Asymptotic estimated digestibility, a new indicator of black soldier fly (Hermetia illucens) conversion efficiency in relation to larval density

Abstract

Black soldier fly larvae (BSFL) are highly efficient at converting organic substrates into insect biomass suitable for livestock nutrition. BSFL conversion efficiency depends on the nature of their feed, and formulation approaches use macronutrient contents to reach a targeted performance. However, digestibility varies between ingredients and BSFL digestibility data is lacking due to methods unsuitable for insects living in their feed. This study proposes to work under a range of larval densities to reach total ingestion of the feed, and defines Estimated Digestibility (ED) as the difference between distributed feed and frass macronutrient weight, divided by macronutrient weight in distributed feed. Using the same amount of chicken feed, same feeding time and 0 to 29 larvae/cm2, ED of dry mass (DM), starch, nitrogen, ether extract (EE), neutral detergent fibre, acid detergent fibre, acid detergent lignin, ash and energy were evaluated, along with survival rate, feed conversion ratio (FCR) and larval individual mass gain on a fresh and dry basis. ED of DM, starch, nitrogen, EE and energy increased with larval density following an asymptotic trend. ED of ash increased between 0 and 11.4 larvae/cm2 but decreased for higher densities. Using a logistic model, asymptotic ED of DM (80.3±1.3%), starch (99.0±2.3%), nitrogen (78.6±1.1%), EE (95.3±1.5%), ash (58.4±1.0%) and energy (80.6±1.2%) were determined. No effect of areal density on ED of fibre fractions was detected. Survival rate and individual mass gain peaked at densities below 5 larvae/cm2, FCR (dry to fresh) was improved with increasing density, while FCR (dry to dry) was optimised at 11.4 larvae/cm2. Asymptotic ED is a new indicator corresponding to the maximal fraction of macronutrients potentially digestible by BSFL. It should be used to develop a diet formulation approach based on digestible rather than crude macronutrient contents. Further studies should focus on evaluating asymptotic ED in different ingredients.