The use of dietary acidifiers in tilapia nutrition and feeding

Routine use of antibiotics as growth promoters is a matter of debate in the animal farming industry. The EU has banned all antibiotic growth promoters (AGP) from livestock production with effect of January 2006. Public opinion and regulation authorities in most of the export countries focusing now on the misuse of antibiotics in aquaculture and public attention have shifted towards production methods. Therefore, alternatives to AGP are worldwide sought in a variety of forms. Acidifiers consisting of organic acids and their salts present a promising alternative and they have received much attention as a potential replacement in order to improve the performance and the health of treated animals.

Acid preservation of fish and fish viscera to produce fish silage has been a common practice and its final product has been widely used in fish feeds with reported beneficial effects (Gildbert and Raa 1977; Åsgård and Austreng 1981).

The beneficial effects of acid preserved products caught the attention of the scientific community to investigate the effects of these short-chain acids onto the fish feed directly. Several studies have been conducted with different tropical species, especially tilapia.

Ramli et al. (2005) tested potassium diformate (potassium salt of formic acid) as a growth promoter in tilapia grow-out in Indonesia (Table 1). In this study, fish were fed over a period of 85 days 6 times a day diets containing different concentrations of potassium diformate (0%, 0.2%, 0.3% and 0.5%). The diets contained 32% crude protein, 25% carbohydrates, 6% lipids and 10% fibre. The fish were challenged orally starting day 10 of the culture period with Vibrio anguillarum at 10⁵ CFU per day over a period of 20 days.

Over the entire feeding period from day 1 to 85, potassium diformate significantly increased feed intake (P<0.01) and weight gain (P<0.01) as well as improved the feed conversion ratio significantly (P<0.01). Furthermore, protein efficiency ratio was also significantly improved due to the addition of the formic acid salt (P<0.05). The improvement was best with 0.2% and 0.5% addition of the formate. Survival rates of fish after the challenge with V. anguillarum on day 10 were also significantly higher compared to the negative control and the effect was dose dependent (P<0.01).

Table 1: Effects of potassium diformate supplementation in diets on performance of tilapia challenged with V. anguillarum (modified from Ramli et al. 2005)

^abcwithin rows, means without common superscripts are significantly different (p<0.05)

The authors concluded that the application of potassium diformate at 0.2% is an efficient tool to control bacterial infections in tropical tilapia culture.

Similar results were achieved by Zhou et al. (2008), who tested hybrid tilapia (Oreochromis niloticus x Oreochromis aureus) fingerlings (2.7 g initial weight) in a dose response study with potassium diformate (0%, 0.3%, 0.6%, 0.9% and 1.2%), while also comparing the results with an antibiotic growth promoter (8 mg / kg Flavomycin). During the 56 day trial period, tilapia fed all the potassium diformate enriched diets grew faster than the negative control (an increase of up to 11.6%), while fish fed 0.3% and 0.6% potassium diformate achieved even better weight gain than the fish in the positive control group. The authors speculated that dietary potassium diformate could stimulate a beneficial bacterial colonization of the intestine.

Another study on tilapia (Oreochromis niloticus) searched on stimulating the feeding behaviour of the fish with different organic acids (Xie et al. 2003), as this is sometimes reported with different organic acids or their salts in pigs (Paulicks et al. 1996). The result showed that citric acid at a concentration of 10^-2 to 10^-6 M and lactic acid at 10^-2 to 10^-5 M stimulated feeding, as automatically recorded via the frequency of feeding "bites" of tilapia. On the other hand, fish tended to avoid acetic acid at 10^-3 M. The inclusion of acetic acid at 10^-5 M had no significant effects on fish feeding.

A more recent trial (Petkam et al. 2008) determined the effects of an acid blend, containing of calcium formate, calcium propionate, calcium lactate as well as calcium phosphate and citric acid at different levels (0.5%, 1.0% and 1.5%) on the growth performance of tilapia (Oreochromis niloticus). Fish were fed to satiation two times a day during an 8 week period, using a pelletized diet containing 31% crude protein. Despite a lack of statistical significant data on growth and FCR, the inclusion of the acidifier at 1.5% of the diet resulted in a numerical 11% increase in body weight when compared to the negative control and achieved similar results as the AGP-supplemented diet (0.5% Oxytetracycline) - (Table 2).

Organic acid salt may be therefore especially during the grow-out period of high importance for tilapia culture (Lückstädt 2008).

Though there are only a limited number of published studies on the use of acidifiers for growth promotion, feed efficiency as well as mineral absorption and disease prevention in tilapia aquaculture, results from those studies indicate promising potential and compel aqua feed manufacturers to consider using acidifiers in their diets. The use of acidifiers can be therefore an efficient tool to achieve sustainable, economical, and safe tilapia production (Lückstädt 2007).