Evaluation of the ability of barley genotypes containing different amounts of β-glucan to alter growth and disease resistance of rainbow trout Oncorhynchus mykiss

A feeding trial was performed to screen three barley genotypes containing different levels of β-glucan for their ability to influence growth, immune function, and disease resistance of rainbow trout. Three experimental diets were prepared by substituting each of three barley genotypes containing different amounts of β-glucan, low (38 g/kg), average (52 g/kg) and high (82 g/kg) β-glucan barley, respectively, for the entire wheat portion (321 g/kg) of the diet. An additional test diet which consisted of the control diet supplemented with a commercially available yeast β-glucan product (MacroGard) at the manufacturer's recommended level of 2 g/kg also was evaluated. Juvenile rainbow trout (approximately 14.3 g initial weight) cultured in 145 L fiberglass tanks (three tanks/diet; 50 fish/tank) in a fresh water flow through system were fed the test diets by hand to satiation for 9 weeks. At 3 and 9 weeks post weighing, fish were sampled for determination of respiratory burst activity, and lysozyme and TNF-α expression. At the conclusion of the feeding trial, fish remaining after sampling were pooled by diet; one subsample was examined for their ability to respond humorally to infectious hematopoietic necrosis virus and two other subsamples were challenged by intraperitoneal injection with a virulent strain of the same virus. Substituting barley flour for wheat flour in a fish-meal based diet did not substantially alter weight gain or proximate composition of rainbow trout. In contrast, positive effects of barley genotype on disease resistance were observed. Trout fed the average or high β-glucan barley genotypes had survival rates similar to those fed the commercially available yeast β-glucan supplemented diet and higher than trout fed the wheat control diet following viral challenge. The results indicate that select barley genotypes can be substituted for wheat without significant detrimental effects on production efficiency while potentially increasing resistance to viral pathogens.