of apparent ileal protein by 7% and amino acids from 4.9% to 12.8% in early weaned piglets with 2% fumaric acid. In case of grower pigs, Mosenthin et al. (1992) found improved ileal digestibility of certain essential amino acids but reported no effect on the apparent ileal digestibility of dry matter, organic matter, crude protein or ash when 2% propionic acid was added to a barley–soybean-meal-based diet. However, with dietary inclusion of formic acid (1.4%), fumaric acid (1.8%) or n-butyric acid (2.7%) in grower pigs (Mroz et al., 2000) illustrated a significant increase of 6% in the apparent ileal digestibility of protein and several essential and non-essential amino acids.
In case of broilers, Ghazala, Atta, Elkloub, Mustafa, and Shata, (2011) showed improved metabolizable energy (ME) and nutrient digestibility of crude protein (CP), ether extract (EE), crude fibre (CF) and nitrogen-free extract (NFE) through dietary fumaric (0.5%) or formic acid (0.5%) and acetic (0.75%) or citric acid (2%). The improved CP and ME digestibility through organic acid supplementation is also related to the control of microbial competition for host nutrients, endogenous nitrogen losses and ammonia production (Omogbenigun, Nyachti, & Solminski, 2003). As low ME of soybean meal is related to its poor digestibility of carbohydrate portion in chicken, inclusion of 2% citric acid in soybean meal found to improve α-galactosidase activity and decreased the crop pH (Ao, 2005). Smulikowska (Smulikowska, Czerwiński, Mieczkowska, & Jankowiak, 2008) illustrated increased nitrogen retention in the host supplemented with fat coated organic acids, because of their enhanced bioavailability in the distal digestive tract and the greater epithelial cell proliferation. In broiler chickens, Ndelekwute and Enyenihi (2017) reported that citric and ascorbic acids of lime juice improved digestibility of nutrient at 7 weeks of age of broiler chickens. Also, Ndelekwute et al. (2018) found that digestion coefficients of protein, fibre and ether extract were significantly improved by addition of organic acids in drinking water (p < .05). But, nitrogen-free extract and dry matter digestibility were significantly reduced by organic acid supplementation (p < .05). On the same context, Ndelekwute et al. (2019) found that the per cent of faecal moisture and nitrogen-free extract digestibility were reduced by organic acid supplementation. But, digestibility of protein, crude fibre and ether extract were improved due to organic acid in comparison with control (Ndelekwute et al., 2019). Yang et al. (2019) evaluated the impact of protected organic acids on pig performance, faecal microbial counts and nutrient digestibility, and they found that 0.2% of protected organic acids increased the apparent digestibility of dry matter compared with control (p < .05). Supplementation of protected organic acid blends to pig diets revealed beneficially affects ileal noxious gas and the nutrient digestibility, (Devi, Lee, & Kim, 2016).
The dietary organic acids complex with minerals have been found to improve digestibility and reduced excretion of supplemental minerals and nitrogen, thereby controlling their discharge into environment. The acidic anions have been found to promote the cation absorption of minerals such as calcium, phosphorus, magnesium and zinc (Edwards & Baker, 1999). In contrast, addition of 15 g of citric acid has shown to alleviate the symptoms of parakeratosis in pigs fed with suboptimal level of zinc but no appreciable effects were found on the apparent absorption and digestion of any of the minerals (Hohler & Pallauf, 1994). Boling, Webel, Mavromichalis, Parsons, and Baker (2000) stated that citric acid could effectively improve the utilization of phytate phosphorus but the response was found much smaller in pigs than chicken. Nourmohammadi, Hosseini, Farhangfar, and Bashtani (2012) demonstrated supplementation of 3% citric acid along with microbial phytase enzyme in broiler chicken caused better ileal nutrient [CP, apparent metabolizable energy (AME), Ca and total P] digestibility and increased mineral retention. It was reported that lower pH facilitates the P solubility and the microbial phytase was more active through acidification resulting in improved P absorption. The organic acid supplementation together with the developing desirable gut microflora was found to contribute for mineral retention and bone mineralization through increased digestibility and availability of nutrients as stated by Ziaie et al. (2011).
4.2 Impact on performance and production
A meta-analysis of Partanen and Morz (1999) stated that the growth performance in the weaned piglets did not show much difference among the formates, fumarates and citrates, whereas the formates showed better performance in fattening pigs followed by fumarates. In comparison, Suryanarayana, Ravi, and Suresh (2010) showed greater average daily gain (g) and feed: gain ratio in grower pigs augmented with 0.9% sodium formate. When 0.8% or 1.2% potassium diformate were added in the diets of primiparous and multiparous sows, the author found positive effect on the back fat thickness of sows in gestation with no change in average daily feed intake or body weight gain. Irrespective of the dose, the piglets born to the supplemented sows exhibited increased birthweight, weaning weight and average daily gain (Overland et al., 2009). Similarly, sows fed with 0.8% potassium diformate (Lückstädt, 2011) demonstrated higher feed intake from third day post-farrowing, reduced weight loss during weaning time and significantly lower back fat reduction. As per Xia et al. (2016) potassium diformate boost the secretion of hydrochloric and lactic acid through increased mRNA expression of H+-K+-ATPase and gastrin receptors in the oxyntic mucosa of stomach. Dietary incorporation of sodium butyrate in pregnant sow and post-weaning piglets increased their growth performance with beneficial impact on muscle and adipose tissue oxidative genes.
