Effect of feeding level on the digestibility of alternative protein-rich ingredients for African catfish (Clarias gariepinus)

Apparent digestibility coefficients (ADC) of nutrients and individual amino acids (AA) for 13 feed ingredients as affected by feeding level were determined for African catfish, a species of economic importance in Africa. Results from two trials are reported. In each trial, ADC were determined using a reference diet and test diets with yttrium oxide as indicator. Juvenile African catfish (averaging 53.9 g, trial 1; 40.4 g, trial 2) were stocked in tanks connected to a common recirculation aquaculture system. Ingredients tested included hydrolysed feather meal (HFM), fishmeal (FM), insect meal (IM), soybean meal (SBM), sunflower meal (SFM), poultry meal (PM), corn dried distillers grains with solubles (CDDGS), faba beans (FB), lupine meal (LM), pea protein (PP), guar meal (GM), canola meal (CM) and yeast meal (YM). The effect of feeding level on ADC was determined by feeding fish restrictively (80% satiation) for 5 weeks and subsequently to apparent satiation for 2 weeks. Inclusion of yeast meal at 30% resulted in low palatability. ADC of nutrients were significantly affected by feeding level (except for fat and carbohydrate), but the effect was ingredient-dependent. African catfish was able to digest protein very effectively in almost all tested ingredients with ADC values ranging from 85.6 to 105.1% across feeding periods. Several ingredients tested, including animal protein ingredients and YM had similar high ADC for dry matter as FM. However, the ADC of AA differ among ingredients, indicating a need for digestible amino acid profile data. Methionine (Met) was the first limiting essential amino acid in HFM, FB, and LM with values ranging from 5−6 g/kg, expressed as digestible Met (dMet) per unit of digestible protein (DP), compared to FM (27 g dMet/kg DP). IM had comparable and sometimes higher overall digestible essential AA values compared to FM, except for methionine and lysine. For oilseeds and legumes, SBM tended to be the best quality AA source, as it had the highest digestible essential amino acid profile. These data provide information concerning nutrient and digestible AA values, which will allow a more efficient use of alternative ingredients in African catfish diets. Formulating diets based on the digestible AA in ingredients will aid precise feed formulation, thereby minimising economic losses and reducing the environmental footprint of aquaculture production.