The Impact of Acidifiers in Replacement of Antibiotic Growth Promoters

After 50 years of routine usage, in-feed antibiotic growth promoters (AGPs) have been banned from animal feeds within the EU since the end of 2005. During this period of routine use management systems for pigs and poultry have changed dramatically and there was considerable concern that the in-feed ban would result in poorer performance and increased health problems. Consequently over the past several years much research effort has been devoted to investigating alternative feed additives which might fill the gap left by the withdrawal of AGPs and prevent widespread production problems. One group of feed additives which has demonstrated some potential in this regard is the feed acidifiers, which have been shown to be particularly effective in weaner pig diets.

Feed acidifiers are generally organic acids, organic compounds which have a carboxyl group (-COOH). This dissociates in solution releasing a proton hence giving the compound acidic properties (Voet and Voet, 1995). Although there are an enormous number of organic acids, many of which are found naturally occurring in the body, eg fatty acids, amino acids, nucleic acids, only relatively few of these are used as feed acidifiers. These are characterised by short carbon chain lengths of up to seven and may include more than one carboxyl group (see Table 1). All can occur naturally in the body and as such can readily be metabolised (Voet and Voet, 1995).


Mode of action

REDUCTION OF GASTRIC PH

Stomach acid provides a natural barrier to infection; for example E.coli growth is inhibited below pH4 (Dirkzwager et al., 2005) and enables protein digestion, however the newly weaned pig has limited ability to secrete acid (Cranwell and Moughan, 1989) and hence may benefit from additional acid in the diet to lower stomach pH.


INHIBITION OF BACTERIAL GROWTH

In addition to inhibitory effects due to low pH, organic acids have a direct bactericidal action: undissociated acid can be taken up by the bacteria and then dissociates within the cell causing pH reduction, inhibition of metabolic enzyme systems and cell death (Russell and Diez-Gonzales, 1998).


ENHANCED DIGESTIBILITY

Organic acids appear to enhance diet digestibility by extending stomach retention time, increasing pancreatic enzyme secretion and increasing mineral absorption (Roth et al., 1998).


ENERGY SUBSTRATE FOR ENTEROCYTES

Some organic acids, most notably butyrate, can be directly metabolised by the enterocytes lining the gut and may enhance growth and development of the gastrointestinal tract (Miller and Slade, 2006).



Table 1. Organic acids commonly used in piglet diets


Adapted from Mroz (2003)



INHIBITION OF BACTERIAL GROWTH

In addition to inhibitory effects due to low pH, organic acids have a direct bactericidal action: undissociated acid can be taken up by the bacteria and then dissociates within the cell causing pH reduction, inhibition of metabolic enzyme systems and cell death (Russell and Diez-Gonzales, 1998).


ENHANCED DIGESTIBILITY

Organic acids appear to enhance diet digestibility by extending stomach retention time, increasing pancreatic enzyme secretion and increasing mineral absorption (Roth et al., 1998).


ENERGY SUBSTRATE FOR ENTEROCYTES

Some organic acids, most notably butyrate, can be directly metabolised by the enterocytes lining the gut and may enhance growth and development of the gastrointestinal tract (Miller and Slade, 2006).

Efficacy of organic acids as replacers of AGPs

In 1953, a review of the available literature led Braude et al. to conclude that AGPs increased growth rate by 10 to 15% and feed efficiency by 2 to 5% and a much more recent update led Mroz (2003) to similar conclusions: 4 to 15% and 2 to 6% for improvements in gain and feed efficiency respectively.

Organic acids do generally enhance pig performance with this effect being particularly marked in weaner pigs. In an extensive analysis of published data on formic, fumaric and citric acids, Partanen and Mroz (1999) found that these acids generally enhanced average daily gain of weaned piglets and improved their feed conversion efficiency. However responses were highly variable with effects on daily gain, for example, varying between -58 g/d to +106 g/d compared to the non-acid fed control. They noted that the growth promoting effects of the organic acids were not as great as those of AGPs, as indicated in Table 2 where it can be see that acids have a more variable effect than AGPs with only 30% of trials producing positive results compared to 62% of trials testing AGPs.



