Availability of phosphorus and trace elements in low-phytate varieties of barley and corn for rainbow trout (Oncorhynchus mykiss)

Approximately two-thirds of phosphorus in various grains is present as phytate, which is not well-utilized by fish and other monogastric species. Besides its low availability of phosphorus, phytate is reported to reduce the availability of other dietary nutrients to animals. Single-gene, non-lethal low phytic acid (lpa) mutations in corn and barley cause the seed to store most of the phosphorus as inorganic phosphorus instead of as phytate phosphorus. Theoretically, using these mutant grains containing lower levels of phytate in animal feeds should reduce phosphorus excretion by the animals, provided that available phosphorus levels in feeds containing these grains are appropriately adjusted downward. This study was conducted as a first step to determine if the biological availability of phosphorus in the low-phytate mutants of barley, dent corn and flint corn differed significantly from that in ordinary grains for fish. Also of interest was the effect, if any, on the availability of other minerals in formulated feeds containing the low-phytate gains. Feeding trials demonstrated that the apparent availability of phosphorus in low-phytate grains was significantly higher than that in ordinary grains when they were combined with low-ash ingredients. Fecal phosphorus content (on average) decrease 50.2% (in phytate-phosphorus) or 42.9% (in total phosphorus) by replacing ordinary grains with low-phytate grains in the low-ash diets. The apparent availabilities of calcium, iron, zinc and strontium also were significantly higher in the low-ash diet containing low-phytate dent corn than that containing ordinary dent corn. However, no such increase was observed with low-phytate barley or low-phytate flint corn over their counterpart grains in either calcium, iron or zinc. The apparent availabilities of copper, manganese, magnesium, potassium and sodium were not significantly different between ordinary and low-phytate grains. The apparent digestibility of dry matter also was not significantly different between ordinary and low-phytate grains. The results of this study suggest that a substantial reduction of phosphorus discharge from fish, poultry and animal farms could be achieved simply by replacing ordinary grains with low-phytate mutant grains in low-ash feeds.