Dietary selenium requirement for on-growing grass carp, Ctenopharyngodon idellus

Selenium (Se) is an essential trace element for fish. Appropriate dietary Se supplementation is essential for fish's normal growth and health. This study was conducted to explore the dietary Se requirement of on-growing grass carp (Ctenopharyngodon idellus) (73.27 +/- 0.12 g) using selenium yeast (Se-yeast) as the Se source. Graded levels of Se-yeast were added to the basal diet at 0, 0.125, 0.25, 0.5, 1.0, 2.0, and 4.0 mg Se/kg diet providing the analyzed Se of 0.05, 0.18, 0.30, 0.57, 1.05, 2.21 and 4.21 mg/kg, respectively. After 56-day culture, the final body weight and weight gain rate (WGR) were significantly increased in fish fed with 0.57 to 2.21 Se/kg diet while feed conversion ratio (FCR) showed the inverse pattern (P < 0.05). The specific growth rate (SGR) was significantly increased with incensing dietary Se levels up to 2.21 mg Se/kg diet (P < 0.05). Antioxidant enzyme activities in the liver and plasma were significantly increased in fish fed with 0.57-4.21 mg Se/kg diet (P < 0.05). Malondialdehyde (MDA) contents in the liver and plasma were significantly decreased and then increased with dietary Se supplementation (P < 0.05). Transcript levels of 6 selenoprotein genes in the liver (gpx1a, gpx4a, selp, selw, selk, and dio1) and 7 selenoprotein genes in muscle (gpx1a, gpx4a, gpx4b, selp, selw, selk, and dio1) were significantly increased with increasing dietary Se levels (P < 0.05). Based on broken-line regression analysis of WGR, SGR, and FCR, the optimal dietary Se requirement for on-growing grass carp was 0.915-1.026 mg Se/kg diet. A dietary Se level of 4.21 mg/kg could be toxicity for on-growing grass carp characterized by depressed growth and increased MDA content.