INFLUENCE OF VITAMIN-C ON THE FOLATE REQUIREMENT OF CHANNEL CATFISH, ICTALURUS-PUNCTATUS, FOR GROWTH, HEMATOPOIESIS, AND RESISTANCE TO EDWARDSIELLA-ICTALURI INFECTION

Channel catfish were fed a semipurified basal diet containing 0, 0.4, or 4.0 mg folic acid/kg with 0, 20, or 200 mg vitamin C/kg in a factor ial design in aquaria (29 +/- 2 degrees C) for 25 weeks, and growth, s urvival, and hematological measurements were determined. The remaining fish were overwintered (25 weeks) in circular raceways at ambient out door temperature, fed the same experimental diets and challenged with Edwardsiella ictaluri when the water had reached 26 degrees C. Dietary supplementation of folic acid had no effect on any measurement except hematocrit without dietary vitamin C. When the marginal level of vita min C (20 mg/kg) was fed, hematological measurements increased maximal ly with the low or high supplement of folic acid (0.4 or 4.0 mg/kg), b ut weight gain and RBC mean corpuscular volume (MCV) did not respond t o folic acid supplementation. When the high level of vitamin C (200 mg /kg) was fed, weight gain improved and MCV decreased maximally with th e highest supplement of folic acid. Fish challenged with E. ictaluri s howed maximum survival and antibody production when 0.4 or 4.0 mg foli c acid/kg was fed with the high level of vitamin C, but only the highe st supplement of folic acid improved survival when the diet contained the marginal level of vitamin C. Simultaneous supplementation of both vitamins at the highest levels resulted in the lowest incidence of mor phologically abnormal blood cells. These results indicate that the die tary concentration of vitamin C influences the response of channel cat fish to different dietary levels of folic acid.