Enzyme pretreatment of fibrous ingredients for carnivorous fish: Effects on nutrient utilisation and technical feed quality in rainbow trout (Oncurhynchus mykiss)

Carnivorous fish have limited ability to utilise fibrous structures as a source of energy. Treatment with exogenous enzymes targeted at degradation of non starch polysaccharides (NSP) could help to improve the nutritive value of these components. Two experiments were conducted. In the first in vitro experiment, three defatted oilseeds; soybean meal (SBM), rapeseed meal (RSM) and sunflower cake (SFC) as well as green field peas were incubated individually at semi-moist conditions (45% water, 45 min, 45 °C) with or without NSP degrading enzymes (RONOZYME® VP). The amount of total NSP was significantly (P < 0.05) reduced after enzyme treatment of SBM and SFC, but no effect was seen for RSM or field peas. A prolonged wet incubation (85% water, 180 min) with SBM and RSM at three different temperatures (30, 45, 60 °C) was also conducted with RONOZYME® VP. An overall significant (P < 0.05) effect on degradation of total NSP was seen for SBM at wet conditions, but there was no significant difference between the three temperatures, and increased incubation time or moisture level did not improve the degree of degradation any more than the semi-moist treatments. In the following fish experiment three diets were made with combinations of SBM, RSM and SFC: Diet A (SBM + RSM), diet B (SBM + SFC) and diet C (RSM + SFC). In addition to a conventional feed production, the plant ingredients in the three diets were pretreated (45 min, 45% moisture, 45 °C) in the presence of RONOZYME® VP, and mixed with the remaining dry ingredients prior to conditioning and extrusion. Diet A was also pretreated without enzyme as a sham treatment. The temperature in the extruder barrel decreased in the feeds that had been pretreated, but there was no negative effect on the pellet durability index (PDI) or water stability index (WSI). Each of the seven dietary treatments was fed to triplicate groups of rainbow trout raised in seawater tanks (average initial weight, 205 g). The fish body weight was more than doubled during the experimental period. The enzyme pretreatment did not improve the nutrient digestibilities, and the overall feed conversion ratio (FCR) increased significantly (P < 0.05) from 1.04 to 1.10. We conclude that treatment with NSP degrading enzymes has significant effects on reductions in total NSP in vitro, but the released substrates did not contribute to an improvement in fish performance under the current experimental conditions.