Organic acids in animal nutrition

The application of organic acids and their salts to diets for pigs has been studied extensively. They have proved especially effective in maintaining growth performance since the ban on antibiotic growth promoters came into effect in Europe in 2006. Numerous trials have demonstrated their mode and magnitude of action and have established effective doses for piglets, fattening pigs and sows. The use of formic acid and its double potassium salt in particular have been the subject of intense investigation, with the result that we now know its dose-dependent effect on growth performance and feed conversion in pigs under a range of different environmental conditions and feed formulations. Its main mode of action is its antimicrobial effect, which makes it comparable with antibiotic growth promoters; however, organic acids also reduce pH in the stomach, which optimises conditions for pepsin activity, and increases the digestibility of nitrogen, phosphorus and several minerals. This is not only beneficial in sparing nutrients, but also prevents losses that might otherwise contribute to environmental pollution. More recently, the use of acids in general and diformates in particular has spread to the poultry and aquaculture industries. Its performance-enhancing effects in poultry and fish are documented. With a growth-promoting effect similar to that of antibiotic growth promoters, switching from growth promoters to organic acids, especially potassium diformate, can be achieved without detriment to profitability.

Introduction

Organic acids have been used for decades in commercial compound feeds, mostly for feed preservation, for which formic and propionic acids are particularly effective. In the European Union, these two organic acids and several others (lactic, citric, fumaric and sorbic acids) and their salts (e.g., calcium formate, calcium propionate) are used under the classification "feed preservatives". One such organic acid salt is also approved for use as a zootechnical additive (products that formerly included antibiotics and other growth promoters) for enhancing performance in pigs. To achieve this status for each lifestage of a species, evidence of significant performance enhancement (feed conversion ratio, live weight gain) must be shown in three separate trials for each recommended dosage.

Experience has shown that acidifiers are the most reliable product group of the non-antibiotic growth promoters available in Europe and can also be used safely and effectively with other additives. The main mode of action of organic acids is through their antimicrobial effects, the magnitude of which is dependent on the chemical properties of the individual acid or acid salt.

The market for organic acids in Europe is expected to continue to grow, especially in southern and Eastern Europe, reflecting the industry´s move away from antibiotic growth promoters. Northern Europe was already beginning to adopt these products before the EU-ban, based on the expectation that acids would emerge as a ‘cost-effective, performance enhancing option for feed industry (Kochannek, 2011).

Mode of action of organic acids for pig diets 

Since the ban of antibiotic growth promoters in the European Union, organic acids have been used increasingly, not only because of their preservative qualities, but also for their nutritive properties.

They are both bacteriostatic and bactericidal. As undissociated organic acids are lipophilic, they can cross the cell membrane of Gram negative bacteria, such as Salmonella. Once inside the cell, the higher cytosolic pH causes the acid to dissociate, releasing hydrogen ions, which consequently reduces the intracellular pH. Microbial metabolism is dependent on enzyme activity,which is depressed at lower pH. To redress the balance, the cell is forced to use energy to expel protons out across the membrane via the H+-ATPase pump to restore thecytoplasmic pH to normal. Over a period of exposureto an organic acid, this can be sufficient to kill the cell. Expelling protons also leads to an accumulation of acid anions in the cell (Lambert and Stratford, 1998), which inhibits intracellular metabolic reactions, including the synthesis of macromolecules, and disrupts internal membranes. Lactic acid bacteria are less sensitive to the pH differential across the cell membrane, and thus remain unaffected. Inhibition of microbial growth by this mode of action has been exploited for thousandsof years in food preservation; organic acids are natural by-products of microbial metabolism. 

Studies have quantified the effects of several organic acids against different microbes in vitro (e.g., Strauss and Hayler, 2001; Figure 1). Minimum inhibitory concentration (MIC) is commonly used as an index of the efficacy of antimicrobial substances and reflects the minimum concentration of a substance needed to inhibit bacterial growth.

Figure 1. The antimicrobial effects (minimum inhibitory concentration, MIC) of formic, propionic and lactic acids against various bacteria (from Strauss and Hayler, 2001).

Addition of organic acids to feed combats susceptible microorganisms, including pathogenic bacteria and some fungi, which would otherwise cause spoilage and reduce its nutritive value by metabolizing the starch and protein therein.

