Efficacy of dicalcium and defluorinated rock phosphates as dietary phosphorus sources for channel catfish ( Ictalurus punctatus)

Two experiments were conducted to evaluate the efficacy of dicalcium phosphate (DCP) and defluorinated phosphates (DFP) as dietary phosphate sources for channel catfish, and to test the effect of their solubility in neutral ammonium citrate (NAC) on phosphorus utilization. The following phosphates were tested: DCP with a NAC solubility of 90.7%, DFP with a high NAC solubility (HDFP, 85.4%), a DFP with a medium NAC solubility (MDFP, 62.7%), and a DFP with a low NAC solubility (LDFP, 44.6%). In Experiment 1, 12 purified diets (32% protein and 2.9 kcal digestible energy g −1) containing either DCP, HDFP, or MDFP and four levels of total phosphorus 0.2, 0.3, 0.4, or 0.8% were tested. In Experiment 2, four purified diets containing either DCP, HDFP, MDFP, or LDFP at 0.49 total phosphorus (0.40% available phosphorus based on DCP) were tested. Juvenile channel catfish (4.8 g fish −1 and 3.8 g fish −1 for Experiments 1 and 2, respectively) were raised under laboratory conditions and fed twice daily to approximate satiation for 12 weeks. In Experiment 1, weight gain and feed consumption increased and feed conversion ratio (FCR) decreased linearly as dietary phosphorus concentration increased regardless of phosphate type. Fish fed diets containing HDFP consumed significantly more feed, had a lower FCR, and gained more weight than fish fed diets containing DCP and MDFP regardless of dietary phosphorus concentration. Bone ash and phosphorus concentrations showed a similar pattern as weight gain in regard to response to dietary phosphorus concentration. As dietary phosphorus concentration increased, bone ash and phosphorus concentration increased linearly regardless of phosphate type. Fish fed diets containing HDFP had higher bone ash and phosphorus concentration than those fed diets containing DCP and MDFP regardless of dietary phosphorus concentration. In Experiment 2, fish fed the HDFP diet consumed significantly more feed than other groups of fish and gained significantly more weight than fish fed diets containing DCP and MDFP, but not more than fish fed the diet containing LDFP. Feed conversions were the same for all fish regardless of treatment. There were no significant differences in bone ash and bone phosphorus concentrations among the treatments. These data indicate that DCP and DFP are equally efficacious for use as phosphorus sources for channel catfish. The data do not support the contention that a higher NAC solubility correlates to a higher utilization o