| Summary: | Two laboratory feeding trials were conducted to quantify differences in the digestibility and absorption of dietary amino acids provided in intact protein or in purified amino acid mixtures. In the first experiment, 36 channel catfish (346 ± 47 g) were force-fed one of six practical feed ingredients (blood meal, corn meal, fish meal, meat and bone meal, soybean meal or wheat middlings), or a mixture of crystalline amino acids designed to duplicate the amino acid composition of each of the practical ingredients. Apparent digestibility coefficients (ADC) were calculated for 15 amino acids in each practical ingredient and in each amino acid mixture. An average ADC of all amino acids in each treatment also was calculated. Very few differences in the ADCs of individual amino acids were observed. However, in all ingredients but two, the mean ADCs of amino acids in fish fed purified amino acids were significantly higher than the mean ADCs of amino acids in fish fed intact protein. In the second experiment, 468 channel catfish (396 ± 49 g) were force-fed the same ingredients and amino acid mixtures used in the digestibility trial, and blood was collected from the hepatic portal vein at 1-h intervals for 12 h after feeding. In four of the six ingredients tested, postprandial concentrations of 2-8 amino acids in blood plasma were significantly higher and reached peak levels earlier (1-3 h after feeding) in fish fed purified amino acids than in fish fed intact protein. Results indicated that protein was well digested by channel catfish regardless of its source. However, soybean protein appeared to be digested more slowly than the other proteins tested. The efficiency with which supplemental amino acids are utilized might be reduced if slowly digested proteins compose a major portion of the diet, because of temporal differences in the absorption of amino acids from different dietary sources. If so, the effectiveness of amino acid supplementation could vary with the ingredient composition of the diet to a greater extent than has previously been recognized.
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