Table 2. Results of Danish weaner trials investigating the effects of Antibiotic growth promoters (AGPs) and acids / acid salts





The majority of weaner diets now contain some form of organic acid supplementation plus a variety of other alternative products such as pre and pro biotics.

Organic acids are generally less effective in grower finisher diets than in weaner diets although they can show beneficial effects depending on choice of acid and particular farm circumstance (Mroz, 2003). Some acids have specific bactericidal effects against Salmonella and hence are being increasingly used in Salmonella control (see below).

Organic acids have less predictable performance effects in poultry (Dibner and Buttin, 2002) and are less widely used.


Choice of acid

As a result of their acidic nature organic acids are corrosive; some of them also have a strong and, in certain cases unpleasant, smell. Probably the worst offender is butyric acid which is extremely aromatic and smells of rancid butter. Hence acids in their pure form are unpopular and difficult to handle in the feed mill. In addition the shorter the chain length and the more volatile the product, hence there is continuous loss of acid from processed feed and so the storage life of the product is limited. To counteract these negative effects a variety of alternative forms of acid exist, for example, salts, glycerides and microencapsulated products. These facilitate handling and processing and are generally more stable in processed feed. It has also been suggested that palatability of the resulting diets may be improved when the acid salt is used rather than the free acid. In a choice situation piglets refused diets acidified with pure acid but showed no aversion to the corresponding acid salt (Ettle et al., 2004). However, when no choice is offered piglets will eat an acidified diet normally regardless of whether it contains acid salt or free acid.

The short carbon chain length combined with the acidic conditions in the stomach mean that organic acids are quite rapidly absorbed across the gastrointestinal wall. This in turn means that the effects of supplementation with the free acid will be limited to the upper part of the digestive tract which in weaner pigs may be most appropriate part of the tract for acid action. Acid incorporated into the diet as salts with potassium, sodium or calcium; mono-, di- and triglyceride forms, and encapsulated free acids will each have different dissociation rates compared to the free acid. In addition the site of action of the acid may be moved from the stomach and upper small intestine to further down the small intestine; this is particularly the case for encapsulated forms which release the free acid slowly along the small intestine and into the hind gut (Piva et al, 1997). It is therefore important to understand the mode and site of action of the acid required when choosing which form of which acid to include in diets.


Control of Salmonella

By 2007 all member states of the EU will have to screen their pig herds for Salmonella; Salmonella testing is already in place in Denmark and UK.

Acidification of the feed reduces both Salmonella contamination of the feed and Salmonella infection levels in the pig. Formic acid has the most potent antibacterial action against Salmonella species. Widespread commercial trials have shown significant reductions in ZAP scores when formic acid or its dipotassium salt are included in grower finisher rations (see Figure 1).

Organic acids do not appear to be as effective against Salmonella in poultry (Waldroup et al, 1995; Heres et al , 2004).





Figure 1. Effect of introduction of dietary di-potassium formate on the prevalence of Salmonella in Salmonella positive herds



Effect of other dietary components

Whether or not an acid is effective depends to some extent on the characteristics of the diet and other dietary components. In a trial at the University of Leeds addition of increasing levels of sodium butyrate (0, 1.5, 3, 6 g/kg) to weaner diets produced a positive dose response in terms of growth rate when the diets also contained 3,100 ppm zinc oxide but a negative dose response after the 1.5 g/kg level in the absence of zinc oxide (Miller and Slade, 2006). High levels of lactose in the diet may be fermented in the stomach hence generating lactic acid and negating the requirement for additional dietary supplementation of organic acids.


Conclusion

Dietary inclusion of organic acids does generally improve performance of weaner pigs, and may also improve performance of grower finisher pigs and poultry although results are more variable in these animals.

Performance improvements are generally smaller than for AGPs.

Care must be taken in choice of product to use depending on what problem is being addressed.

Organic acids, notably formic acid, included in the diets of grower finishers are effective in lowering Salmonella scores.



References