Mroz, Grela, Krasucki, Kies, and Schoener (1998) demonstrated that formic acid has anti-agalactia properties in lactating sows. A few studies on other organic acids like acetic acid, lactic and sorbic acid have also shown an equivalent growth promoting effects in swine (Roth & Kirchgessner, 1988). Eckel, Kirchgessner, and Roth, (1992) recommended that the strong odour and flavour of acids like tartaric and formic acid may lead to lower daily gain in piglets corresponding to reduced feed intake when their threshold dose in their diet exceeds. This suggests that the palatability of the diet can influence the growth performance and hence minimum effective concentration of each acid should be established (Partanen & Morz, 1999). Inclusion of salts of organic acids can also be a solution, as they are tasteless and do not influence the feed intake.
In chicken, Brzóska, Śliwiński, and Michalik-Rutkowska (2013) showed growth-enhancing and mortality-reducing effect in broiler chicken using dietary organic acid (0.3%–0.9%) but found no significant influence on carcass yield or individual carcass parts. On the contrary, Fascina et al. (2012) reported better performance and carcass characteristics by incorporation of organic acid mixture (30.0% lactic acid, 25.5% benzoic acid, 7% formic acid, 8% citric acid and 6.5% acetic acid) in broiler diets. The superior growth performance can be attributed to low pH in diet and digestive tract acting as microbial barrier, reduced buffering capacity and improved nutrient digestibility. Broiler birds fed with 0.5% citric acid has shown progress in weight gain, feed intake, tibial ash deposition, carcass weight and non-specific immunity through increased density of lymphocytes in lymphoid tissues (Haque et al., 2010).
A study on acidifying drinking water of broilers with citric acid (pH 4.5) demonstrated improved gut modulation, liver health and thyroid hormones (T3 and T4) with respect to lipid profile showing equilibrium in internal homeostasis (Abdelrazek, Abuzead, Ali, El-Genaidy, & Abdel-Hafez, 2016). But the same study with acetic acid worsened the performance and gut health of broilers. Evaluation of encapsulated acidifier or herb-acidifier blend in the broiler diet revealed better performance in terms of intestinal histology, intestinal pH, serum total protein, serum albumin and gut eubiosis than without encapsulation (Natsir, Hartutik, Widodo, & Widyastuti, 2017). A latest investigation on resistance to heat stress by drinking water acidification with sodium butyrate in broilers suggested alleviation of detrimental effects of heat stress on growth, carcass quality, haematological, biochemical traits, inflammatory markers, oxidative stability and histology of liver and immune organs. Sodium butyrate and acidifier blends were also proved to be antioxidants to check free radical injury due to heat stress (Awaad et al., 2018). With regard to layer chicken, employment of acidifiers markedly increased the egg production (Yesilbag & Colpan, 2006). Dietary inclusion of 0.2% protected organic acid to pigs has the potential to enhance the growth rate. Feeding protected organic acid rations to piglets improved the average body gain during 0–2 weeks and overall period (0–6 weeks) (Yang et al., 2019). In broilers, the dietary supplementation with organic acid improved body weight gain and feed efficiency when compared with control. But, dietary organic acids had no significant impacts on feed intake or relative organ weights (Basmacioğlu-Malayoğlu, Ozdemir, & Bağriyanik, 2016).
4.3 Carcass and meat quality
The relative weights of carcass depot fat, leg and breast muscles, liver and gizzard were not affected by dietary acidifier at 3, 6 and 9 g/kg of the diet. Breast and leg muscles represented 27.9% and 20.7% (acidifier groups) and 27.7% and 21.5% (control group) of the carcass weight respectively. On the other hand, dietary acidifier treatments did not affect chemical composition of leg and breast muscles, including content of dry matter, fat and protein (Brzóska et al., 2013).
On the same trend, in broiler chicks, carcass traits (breast, thigh, liver, heart and gizzard) were not significantly affected by the acidifier (1 ml/L of NufocidL as an organic acid supplement in the drinking water) supplementation (Heidari, Sadeghi, & Rezaeipour, 2018). Also, supplementation of acidifiers, Bacillus subtilis and their combination did not affect (p > .05) carcass yield, dressing % and the relative weight of internal organs. The Bacillus subtilis group showed the highest value of breast weight when compared with the acidifier groups (Malik et al., 2016).
