DETERMINING THE NUTRIENT AVAILABILITY OF COMMERCIAL TILAPIA FUNCTIONAL FEEDS UNDER FARM AND LABORATORY CONDITIONS USING ACID-INSOLUBLE ASH AS A NATURAL INERT MARKER
The rapid expansion of tilapia production is due in large part to the intensification of culture systems made possible with the use of commercial pelleted feeds. Functional feeds are feeds with additional function, usually with health-promoting or disease preventing properties beyond their nutritional value. These feeds are now considered the new generation of aquafeeds. Our research into the use of organic acids as a functional feed additive in tilapia feeds as an alternative to the use of harmful antibiotics will be presented. Apart from their potential role in enhancing resistance to tilapia diseases, dietary organic acids are also known to improve nutrient digestibility of aquafeeds. The present study attempted to determine the impact of dietary organic acids on nutrient availability under commercial farm conditions and compared the results with that obtained under controlled laboratory conditions using dietary acid-insoluble ash as a natural inert marker.
Four 20-meter diameter plastic circular floating cages (2 replicates per treatment) in a tilapia farm were used in the study. All male Nile tilapia fingerlings (~25 g) were stocked at a density of 60,000 fish per cage, giving a stocking density of 16 fish/ m3. The tilapia feeds, containing no added organic acids or 2% of a prototype organic acids blend were produced and purchased from Cargill Sdn. Bhd., Malaysia. With the exception of the added organic acids, both diets were similarly formulated. Feeds were mechanically dispersed to the respective cages two times daily. Two to four weeks before the final harvest, 85 large fish were randomly sampled using a dip net from each cage. All fish were dissected and fecal samples were extracted from the terminal segment of the tilapia gut and were considered to contain almost fully digested feeds just before expulsion as fecal matter.
In the controlled laboratory trial, eight groups of 15 tilapia fingerlings (~ 16 g) were randomly allocated into each 95-L glass aquarium. Each commercial tilapia feed was fed to four replicates for seven weeks twice daily. After four weeks into the feeding trial, feces were collected once daily from each aquarium and pooled. Only intact fecal strands were carefully siphoned into a fine mesh net and collected.
The collected fecal samples were then analyzed for ash, protein, lipid, phosphorus and acid-insoluble ash (AIA) content to determine the apparent digestibility of nutrients.
Results indicated that AIA can be used to estimate nutrient digestibility of commercial feeds under commercial farm conditions. Dietary organic acids were shown to significantly enhance (P<0.05) dry matter, ash, phosphorus and protein digestibility of feeds in the laboratory trial but significant differences between the two commercial feeds were not detected from the farm trial despite showing similar trends for dry matter and protein digestibility values. Challenges faced in determining nutrient digestibility of commercial feeds under commercial farm conditions will be discussed.