In pig diets, organic acids and their salts also take effect in the gastrointestinal tract, mainly in the proximal tract - the stomach and small intestine. Firstly, organic acids lower the pH of the stomach contents, which can be especially beneficial at weaning, when the gastric acid secretion capacity of the animal´s stomach is often insufficiently developed. Although pH reduction can inhibit pathogen growth and optimise pepsinactivity, pH alone does not account for the numerous benefits reported when organic acids are included in diets for pigs. Inorganic acids, such as hydrochloric or phosphoric acid (both of which reduce stomach pH), do not improve growth rate or feed conversion of pigs in vivo (Metzler and Mosenthin, 2007; Table 1). Supplementation of diets with organic acids reduces the pH in the stomach, especially in weaning pigs, where it stimulates the conversion of inactive pepsinogen to active pepsin. This may improve protein digestibility and decrease the rate of gastric emptying. Organic acids also stimulate exocrine pancreatic secretion of enzymes and bicarbonate, thus assisting with protein and fat digestion.

Furthermore, organic acid anions can complex with calcium, phosphorus, magnesium and zinc, improving the digestion of these minerals and reducing the excretion of supplemental minerals and nitrogen (Roth et al., 1998a, b). This is particularly useful from the perspective of European pig production systems, which have come under increasing scrutiny from legislators because of their emissions into the environment. The bacteriostatic or bactericidal effects of organic acid anions also take effect in the proximal gastrointestinal tract. Here, the differential inhibition of pathogens compared to beneficial bacteria such as lactobacilli and bifidobacteria improve the microbial load (eubiosis) in the tract, preventing post-weaning diarrhoea. As several studies have shown that the bactericidal effect of organic acids persists in the absence of a significant decrease in pH, they are also useful in combating bacterial pathogens in grower-finisher pigs and sows(e.g., Kirchgessner and Roth, 1987; Kirchgessner et al., 1992).

It should be noted that whereas organic acids lower gastric pH, organic acid salts do not (Eidelsburger et al., 1992a). Therefore, the improvements in growth performance resulting from dietary inclusion of organic acid salts are due to an antimicrobial effect, as demonstrated by Kirchgessner et al. (1992). 

Other studies have demonstrated that adding organic acids to diets for pigs stimulates secretion of pancreatic enzymes - especially butyrates and propionates (Mosenthin, pers. comm.) - and may influence gut morphology and intermediary metabolism via metabolic enzyme activity. Butyric acid for instance, is the main energy source for the epithelial cells of the large intestine and is considered to be effective for promoting epithelial growth (Gálfi and Bokori, 1990).

 

 

 

 

 

Table 1. Effects of organic acids on the apparent total tract digestibility of crude protein and energy and on nitrogen (N) retention in pigs (adapted from Metzler and Mosenthin, 2007).

Achieving best results in pig performance 

Although growth performance benefits have been shown in numerous studies over the past half-century (Cole et al., 1968), the ban on antimicrobial growth promoters in the European Union in 2006 resulted in an increased scientific focus on organic acids. Even before the ban, a double salt of potassium formate and formic acid had generated sufficient data to support its approval as a ‘growth promoter´under Council Directive 70/524/ EEC in 2001 (Øverland, 2001). Achieving this approval required that the growth-promoting effects had been established under a range of practical conditions across Europe.

At weaning, piglets are particularly susceptible to infection with intestinal pathogens, as well as being inadequately equipped physiologically to deal with solid feed. The buffering capacity of weaning feeds is also high, compounding the problem through a negative effect on pepsin activity in the stomach (Eidelsburger et al., 1992b), a problem that is addressed through the acidification of the diet.

In diets for grower-finisher pigs, the antimicrobial effect of organic acids in the feed (hygiene), stomach and small intestine is largely responsible for their performance-enhancing benefits. This has been shown repeatedly in trials under European conditions. Effective doses have been established that can improve productivity of pigs to levels comparable with antibiotic growth promoters (Øverland et al., 2000; Table 2). 

More recent studies have demonstrated benefits of adding organic acids to diets for sows. Øverland et al. (2009) added 0.8% or 1.2% potassium diformate to diets for primiparous and multiparous sows from one mating to the next, feeding the acidifier through gestation and lactation. The performance of the piglets of these sows was also recorded and compared. The authors found that sows fed potassium diformate had increased back fat thickness during gestation, although daily feed intake and body weight gain did not change. Feeding potassium diformate also tended to be associated with a heavier birth weight of piglets, irrespective of dose. It also improved average daily gain, resulting in a greater weaning weight. Sows fed the diets containing potassium diformate tended to have increased milk fat content on day 12 post-farrowing. On the other hand, sows fed potassium diformate at a dosage of 0.8% under tropical conditions (Lückstädt, 2011) tended (P < 0.1) to have a higher feed intake from 3 days after farrowing onwards. Furthermore, reduced weight loss (P = 0.06) during the weaning period and lower back fat loss (P = 0.05) was observed.

Salmonella control has a high priority in European pork production systems. It is a significant cause of human salmonellosis and causes major economic losses in thepork production chain through reduced productivity and increased veterinary and hygiene control costs. Preventing the spread of salmonella to the consumer requires special control measures during slaughter and processing. The extra cost of these controls is increasingly being transferred back to the producer in the form of financial penalties or as a loss of income for contaminated pigs.