Youssef et al. (2017) found that the percentage of carcass yield did not show any significant effect due to dietary treatments, but exhibited a numerical improve in probiotic group (72.84%), followed by lactic acid and antibiotic groups (71.45%) in comparison with the normal group (70.35%). On the same context, the relative weights of gizzard, breast, liver, proventriculus and heart were not affected by the dietary supplements (lactic acid, antibiotic and probiotics) when compared with the control (p > .05). Acidifier's impacts are supported by the findings of other studies which reported that the acidifiers did not affect carcass characteristics and dressing yield of broiler chickens (Ghasemi, Akhavan-Salamat, Hajkhodadadi, & Khaltabadi-Farahani, 2014; Kopecký, Hrnčár, & Weis, 2012).
4.4 Impact of acidifier on immunity
The immune system plays a key role in regulating the bird's health (Gadde, Kim, Oh, & Lillehoj, 2017; Yan et al., 2018). In this trend, the use of acidifiers in the poultry diets plays a critical role in enhancing the immunity system ((Dibner & Buttin, 2002a, 2002b). An improvement in the immunological status was observed when broiler chickens fed 0.5% citric acid (Chowdhury et al., 2009). Similarly, Abdel-Fattah, Ei-Sanhoury, Ei-Mednay, and Abdul-Azeem (2008) showed an improvement in the immune response of broilers. Furthermore, the weight of lymphoid organs was increased by the action of acidifiers; in this sense, Yan et al. (2018) observed an increase of the spleen weight in birds that consumed 0.30 g/ kg of sorbic acid, fumaric acid and thymol throughout the grower and finisher period. In addition, on day 42, in the ileal and duodenal mucosa, higher levels of immunoglobulin A were recorded. With regard to layer chicken, employment of acidifiers markedly increased serum protein and serum albumin concentration (Yesilbag & Colpan, 2006). Devi et al. (2016) studied the effects of blends of dietary protected organic acid supplementation on growth parameters, digestibility of nutrient, gas emission, faecal microflora and blood constituents of pigs. The authors found that white blood corpuscles (WBC), immunoglobulin G level and lymphocyte % were improved with protected organic acid groups (0.1% and 0.2%) in sucking piglets and lactating sow. Emami, Daneshmand, Naeini, Graystone, and Broom (2017) studied influence of three commercial organic acids on growth parameters, caecal microbiology, immunity and intestinal morphology of Escherichia coli K88-challenged (ETEC) broiler chickens. The authors found that dietary supplementation of these organic acids can enhance the ileal morphology and immunity of ETEC- challenged broilers.
Also, Lee et al. (2017) evaluated the beneficial effects of organic acids on immune responses against viral antigens (H9N2) in broiler chickens and they found that the CD4+ CD25+ T-cell percentage was higher in the OV group (diet supplemented with organic acids and administered a H9N2 vaccine [OV]) than in the control, demonstrating the potential induction of regulatory T cells by feed additive. Liu et al. (2017) used 450 1-day-old Cobb 500 chicks to evaluate the beneficial role of protected organic acids and essential oils mixture product at 0.30 g/kg. Authors pointed out that supplementation of the product improved crypt depth and villus height of the jejunum, and spleen index at 42 days as compared to control (p < .05). Furthermore, trypsin and chymotrypsin activities of intestinal tract and secretory immunoglobulin A concentration of ileal mucosa were higher in the organic acids and essential oils treatment.
4.5 Impact on intermediary metabolism
Most organic acids contribute a substantial amount of energy during metabolism and should not be overlooked in energy estimation of feedstuffs. Being the intermediates of citric acid cycle, they act as energy source after getting absorbed through the gut epithelium by passive diffusion. As 1 m of fumaric acid generates 18 m ATP or 1,340 kJ, it necessitates approximately 74.3 kJ per m ATP which is comparable with glucose. The same works with citric acid, while the acetic and propionic acid require 18% and 15% more energy per m ATP respectively. Sofos and Busta (1993) elucidated that sorbic acid and long chain fatty acids are metabolized through ß-oxidation of fatty acids. Giesting and Easter (1985) assured that weaned piglets under stress may get the advantage of glucogenic, tricarboxylic acid cycle intermediates such as citric or fumaric acid that restrain some tissue breakdown resulting from gluconeogenesis and lipolysis. The growth enhancing effects of acidifiers were also ascribed to their energy contribution. Blank et al. (1999) illustrated that fumaric acid as a readily available energy source may also have affinity to the small intestinal mucosa and enhance their absorptive surface and capacity due to the rapid recovery of the gut epithelial cells of pigs after weaning.
