Dietary glycerol inclusion decreases growth performance and nitrogen retention efficiency in rainbow trout (Oncorhynchus mykiss)

Abstract

Glycerol is an inexpensive source of dietary energy, although its inclusion in compound feeds has been barely investigated in fish. The effects of dietary refined glycerol inclusion on growth performance, nitrogen and energy balance, as well as its digestibility and intestinal digestive enzyme activities was studied in rainbow trout. Fish (initial BW 20.2 ± 0.1 g, mean ± SE) were divided into nine tanks (1.66 kg m−3), each one assigned to the experimental diets, in triplicates. Fish were fed twice a day to apparent satiation for 60 days three diets including 0, 25, or 50 g kg−1 of refined glycerol (G0, G25, and G50, respectively) with similar digestible protein to energy ratios (22 mg kJ−1). A digestibility trial was conducted in parallel to the growth trial by using a Choubert System to collect feces, in triplicates. Refined glycerol was well digested (ADCglycerol > 99.7%) when included up to 50 g kg−1. Specific growth rate (SGR) decreased as glycerol inclusion levels increased in the diet (P = 0.029). Feed intake was enhanced with increasing dietary inclusion levels of glycerol (P = 0.013). The feed efficiency decreased linearly as the glycerol inclusion levels increased in the diet, reflected in a higher FCR ratio, particularly in the G50 group compared to the G0 or G25 groups (1.04 ± 0.02, 0.90 ± 0.02 and 0.91 ± 0.02, respectively). Total ammonia nitrogen excretion rate (TANER) was measured in trout at the end of the growth trial, displaying increased values with higher dietary glycerol inclusion levels (P < 0.001). As a result, trout fed G25 or G50 diets displayed an increased energy intake, which was not reflected in a reduction of the nitrogen excretion rate. Contrary to expected, glycerol had a negative effect on nitrogen efficiency, indicating that dietary inclusion of this compound does not spare the use of proteins as a metabolic fuel. In addition, amylase, but not proteases and lipase, was altered in the intestine of trout by refined glycerol inclusion, as this digestive enzyme activity increased 6 h post-feeding in trout fed G25 and G50 diets. In conclusion, dietary refined glycerol inclusion on rainbow trout was not reflected in beneficial effects on fish performance or in the quantity of nitrogen waste products excreted to the environment by this carnivorous species.