Mineral availability from barley low phytic acid grains in rainbow trout (Oncorhynchus mykiss) diets K. OVERTURF 1 , V. RABOY 2 , Z.J. CHENG 3 & R.W. HARDY 3 1 USDA/ARS, Hagerman Fish Culture Experiment Station, National Fish Hatchery Road, Hagerman, ID, USA; 2 USDA/ARS, National Small Grains and Potato Germplasm Research Facility, Aberdeen, ID, USA; 3 University of Idaho, Hagerman Fish Culture Experiment Station, National Fish Hatchery Road, Hagerman, ID, USA Abstract Phosphorus storage within plant seeds occurs mainly as phytic acid, which has profound implications on the use of seeds as food material. Phytic acid phosphorus is unavail- able to non-ruminants, leaches ionic minerals during digestion, and is excreted at elevated levels as a waste product. This presents a problem in nonruminant livestock production including current efforts to develop renewable grain and legume products for use in fish feeds. The de- velopment of lines of several cereal grain species that have reduced seed phytic acid concentration provides a novel approach to dietary and environmental problems associated with seed phytic acid. Utilizing four isogenic strains of barley (one normal for seed phytic acid, and three low phytic acid lines that produce seeds with approximately 50, 70 and 95% reductions in phytic acid) the apparent digestibility of nutrients in formulated diets containing these barleys at a level of 30% was measured using rainbow trout (Oncorhynchus mykiss). Also examined was the apparent availability of several minerals including phytate phospho- rus and total phosphorus. Results from these studies cor- responded well with the results of other animal studies that evaluated low phytic acid cereal grains. With increasing reductions in seed phytic acid, seed available phosphorus increased and faecal phosphorus was reduced by up to 50%. Calcium availability increased, copper and sulphur decreased, and the other tested minerals demonstrated either increased or decreased availability in a manner uncorrelated to grain phytic acid concentration. KEY WORDS KEY WORDS: barley, fish feed, minerals, phosphorus, phytic acid, rainbow trout Received 2 May 2002, accepted 12 August 2002 Correspondence: K. Overturf, USDA/ARS Hagerman Fish Culture Experiment Station, 3059-F National Fish Hatchery Road, Hagerman, ID 83332, USA. E-mail: kennetho@uidaho.edu Introduction The adjustment of dietary components for the modification of nutritional intake and excretion in feed for livestock is be- coming an issue of increasing interest. The rearing of nonru- minant livestock such as poultry, swine and fish have intensified during the last 50 years, and with this, the production of large volumes of waste has also increased. The mineral content of this waste is important in determining how it can be utilized or how it will be disposed. Recently the phosphorus (P) level found in animal production waste, including the waste water from aquaculture production, has become an important envi- ronmental issue, as it might contribute to water pollution and eutrophication (Miller etal. 1974; Beveridge 1984; Boyd 1990; Sharpley et al. 1994). A large part of this waste problem stems from the fact that most livestock are raised on diets containing excess total P. Specifically, most P in grain and legume feed components is found as phytic acid P, which is not readily utilizable by monogastric animals. This makes it necessary to increase total dietary P to ensure that adequate dietary levels of available P are fed, while the unused phytate-P is passed to the environment through the animal’s waste. Approximately 65–85% of the total P found in seeds accumulates as phytic acid (NRC, 1993; Eeckhout & De Paepe 1994; Raboy 1997). Phytic acid is a polyanion at physiological pH and an effective chelator of nutritionally important mineral cations such as calcium, zinc, magnesium and iron. Once consumed, phytic acid binds to these minerals and other cations in the intestinal tract to form mixed salts. These mixed salts are largely excreted. This phenomenon reduces the availability of grain and legume-derived P in nonruminant diets (Pointillart et al. 1984) and, when 239 Aquaculture Nutrition 2003 9 ; 239^246 .............................................................................................. .............................................................................................. Ó 2003 Blackwell PublishingLtd