Comparison of water and feed administration of formic, acetic, and citric acids: effects on performance, gut health, and nutrient digestibility in growing rabbits.

Growing rabbits have an immature gastrointestinal system that increases their susceptibility to digestive disorders. Organic acids (OA) may support gastrointestinal health through their acidifying and antimicrobial effects, but their efficacy depends on acid type and administration route. Therefore, this study evaluated the effects of supplementing formic (FOR), acetic (ACET), or citric (CIT) acids administered via feed (F) or drinking water (W) on growth performance, gastrointestinal health, and nutrient digestibility in growing rabbits. Seven treatments were tested: a control (CON) and three OA administered via feed or water (F-FOR, F-ACET, F-CIT, W-FOR, W-ACET, and W-CIT). A total of 1 624 rabbits, both male and female, were studied from 28 to 56 days of age. OA in drinking water were adjusted to pH 4, and feed inclusion levels were set to achieve a similar intake. Growth performance, mortality, gastrointestinal pH, pepsin activity, caecal fermentation, and ileal nutrient digestibility were evaluated. As regards OA type, rabbits receiving W-FOR showed a higher final BW (+60 g; P < 0.05) and increased average daily feed intake (ADFI) and average daily gain (ADG) (+4.0 and +1.8 g/day, respectively; P < 0.05), but also increased mortality (+7.3 percentage points; P < 0.05) compared to CON. This observation may reflect a survivor effect, whereby more robust animals remain, and the higher ADFI is often associated with increased mortality in rabbits. Rabbits fed W-CIT showed higher ADG and lower feed conversion ratio (FCR) than CON (+1.5 g/day and -0.1, respectively; P < 0.05), while F-FOR and W-ACET also reduced FCR (average -0.1; P < 0.05) without affecting mortality. OA administered via water increased final BW (+50 g; P < 0.01), global ADG (+1.7 g/d; P < 0.001) and ADFI (+2.9 g/d; P < 0.01), and improved overall FCR (-0.07; P < 0.05) compared with feed administration. Mortality was also higher with water than with feed administration (P < 0.01). F-ACET and F-CIT increased ileal CP digestibility (average +6.6 percentage points; P < 0.05), whereas water supplementation reduced CP digestibility (-6.1 percentage points; P < 0.001) and increased ileal DM and CP flows (+17.1 g DM/d, P < 0.01; +1.46 g CP/d, P < 0.05). OA supplementation altered caecal fermentation, increasing acetic acid and reducing propionic, butyric, and n-valeric acid proportions, and decreased ammoniacal nitrogen concentration compared to CON (P < 0.05). The effects of OA depended on both acid type and administration route. According to our results, dietary supplementation with formic or citric acid can be recommended under practical rabbit production conditions to improve digestive efficiency and gastrointestinal environment without compromising productive performance and mortality.