Effects of dietary soybean protein levels on metabolic response of the southern catfish, Silurus meridionalis

A closed respirometer was used to measure oxygen consumption of the southern catfish Silurus meridionalis fed with six isonitrogenous (48% crude protein) diets replacing 0%, 13%, 26%, 39%, 52% and 65% fish meal (FM) protein by soybean meal (SBM) protein, in order to investigate the effects of dietary soybean protein level (SPL) (replacing FM) on metabolic rates of the southern catfish. The results showed that there were no significant differences in routine metabolism among dietary treatments. Either the total metabolic rate or specific dynamic action (SDA) was positively correlated with assimilated food energy at each diet, respectively (P<0.05). The SDA coefficient (means the energy spent in metabolism per unit of assimilated dietary energy) significantly increased with increasing dietary SPL (P<0.05). Fish fed the diet with 13% SPL had a significantly lower SDA coefficient (0.1528) than fish fed the diet with 52% or 65% SPL (0.1826 or 0.1932) (P<0.05). However, there were no significant differences in SDA coefficient among fish fed the diets with 13%, 26% and 39% SPL (P>0.05). Results of the present study suggested that an imbalance of essential amino acids at higher dietary SPL resulted in more energy channeled into metabolism, and subsequently increased the SDA coefficient.

Effects of dietary soybean protein levels on energy budget of the southern catfish, Silurus meridionalis

Energy budgets were calculated for the southern catfish, Silurus meridionalis, fed diets replacing 0%, 13%, 26%, 39%, 52% and 65% fish meal protein with soybean meal (SBM) protein with or without methionine supplementation to apparent satiation at 27.5 degrees C. With increasing dietary soybean protein levels (SPL), the feed energy lost in feces, excretion and metabolism increased, while that available for growth decreased (P<0.05). When 0.12% or 0.26% methionine at 39% SPL was added to reach that in body carcass or the control group (0% SPL), no significant differences were found in each component of energy budgets. When 0.21% or 0.33% methionine at 52% SPL was added to reach the content of methionine in body carcass or the control group, energy spent on growth increased, but that on excretion and metabolism decreased (P<0.05). These results suggested that the differences in growth rate among the southern catfish fed the diets with different SPL were due to decreasing absorption rate, increasing excretion and metabolism with increasing dietary SPL. The most important factor limiting the use of soybean protein was the imbalance of essential amino acids, which resulted in more energy spent on metabolism and excretion, less energy on growth. Supplementation of methionine produced a relatively better amino acid profile and subsequently improved the utilization of soybean protein at high SPL, which resulted in less energy used for metabolism or lost in excretion and more energy available for growth.