4.6 Preservation of feedstuffs
For successful livestock farming, continuous supply of good quality feedstuff should be ensured all year round. Even under hygienic conditions, factors like high moisture and warm environment can incur growth of certain fungi, yeast or bacteria, minimizing the feed nutritive value by metabolizing its starch and protein. Depending on the type of organism and the level of infection, conservative agents inhibit the microbial growth and lessen the pathogen uptake by the animal which otherwise cause acute health risks. Formaldehyde, which acts as a potent mould inhibitor (Spratt, 1985), enhances the keeping quality of feed for approximately 20 days and is commonly sprayed over the finished feed before package. The buffered propionic acid combined with sodium benzoate in preservation of wet corn showed appreciable difference between the treated and untreated corn (Luckstadt, Zhang, & Wu, 2006). The range of fungal counts was only 0 to 30 CFU/g in treated corn while that of untreated group resulted higher levels of 2.6 and 2.7 × 106 CFU/g in the low and high moisture content respectively. The authors have also found reduced yeast counts in treated corn and elevated mycotoxins (aflatoxin B1 and zearalenone) in the untreated group.
Use of silage additives has also been intensified over the recent years with acidification being made to enhance fermentation, aerobic stability and nutritive value. Moreover, addition of organic acids and their salts exhibit their antimicrobial activity through undissociated acids as discussed earlier and reduce protein losses by rendering the plant protease enzymes inactive. Formic acid has been used conventionally for ruminant silage while propionic acid is known for its antifungal activity and aerobic stability of silages. The expense of propionic acid has restricted its use to the last few loads at the top of conventional or bunker silos thereby reducing the surface spoilage (Aragon, 2007).
In poultry feed, organic acids or salts can be included at 0.5 kg/ton for mould control and at 2.5 to 3.0 kg/ton for pH reduction and salmonella control (Banupriya, Kathirvelan, & Patric Joshua, 2016). In chicken, addition of formaldehyde had no effect on feed intake and growth rate (Regal, 2014). Acidifiers are added at a lower level for conserving purposes than that for enhanced performance. Other choice of acid preservatives includes benzoic acid, acetic acid and their salts like sodium benzoate and sodium acetate. Salts can be used to overcome the corrosive action of acids but cost restrains are to be considered. Most commercial products use the combination of acids and/or salts to accomplish their synergistic effects. However, it is important to consider the specific inhibitory effects of each acid before supplementation (Gonzalez-Fandos & Herrera, 2014; Sansawat, Lee, Singh, Ha, & Kang, 2019) (Figure 2).
4.7 Factors influencing efficacy of acidifier
With the increasing knowledge about the different actions of acidifiers in animal nutrition, a handful of aspects are yet to be inferred. Despite the improved performance recorded in many publications, there exist some conflicting results due to some attributes which could be associated with acid, animal or the dietary factors. This includes the following: the chemical nature (acid, salt, coated or uncoated), pK value of acids, molecular weight, inclusion level and minimum inhibitory concentration of acid, type of microorganism, species and their concentration, animal species, site of action, dietary composition and buffering capacity of feed.
5 CONCLUSIONS AND FUTURISTIC PERSPECTIVE
The findings of various scientific works on feed acidifiers reveal that organic acids and their salts represent an effective substitute for the sub-therapeutic in-feed antibiotic growth promoters in livestock nutrition. Their apparent actions include improved feed hygiene, lowering of gastric pH and inhibition of pathogens without affecting the beneficial bacteria, stimulation of pancreatic secretions and energy source during intermediary metabolism, enhanced nutrient digestibility, improved growth performance and immunity. The utilization of acidifiers also promotes economic benefits of higher feed efficiency, improved daily gain leading to reduction in feed costs and shorter marketing time. Besides the promising responses, inconsistent results of acidifiers related to the uncurbed factors including the buffering nature of dietary ingredients, presence of other antimicrobial compounds, hygiene of the production environment and heterogeneity of gut microbiota have to be addressed. Supplementary researches are demanded to clarify and minimize the impact of these factors to accomplish the maximal benefit of acidifiers in livestock.
CONFLICTS OF INTEREST
The authors declare no conflict of interest.
AUTHOR CONTRIBUTIONS
Gopi Marappan conceived the idea for the manuscript and led preparation of the manuscript. Beulah V. Perlin performed the bibliographic research and drafted the manuscript. Prabakar Govindasamy, Manojkumar Villavan and Shanmathy Muthuvel critically revised the manuscript. Mahmoud Alagawany, Mayada Ragab Farag and Kuldeep Dhama updated different sections, designed figures and edited the final version of the manuscript. All the authors read and agreed with the final version of the manuscript.
ANIMAL WELFARE STATEMENT
All the authors confirm that the ethical policies of the journal as mentioned in the journal's author guidelines have been adhered to. No ethical approval was required as this is a review article with no original research data.
