Introduction
Soybean meal (SBM) is a predominant plant-based protein in fish feed due to its suitable protein content (~48%) and amino acid profile (Lekva et al., 2010). The aim of this study, therefore, is to evaluate the effects of replacing SBM with CAP on the growth performance of rainbow trout while accounting for the dilution effects on nutrient density.
Materials and methods
The growth trial was conducted for 67 days in a recirculating aquaculture system (RAS) at Qinghai University, Qinghai, China. A total of 840 fish (initial body weight: 84.10 ± 0.08 g) were acclimated for three weeks to adapt to the experimental conditions. Three iso-nitrogenous and iso-lipidic experimental diets were formulated: (1) control: 20% SBM and 0% CAP; (2) CAPS6: 10% SBM and 6% CAP; and (3) CAPS12: 0% SBM and 12% CAP. The control diet contained similar composition and nutritional profile as commercial diets. Appropriate amounts of lysine, methionine, and threonine were added to balance the amino acid profile, and MCC was included as an inert filler to adjust all diets to a standardized total of 100%. Fish were hand-fed to apparent satiation twice daily (at 09:00 and 17:00), and daily feed intake was recorded. All water quality parameters were maintained within the optimal range to support fish growth. Growth performance parameters, including final body weight (FBW), feed intake (FI), feed conversion ratio (FCR), and weight gain rate (WGR), were calculated. To correct for the nutrient dilution effects, corrected feed intake (FIc) and corrected feed conversion ratio (FCRc) were also calculated.
Main results and discussions
In present study, no significant differences in FBW, FI, or WGR were observed between CAP inclusion treatments and the control (P > 0.05). However, the FCR increased with rising CAP inclusion levels (P < 0.05) (Figure 1), whereas the FCRc remained comparable to the control (P > 0.05) (Figure 2). The current growth trial showed that CAP had no negative effects on rainbow trout growth. However, the inclusion of MCC may have introduced a nutrient dilution effect, thereby reducing the availability of digestible nutrients (Costa et al., 2022). After correcting for nutrient dilution, FCRc in CAPS6 and CAPS12 was not significantly different from that of the control. Moreover, FIc showed a downward trend from the control to CAPS12, although the differences were not statistically significant.
Conclusions
This study indicated that completely replacing SBM with a novel bacterial protein did not significantly affect FBW, FI, or WGR. However, as the inert filler inclusion level increased, the nutrient density of the experimental diets was reduced, resulting in increased FCR as fish attempted to meet their nutritional requirements. Therefore, the nutrient dilution effects of inert fillers should be further investigated, especially in carnivorous species.
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