Good gut health is increasingly being shown to be effective against intestinal pathogens, a strategy that has only been made possible through the removal of antibiotic growth promoters in feed. Creating and maintaining a healthy intestinal environment has become essential to productivity and food safety programmes alike. 

Salmonella enteritica Typhimurium is the predominant serotype found in pig carcasses in Europe, accounting for about 71% of cases. Several serotypes are resistant to antibiotics, which has put pressure on producers to prevent contamination. While salmonellacannot be wholly eradicated in pig units, it can be controlled to minimise the risk to consumers. Biosecurity plays a significant role in salmonella control. In feed compounding, although heat treatment is effective in reducing contamination of feed leaving the feed mill, this effect does not persist during transport, storage and subsequent outfeeding. When conditions within the feed are less conducive to bacterial infection, salmonella contamination can be reduced. The next critical control point is within the pig´s gut, where conditions for bacterial growth may again be optimal. Salmonella growth requires warmth (35-37 °C is optimal), a moisture content greater than 12% and a pH between 4.5 and 9.0. It is no coincidence that the pig gut can provide salmonella everything they need to thrive.

Table 2. Effect of potassium diformate (KDF) and tylosin phosphate on the performance of piglets (9-21 kg live weight; n = 120) under Danish production conditions (adapted from Danielsen, 1998; Øverland, 2001).

 

 

While biosecurity and hygiene in the feed mill and on farm are essential, organic acids also have benefits for salmonella control. Feed acidification is not only effective within the feed, as reviewed by Stonerock (2007), its biggest benefit may occur within the pig itself. Research trials in the UK, France and Ireland with a 0.6% potassium diformate (KDF) feed additive showed that KDF significantly reduced the salmonella count in the feed as well as in the gut of pigs. This effect is particularly well illustrated by data collected on 12 farms in Ireland (Lynch et al., 2007). Salmonella control has been compulsory under Irish law since 2002 and farm status is categorised by the percentage of positive pigs in a herd according to the Danish mix-ELISA test. Category 3 (>50% positive) farrow-to-finish farms and their associated fattening units were selected for the study. All the farms that were treated with KDF alone or a combination of KDF and improved hygiene and biosecurity measures had notable improvements in both the bacteriological and the serological prevalence of Salmonella spp. All but one farm in which KDF was used ended the trial with a much improved salmonella status and the bacteriological prevalence was also low on most farms. Improved hygiene and biosecurity measures alone also improved salmonella status, but to a much lesser extent than KDF. The reduction in prevalence obtained with KDF alone, compared with the two farms in which KDF and additional hygiene and biosecurity measures were used demonstrates the additive´s efficacy.

A study by Dennis and Blanchard (2004) in the UK and a more recent study by Correge et al. (2010) in France also concluded that potassium diformate is an effective tool in salmonella control strategies on commercial farms, as it reduced the percentage of salmonella-positive pigs by 50% and decreased salmonella ELISA scores in pork meat juice by 46% in grower-finisher pigs. The UK trial also showed an improvement in daily gain of 7.7%, reduced mortality and a reduction in medicinal intervention compared with the rolling average for the unit, which showed an economic benefit to implementing salmonella control measures.

One of the first reports of improved broiler performance when diets were supplemented with single acids was for formic acid (Vogt et al., 1981). Subsequently, Izat et al. (1990a) reported reduced levels of Salmonella spp. in carcass and caecal samples after including calcium formate in broiler diets. Izat et al. (1990b) showed that buffered propionic acid could be used to counteract pathogenic microflora in the intestine of broiler chickens, which resulted in a significant reduction in carcass contamination with Escherichia coli and Salmonella spp. 

The use of pure formic acid in breeder diets reduced the contamination of tray liners and hatchery waste with S. enteritidis (Humphrey and Lanning, 1988). Hinton and Linton (1988) studied salmonella infections in broilers using a mixture of formic and propionic acids. They demonstrated that 6 kg/t (0.6%) of this organic acid blend was effective in preventing intestinal colonization with Salmonella spp. from naturally or artificially contaminated feed. 

Improvements in broiler performance and hygiene in response to organic acids are often reported. However, an important limitation is that organic acids are rapidly metabolised in the foregut (the crop to the gizzard), which will reduce their impact on growth performance. Double salts of organic acids, such as potassium diformate and sodium diformate, which reach the small intestine, have been shown to have a significant impact. Selle et al. (2004) demonstrated the effects of potassium diformate at dosages of 0.3-1.2% until 35 days posthatch on nutrient utilisation (Table 3). Furthermore, diformates reduced numbers of pathogenic bacteria (Salmonella, Campylobacter and Enterobacter) in broiler chickens and increased numbers of Lactobacilli and Bifidobacteria (Lückstädt and Theobald, 2009).

