Effects of culturing rapeseed meal, soybean meal, macrophyte meal, and algal meal with three species of white-rot fungi on their in vitro and in vivo digestibilities evaluated using rainbow trout

Abstract

The present study was conducted in order to improve the nutritive value of plant feed ingredients. Rapeseed meal (RM), soybean meal (SM), algal meal (AM; made from Gelidiaceae algae), and macrophyte meal (MM; made from Egeria densa) were cultured (fermented) with three species of white-rot fungi — Trametes coccinea (+T), Lentinula edodes (+L), and Pleurotus sajor-caju (+P) — for 6weeks at 38°C (+T) or 28°C (+L, +P). The controls were not cultured. The neutral detergent fiber (NDF) content of RM decreased significantly in RM+T (25%) and RM+L (26%) compared with RM control (35%). NDF content of SM decreased significantly in SM+T (5.0%) and SM+L (5.2%) compared with SM control (8.1%). NDF content of AM decreased significantly in AM+T (20%), AM+L (59%), and AM+P (33%) compared with AM control (62%). However, NDF content of MM did not decrease by fungal culture compared with MM control (30%). In vitro degradability of NDF was high in RM+T (49%), RM+L (41%), SM+T (50%), and AM+T (78%) compared with respective controls (0%). Yield rates of the fermented ingredients ranged from 57% (RM+L) to 84% (AM+L). Crude protein (CP) content of RM tended to increase in RM+T (51%) and RM+L (56%) compared with RM (46%). CP content of SM increased significantly in SM+T (62%) compared with SM (51%). CP content of AM tended to increase in AM+T (40%) compared with AM (27%). Among these fermented materials, four with low NDF and high CP content were selected (i.e., RM+T, RM+L, SM+T, and AM+T). These, together with respective controls, were evaluated for their in vivo digestibility using rainbow trout. Digestibilities of CP, organic matter, and dry matter tended to be lower in fungal cultured than control samples. However, the amount of CP absorbed was higher with cultured ingredients than the controls due to the increased CP content by fermentation. Digestibility of phosphorus (P) in RM increased significantly in RM+T (100%) and RM+L (86%) compared with RM (52%). Digestibility of P in SM was 100% for SM+T and SM control. Digestibility of P in AM tended to increase in AM+T (100%) compared with AM (89%). Digestibilities (absorption) of Mg, Zn, and Cu in RM tended to increase by fungal fermentation due presumably to the decrease in phytate content. These results collectively suggest that fungal fermentation of plant feed ingredients is a cost-effective approach to increasing the nutritive value and decreasing the environmental burden of fish feeds.