The effect of dietary protein level on the apparent digestibility coefficient of two selected feed ingredients for Nile tilapia, Oreochromis niloticus L.

The effect of dietary protein level on the apparent digestibility coefficient (ADC) of meat and bone meal and rapeseed meal for Nile tilapia Oreochromis niloticus L. was examined. Three reference diets were formulated to contain 240 (RD24), 350 (RD35) and 450 (RD45) g kg−1 crude protein, and chromic oxide was added at 10 g kg−1 in the reference diets as an inert indicator. Six test diets were formulated by mixing one of the reference diets with each test ingredient at a ratio of 70:30. Fish (initial body weight 101.0 ± 0.6 g) were fed reference diets for 3 days and then fed test diets in the following 4 days cyclically for 4 weeks. Faeces of fish fed each reference diet or test diet was collected from the third day of each diet conversion. Dietary protein level significantly affected the ADC of protein and energy of reference diets, test diets and test ingredients. The ADC of protein and energy of meat and bone meal and rapeseed meal was the highest when these ingredients were mixed with diet RD35. The scope of variation in the ADC of protein and energy of meat and bone meal was 8% (from 69.9% to 78.3%) and 8% (from 70.2% to 78.0%), and the scope of variation in the ADC of protein and energy of rapeseed meal was 4% (from 79.8% to 83.3%) and 5% (from 56.0% to 60.7%) when the dietary protein level was increased from 300 to 480 g kg−1. This indicates that the ADC of protein and energy of meat and bone meal and rapeseed meal for Nile tilapia varied considerably at different dietary protein levels.