Table 3. Effects of potassium diformate (KDF) on growth performance, apparent metabolizable energy (AME) and N-retention of broilers from hatch until 35 days post-hatch (adapted from Selle et al., 2004).

Mikkelsen et al. (2009) showed that 0.45% potassium diformate reduced mortality caused by necrotic enteritis (Clostridium perfringens). After the necrotic enteritis outbreak (day 35 of the trial period), KDF significantly reduced the number of C. perfringens in the jejunum, in agreement with results showing that formic acid inhibits growth of C. perfringens (Mroz, 2005) in vitro.

The reduction in pathogenic bacteria in broilers, together with the improved gut microflora, which resulted in a state of eubiosis in treated chickens, suggests that diformate will also improve bird performance. A trial conducted in Russia (Lückstädt and Theobald, 2011) recorded feed intake, growth and mortality of birds. An economic analysis of these data based on the European Broiler Index (EBI) showed statistically improved live weights with increasing dosage (Table 4).

The mode of action of acidifiers in poultry is mainly antimicrobial, whereas in pigs, a key activity is reduction of stomach pH (Desai et al., 2007). Other trials have shown improved health status in chickens, as demonstrated by improved gut microflora (lowerEnterobacter numbers and high Lactobacilli and Bifidobacteria counts).

Much of the data available on the use of acidifiers in pig diets involves trials conducted under European conditions. However, within Europe, there are great variations in climatic conditions (Spain to Norway), feed ingredients (wheat, soy, maize) and nutrient composition (protein, carbohydrates, fibre, minerals).

Studies have shown that these variables can have a significant impact on the animal´s response to an acidifier. For example, a piglet trial carried out by Paulicks et al. (2000) demonstrated that calcium can influence the efficacy of potassium diformate. High dietary calcium content (14 g/kg) increased the buffering capacity of the diet and reduced the acidifier´s growth-promoting effect, although the effect on pig performance was still noticeable. A calcium content of 7-8 g/kg is desirable and will optimise the acidifier´s growth-promoting capacity. Eidelsburger et al. (2007) tested the effect of potassium diformate on the performance and health of weaner piglets fed two different protein levels, representing the high protein contents typically fed in the UK compared with elsewhere in Europe. Although the acidifier was found to improve feed intake, weight gain and feed conversion, it achieved this irrespective of protein content.

Callesen (1999) observed a synergistic effect on weight gain and feed conversion when potassium diformate and phytase were used together. When phytase is added to diets, it enables phosphorus and calcium levels to be reduced, decreasing the acidbinding capacity of the feed, which increases the utility of adding organic acids to the diet. In addition, the pH reduction conferred by the acidifier helps establish the pH optimum for phytase.

Partanen and Mroz (1999) evaluated the effects of dietary organic acids on the performance of weaned piglets and fattening pigs using a meta-analysis of data from the literature. Their findings are summarised in Table 5. Organic acids improved all performance parameters in weaned and fattening pigs compared with non-acidified control diets. 

Holoanalysis, which makes use of as much of the literature as possible, is less restricted to the experimental parameters, e.g., housing, feed components and acidifier dose, (Mellor, 2008). The holoanalytical model demonstrated that using acids in pig diets improves the productivity parameters of greatest importance to economic success (Rosen, 2008), as shown in Table 6. Moreover, this effect is dose-dependent.  Even when the input data is confined to one acidifier (potassium diformate), the models also show (Table 7) that its inclusion has beneficial effects on feed intake (+3.52%), live weight gain (+8.67%) and feed conversion ratio (-4.20%) compared with other acids and acid salts (Lückstädt and Mellor, 2010).

Table 4. Performance data of broilers with or without dietary diformate.

 

 

Table 5. Multifactorial analysis of the effect of organic acids in piglets (adapted from Partanen and Mroz, 1999).

 

 

 

 

 

 

 

Organic acids and their salts have become common additives in pig diets, where their modes of action and dosages are clearly characterised. Their antibacterial effect has also attracted interest in their application to poultry diets, for which effective salts and dosages are rapidly being established. The use of acidifiers in pig and poultry diets is growing, especially in Europe, but also in the Austral-Asian region.

Table 6. Nutritional responses to acids, salts and their admixtures (Rosen pers. com.).

 

 

 

 

Table 7. Effects of potassium diformate, calcium formate, citric acid, formic acid and fumaric acid in pig diets relative to negative control performance (responses are expressed as percentages of values for the negative control) (from Lückstädt and Mellor, 2010).

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