Pot Marigold (Calendula officinalis) Powder in Rainbow Trout (Oncorhynchus mykiss) Feed: Effects on Growth, Immunity, and Yersinia ruckeri Resistance

Abstract

The current research investigated the effects of pot marigold (Calendula officinalis) powder on growth, biochemical parameters, digestive enzymes, serum and mucus immune responses, antioxidant defense, and resistance of rainbow trout (Oncorhynchus mykiss) against Yersinia ruckeri. Fish (No. 450, $15.06\pm 0.11$  g; $\text{mean}\pm \text{SE}$ ) were randomly distributed to five groups fed with a diet containing 0 (control group), 0.5%, 1%, 1.5%, and 2% of pot marigold powder (MP) for 60 days. Then, fish were challenged with Y. ruckeri infection. Specific growth rate (SGR), weight gain (WG), final weight (FW), feed conversion ratio (FCR), mucus lysozyme (LYZ), mucus protease, serum nitroblue tetrazolium test (NBT), alanine aminotransferase (ALT), and alkaline phosphatase (ALP) values in the 1%, 1.5%, and 2% MP groups significantly increased ( $P<0.05$ ) compared to the other groups. Regression analysis exhibited that rainbow trout growth indices were polynomially linked to dietary MP concentrations. In this regard, the optimal levels of MP, according to growth parameters (SGR and FCR), were 1.31 and 1.4% diet, respectively. The intestinal protease, serum complement component 3 (C3), complement component 4 (C4), and LYZ activities in fish fed with the 1% and 1.5% MP-supplemented regime were higher ( $P<0.05$ ) compared to the other groups. Also, fish fed with diets containing 1.5% MP had significantly higher intestinal lipase and mucus ALP activities than the other groups ( $P<0.05$ ). Aspartate aminotransferase (AST) activity in all MP groups remarkably increased ( $P<0.05$ ) compared to the control. Moreover, fish fed with a diet containing 1.5% MP had remarkably higher ( $P<0.05$ ) lactate dehydrogenase (LDH) and total protein (TP) values than the other groups. Serum myeloperoxidase (MPO), total immunoglobulin (Ig), and mucus peroxidase values in the 1% MP group remarkably increased ( $P<0.05$ ) compared to the other groups. In addition, catalase (CAT) and superoxide dismutase (SOD) activities in the 1.5% and 2% MP groups were significantly higher ( $P<0.05$ ) compared to the others. However, malondialdehyde (MDA) levels in fish fed with the 1%, 1.5%, and 2% MP-supplemented diet remarkably decreased than in the other groups ( $P<0.05$ ). The highest survival rate after a challenge with Y. ruckeri was recorded in the 1.5% group compared to the control group. Our findings revealed that using pot marigold powder in diets, especially at the 1.5% level, has positive effects on growth, digestive enzyme activities, antioxidant capacity, immune response, and disease resistance in O. mykiss.