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Introduction
Before getting into the subject, it is important to point out that the effect of organic acids (OAs) on pigs' gastrointestinal health, digestibility, and production performance depends on several external factors such as the type of diet, the concentration and combination of OAs used, and the pigs' age and health status. These factors must be considered when comparing results from different scientific studies and applying the knowledge on a practical level.
For example, the buffering capacity of the diet impacts the action of organic acids, with a greater effect in lower buffering capacity diets (Blank et al., 1999; Wang et al., 2023). On the other hand, depending on the sampling time used at the gastrointestinal level to evaluate the acidifying capacity of an acid, the results obtained may vary (Canibe et al., 2005). Therefore, to apply scientific knowledge in practice, it is important to have a detailed understanding of the study's methodology and our farm's conditions.
Formic acid
Formic acid (EFSA Registration Number 1k236) is a short chain fatty acid (SCFA) with the chemical formula CH2O2, authorized by EFSA as a technological additive for use in animal feed as a preservative with a maximum inclusion of 10 g/kg in feed with 12% moisture content for pigs.
It is a clear liquid acid, is soluble in water and alcohol, and has a low molecular weight of 46.03 g/mol, a pKa of 3.75, and a logP Kow of -0.54. Thanks to its low pKa value, the acid has a high acidifying capacity at the gastric level, although it is complicated to handle due to its corrosiveness. To reduce corrosiveness, combining formic acid with salts to form formic acid salts such as ammonium formate is a widely used practice. Bear in mind that doing so modifies its mechanism of action at the gastrointestinal level.
Several studies (Table 1, adapted from Luise et al., 2020) have observed improved production performance in the nursery and finishing phases by including formic acid in the feed.
In the nursery, especially in the days after weaning, the use of formic acid is interesting due to its strong acidifying power at the gastric level to facilitate protein digestion and reduce the proliferation of pathogenic bacteria at the gastric level (Tung and Pettigrew, 2006; M. Garcia, 2011). It has been shown that a 0.9% inclusion of a mixture of 75% formic acid and 25% propionic acid in the feed of post-weaning piglets experimentally infected with Salmonella and E. coli reduced the concentration of these pathogens at the gastric level (Taube et al., 2009). At the microbiota level, Maribo et al. (2000) observed that including 0.7% and 1.4% formic acid reduced the concentration of lactobacilli in the small intestine and caecum and coliforms in the stomach in piglets post-weaning. Similarly, Gedek et al. (1992) observed a reduction of lactobacilli in the small and large intestine and a reduction of coliforms in the large intestine independently of the formic acid dose (1.2% to 2.4%), while the level of coliforms in the small intestine was dose-dependent. On the other hand, studies such as that by Gerritsen et al. (2010) did not observe any improvement in production performance but did observe an increase in the in vivo apparent digestibility of crude fiber in post-weaning piglets supplemented with 5 kg/t feed of a mixture of formic acid, propionic acid, lactic acid, citric acid, and sorbic acid.
In finishing, the use of formic acid combined with lactic acid at 0.8% or 1.2% inclusion (50:50 lactic acid:formic acid) has proven to be a good strategy to reduce the prevalence of Salmonella in the last 14 weeks of finishing (Creus et al. 2007).
Table 1. Summary of the effects of formic acid use on production outcomes during nursery and finishing of pigs (table adapted from Luise et al., 2020).
| Formic acid | ||||
|---|---|---|---|---|
| Inclusion, % | Production phase | Average daily gain | Feed conversion ratio | Reference |
| 0.5 | Nursery | = | = | Manzanilla et al. (2004) |
| 0.2 | Nursery | = | = | Kil et al. (2006) |
| 0.6 | Nursery | ↑1 | ↓ | Luise et al. (2017) |
| 0.14 | Nursery | ↑1 | ↓ | Luise et al. (2017) |
| 0.8 | Finishing (growers) | ↑ | Siljander-Rasi et al. (1998) | |
| 0.8 | Finishing (finishers) | = | Siljander-Rasi et al. (1998) | |
| 1.0 | Finishing | ↑ | Øverland et al. (2000) | |
| 1.8 | Finishing | ↑ | Canibe et al. (2005) | |
| 1.0 | Finishing (growers) | = | ↑ | Øverland et al. (2007) |
| 0.8 | Finishing | ↑ | ↑ | Partanen et al. (2002) |
| 1.0 | Finishing (growers) | ↑ | ↑ | Øverland et al. (2008) |
| 1.0 | Finishing (finishers) | = | ↑ | Øverland et al. (2008) |
1 Results after three weeks of administration; no significant differences after six weeks.
Lactic acid
Lactic acid (EFSA Registration Number 1a270) is a SCFA with the chemical formula C3H6O3 authorized by EFSA as a technological additive for use in animal feed as a preservative with a maximum inclusion of 50 g/kg in feed with 12% moisture content for pigs.
It is a yellowish transparent viscous liquid OA, is soluble in water and ethanol, and has a molecular weight of 90.08 g/mol, a pKa of 3.75, and a logP Kow of -0.70. Thanks to its low pKa value, the OA has a strong acidifying capacity at the gastric level in young piglets, making it an acid of choice to reduce the incidence of digestive pathology after weaning, although its effect on promoting production has contradictory results.
In the 1990s, it was published that lactic acid can decrease gastric pH and the proliferation of enterotoxigenic Escherichia coli in piglets (Thompson and Lawrence, 1981; Partanen and Mroz, 1999). A few years later, Tsiloyiannis et al. (2001) observed that including 1.6% lactic acid in the post-weaning feed decreased the percentage of animals with diarrhea, improved production results, and reduced coliform bacterial counts. More recent studies have observed that the inclusion in feed of 3.97 g/kg lactic acid together with 4.16 g/kg fumaric acid in feed in piglets from 7 to 18 kg BW positively modulates the gastrointestinal microbiota by increasing the Bacteroides-Porphyromonas-Prevotella count in the ileum and reducing the concentration of Streptococcus spp in the colon but with no effect on production results (Zentek et al., 2013).
Propionic acid
Propionic acid (1k280) is a SCFA with the chemical formula C3H6O2, authorized by EFSA as a technological additive for use in animal feed as a preservative with a maximum inclusion of 30 g/kg in feed with 12% moisture content in pigs.
It is a clear oily liquid acid, miscible in water and soluble in alcohol, and has a molecular weight of 74.08 g/mol, a relatively high pKa value of 4.88, and a logP Kow of 0.33.
Propionic acid is an effective OA in inhibiting the growth of fungi and bacteria, making it a particularly useful OA as a feed preservative, although it is complicated to handle due to its corrosiveness.
A recent study observed that preserving grain with 4 kg propionic acid/t after harvest positively impacted grain quality in terms of lower mycotoxin concentration, and the performance of 0-35 day post-weaning piglets with increased growth and intake. While no effect on performance was observed, a reduction of pro-inflammatory cytokine expression in the small intestine at day 10 post-weaning was observed when the 4 kg/t propionic acid was dosed in the finished feed. The authors conclude that the improvements in production results obtained may be a consequence of improved grain quality, increased intake, and beneficial impact on the intestinal microbiota (Connolly et al., 2024).
To prepare this data sheet, studies have been selected to independently evaluate the actions of formic acid, lactic acid, and propionic acid on pigs' gastrointestinal health and production parameters. However, in practice, their inclusion is mostly done in combination with other OAs, generating a synergistic or additive effect that enhances their benefits.