Robert C. Reigh and Weibing Yan

Phosphorus is a critical nutrient for plant growth in aquatic environments. Small increases in phosphorus entering a catfish pond can produce algal blooms that degrade water quality and increase off-flavor in fish. The primary source of phosphorus in catfish ponds is feed. This could be reduced by lowering feeding rates, decreasing the amount of phosphorus in the feed or increasing the absorption of dietary phosphorus by the fish.

However, each of these options has problems. Lowering feeding rates is usually impractical because the high stocking densities used by most catfish producers dictate high feeding rates for acceptable yields. Decreasing the amount of phosphorus in feeds is difficult because of the high level of plant products used in catfish diets. A typical catfish feed contains more than 90 percent plant ingredients, primarily soybean meal and corn, which contain phosphorus in a chemical form (phytic acid) poorly digested by fish. Most of the chemically bound phosphorus in feed ingredients passes through the gut unused and is eventually released into the pond by decomposition of fish wastes. Because so little of this phosphorus is used, inorganic phosphorus supplements are added to catfish feed to satisfy dietary requirements.

Increasing the uptake of phosphorus from feed ingredients can be accomplished by supplementing diets with phytase, an enzyme that breaks down phytic acid to release its bound phosphorus. Adding phytase to fish feeds has been shown to improve phosphorus uptake in the few studies conducted to date and, in some cases, to increase use of dietary protein. Several studies have been conducted at the LSU AgCenter’s Aquaculture Research Station to determine the effects of supplemental phytase on breakdown of phytic acid in catfish feed and to evaluate the effects of phytase supplementation on diet digestibility and the use of dietary protein and minerals.

Laboratory feeding trials were conducted with an all-plant diet containing graded levels of phytase (0, 500, 1,000, 2,000, 4,000 and 8,000 units per kilogram). Results indicated that weight gain, feed conversion, diet digestibility, protein availability and use of dietary protein were not improved by phytase treatment, even at the highest level of phytase tested, but adding phytase to the diet at a level of 1,000 units or higher significantly increased the mineral content of bone—especially concentrations of calcium, phosphorus and manganese. Zinc, an essential trace mineral for catfish, was deposited at significantly higher concentrations in bone of fish fed 8,000 units than in bone of fish fed 500 units or less.

Phytic acid was degraded by phytase, primarily in the stomach of catfish, within a few hours after ingestion. Two hours after feeding, stomach contents of fish fed phytase-supplemented diets contained from 5 percent to 90 percent of the amount of phytic acid present in the diet, depending on the concentration of phytase fed. By eight hours after feeding, concentrations of phytic acid in stomach contents of fish fed 1,000 units per kilogram, or more, had fallen to 6 percent or less of initial dietary levels. Phytic acid levels in digesta continued to decrease as the material moved through the intestine.

Results at this time suggest that a phytase inclusion level of 1,000 units per kilogram in the diet is adequate to significantly increase use of dietary minerals and significantly reduce the amount of phytic acid in catfish waste. At this level, the requirement for supplemental inorganic phosphorus is decreased, and the quantity of phosphorus entering the pond environment from feed ingredients is reduced significantly compared to fish fed an unsupplemented diet.

A problem with use of phytase in catfish feeds is the enzyme’s inability to withstand the high temperatures of extrusion processing. Addition of phytase to floating feeds like those commonly used in commercial catfish production will require that methods be developed for applying the enzyme to finished feeds under practical milling conditions; for example, phytase might be applied at the end of the manufacturing process as a top-spray on finished pellets.

Pond feeding trials also are needed to determine if the effects of phytase that have been documented in laboratory feeding trials produce measurable benefits under practical production conditions.
 
(This article appeared in the summer 2001 issue of Louisiana Agriculture.)