Digestibility of common feed ingredients by juvenile mulloway Argyrosomus japonicus

Abstract

Three experiments were done to determine the apparent digestibility of common feed ingredients by mulloway Argyrosomus japonicus. Apparent digestibility coefficients (ADCs) of ingredients were determined using indirect techniques employing chromium oxide as the marker and collecting faecal material by passive settlement. Ingredients tested included two sources of fishmeal (Ecuador or Peru origin), standard and high grade rendered animal by-product meals (poultry meal, hydrolysed feather meal, meat meal, blood meal and three high-protein, low-ash meat meals of ovine or bovine origin), legumes (dehulled lupins and whole field peas), a dehulled solvent extracted soybean meal and sorghum meal. The majority of these ingredients were examined at the 50% inclusion level with the exception of blood meal which was examined at 30% inclusion. The digestibilities of two other cereal products (extruded wheat and 100% pregelatinised unmodified wheat starch) were tested at 20 or 30% and 10, 20 or 30% inclusion rates, respectively, to study the effect of ingredient level on ADC's of typical carbohydrate sources. In all experiments, faecal material was collected from n=3 replicate tanks. Digestibilities of the different fishmeal sources were high, regardless of origin (protein and energy ADC≈97%; fat ADC≈99%). Digestibilities of high grade ovine and bovine meals were also high (protein ADC≈94%; energy ADC≈94%), although the fat ADC of these products was slightly lower (≈80%). The digestibilities of the other rendered animal meals were more variable, with protein ADCs of blood meal, meat meal, poultry meal and feather meal being 90, 84, 76 and 57%, respectively. Energy ADCs for these products were also variable ranging from 61 to 84%, with the lowest value being that for feather meal. The protein from dehulled lupins, whole field peas, sorghum meal and soybean meal was generally well digested, being equal to or greater than 90%. However, the energy ADCs for these products were poor, being lowest in sorghum meal (≈24%) and whole peas (≈40%) and somewhat better in dehulled lupin meal (≈67%) and dehulled solvent extracted soybean meal (≈75). Linear regression analysis indicated that there was a significant negative relationship between the inclusion level of pregelatinised wheat starch (i.e. 10, 20 or 30%) and organic matter or gross energy ADCs (organic matter ADC%=111.22−1.64×%inclusion level, R2=86%; gross energy ADC%=110.42−1.83×%inclusion level, R2=72%). We found no significant difference between organic matter (≈45%), protein (≈88%) or energy (≈45%) ADCs of extruded wheat with respect to inclusion level (i.e. 20 or 30%), however all ADCs were numerically lower at the higher inclusion level. The results from these experiments indicate that mulloway are efficient at digesting protein and energy from fishmeals and good quality rendered animal meals. They appear to be reasonably efficient at digesting protein from plant products commonly used in the manufacture of aqua-feeds, however they are less efficient at extracting the digestible energy from plant products, especially from whole grains. Significant reductions in the organic matter and energy ADCs of pregelatinised wheat starch as a result of inclusion rate indicate that ADCs for carbohydrates such as these may not be additive. The data generated in this study will be useful for formulating research and commercial feeds for mulloway on a digestible protein and energy basis.