Executive Summary: The main objective of the study was to assess the effects of hydrolyzed yeast (HY) supplementation at graded levels of 0, 500, 1000, 2000 g/MT (control, HY0.5, HY1.0, HY2.0, respectively) on the growth performance, immune responses and overall health of Nile tilapia (Oreochromis niloticus). The experiment was conducted at the RAS unit of Maverick Innovation. The facility is a state-of-the-art system consisting of 64, 300-liter tanks.
During the feeding trial, water quality parameters i.e. dissolved oxygen, temperature, and pH were maintained at optimum level for tilapia. Feed intake was normal and there were no variations among the treatments. The mortality was also minimal despite prolonged cold weather during the trial. Despite no significant differences, the final body weight and weight gain was numerically higher in HY treatment compared to the control diet. However, feed utilization parameters such as biological and economical FCR and feed efficiency were improved linearly with the increasing levels of HY. Protein and energy efficiency ratio also showed similar trends and significantly improved with the inclusion of HY. Due to the improved feed conversion, feed cost to produce 1 kg fish also reduced by 0.05 and 0.06 USD for 1 kg and 2 kg HY per metric ton diets, respectively compared to the control diet.
Better protein efficiency ratio resulted in a higher crude protein percentage in fish fed HY supplemented diet. Significant linear increase in protein deposition, protein retention efficiency and energy retention efficiency with the increasing level of HY were also observed. Among the hematological parameters, only hematocrit value was lower in HY1.0 and HY2.0 compared to the control and HY0.5 treatment. No difference among the treatments was observed in other parameters.
Gut histology showed significantly higher villi length in fish fed HY2.0 diets compared to the control diet. Both HY0.5 and HY1.0 diets were also numerically higher, respectively, than the control diet. Total surface volume was also much higher in all three HY supplemented treatments compared to the control diet.
All four target genes were significantly upregulated in HY1.0 and HY2.0 treatments. The expression of two genes supporting growth and ATP production, insulin like growth factor-1 (IGF-1) and glycerol-3-phosphate (G-3-P) respectively, were significantly improved as well as the immune related gene hepcidin. Higher hepcidin is induced by higher production of interlukin-6 resulting in improved ability to reduce inflammation. Ghrelin is often called a ‘hunger hormone’ and also showed significant increase with the inclusion of dietary HY.
In conclusion, dietary HY supplementation improved feed utilization, gut health, better nutrient absorption capacity in terms of average villi surface volume, and improved immunity in Nile tilapia (Oreochromis niloticus). This improved performance resulted in significant savings in feed cost of about 50 USD per MT of fish for the farmers.
