Meta-Analysis of Published Trial Data on the Feed Efficiency Response of Pigs to Feed Acidification with Formic Acid
Feed acidification with formic acid has become a popular alternative to sub-therapeutic antibiotics in parts of the world where their use is prohibited or tightly regulated, such as the EU. The goal of feed acidification is to reduce the pH and buffering capacity of the feed and thus promote better feed hygiene and more efficient feed utilization by the animal. Formic acid, an FDA approved feed acidifier, has been consistently shown to be one of the most potent organic acids for killing potential feed microbial contaminants such as Salmonella, E. coli, Clostridia, and Campylobacter (Strauss and Hayler, 2001; Navarro et al, 2015). In 2006, Tung and Pettigrew conducted a review of acidifiers for the National Pork Board. The review included data on a range of acidifiers, with formic acid being one of the more abundant. Unfortunately, the authors declined to address the feed efficiency in their review. Therefore, the present meta-analysis was conducted in order to build on their work. One additional trial was found and included in this meta-analysis that was published after the previous review was complete.
Since Tung and Pettigrew undertook a comprehensive review of the data, we decided to use their work as a building block. We started by acquiring copies of all of the studies referenced in their review that contained formic acid, and either reported or enabled the calculation of feed efficiency. This resulted in a list of 5 publications (Eckel et al., 1992; Tsiloyiannis et al., 2001; Manzanilla et al., 2004; Jongbloed et al., 2000; Partanen et al., 2002). Next, we collected all of the studies we could find that were published after the original review that met the same criteria set out in the earlier review. This resulted in 1 additional study (Kil et al., 2006).
After securing all of the studies, the next step was to generate a table containing the relevant information describing the results of each trial. Information such as the Reference from which the data was collected, the Experimental Period, Formic Acid concentration, number of Replicate Pens per treatment, and feed efficiency. Studies were inconsistent in their use of feed-to-gain or gain-to-feed to measure feed efficiency. The resulting studies also covered a range of animal ages and body weight ranges, which makes direct utilization of G:F data difficult since feed efficiency decreases as the pigs age (NRC, 2012). For example, the G:F of 6 kg nursery pigs on the negative control from Eckel et al (1992) had a G:F of 0.862, whereas the pigs on the negative control from Partanen et al. (2002) had an initial body weight of 29 kg and a G:F of 0.515. Therefore, we decided to calculate the improvement in gain-to-feed (ΔG:F) relative to the control for each study and use that as the basis for our meta-analysis.
Finally, the resulting data summary was analyzed using a mixed model with the goal of generating an equation capable of predicting ΔG:F. Formic Acid concentration was treated as a fixed effect, Experimental Period nested within Reference was treated as a random effect, and the analysis was weighted by the number of Replicate Pens per treatment within the study.
Figure 1. Change in G:F of pigs by percent formic acid inclusion in the feed. Solid line represent the predicted response based on 6 published trials, the dotted lines represent the 95% confidence limits, and the individual dot (•) represent the observations used to generate the prediction equation and confidence limits.
Results and Discussion
The 6 studies included in this review represent the performance responses of over 400 pigs. Four of the 6 studies look exclusively at the performance of nursery pigs with initial body weights between 6 and 10 kg (Eckel et al., 1992; Tsiloyiannis et al., 2001; Manzanilla et al., 2004; and Kil et al., 2006). Two of the 6 studies both started with grower pigs (Jongbloed et al., 2000; Partanen et al., 2002), with 1(Partanen et al., 2002) continuing into the finishing period. Two of the six studies, representing a third of all animals, utilized what would be considered conventional corn and soybean meal based diets (Kil et al., 2006; Tsiloyiannis et al. 2001), with the rest utilizing more stereotypical European diets (wheat, barley, oats, etc.).
Table 1. Statistical model for the prediction of feed efficiency improvement from acidification of swine feed with formic acid.
Summary of Fit
Root Mean Square Error
Mean of Response
Observations (or Sum Wgts)
Indicator Function Parameterization
Acid Level, %
The data was well described by the model with an R-square of 0.7 (Table 1). The Y intercept is essentially zero, with an increase in G:F of approximately 4%-points per 1% inclusion of formic acid (P < 0.0001). This is illustrated in Figure 1, which shows the resulting prediction equation along with the 95% confidence bands for the effect of formic acid on feed efficiency, and the means used to generate the model. At 0.6% formic acid – half of the maximum inclusion level approved by the FDA (21 CFR §573.480(b)), and the minimum recommended dose in nursery pigs – the feed efficiency of pigs is predicted to improve by 2.2 %-points (95% CI of 1.3 to 3.2). At 1.2% – the maximum inclusion level approved by the FDA – feed efficiency is predicted to increase by 4.7 %-points (95% CI of 3.2 to 6.1). Jendza et al. (2016) reported a 3 %-point improvement in feed efficiency in nursery pigs fed 0.9% of a sodium buffered form of formic acid. That study was excluded due the use of a buffered as opposed to pure formic acid, but results are close to the 3.5 %-point (95% CI of 2.3 to 4.6) improvement predicted by this model for pure formic acid.
Feed acidification with formic acid offers the potential benefit of improved feed efficiency. This meta-analysis puts the feed efficiency benefit at 4%-points per 1% inclusion of formic acid, and is robust in terms of both stage of performance and diet type.
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