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Department of Animal Sciences, University of Illinois at Urbana-Champaign 61801, USA.
Two bioassays were conducted to determine the maintenance requirement and efficiency of utilization of dietary Lys in young chicks. New Hampshire x Columbian males were used in Assay 1 and Avian x Avian males were used in Assay 2. In each assay, chicks were given free access for 10 d to crystalline amino acid (AA) diets containing graded levels of L-Lys.HCl. Doses of Lys represented 5, 40, 55, 70, and 95% of its ideal level in Assay 1; all other AA were set at 100% of their ideal levels, except for the lowest Lys level, in which the other AA were maintained at a 15% excess. In Assay 2, doses of Lys represented 5, 10, 40, 55, 70, and 95% of ideal; all other AA were maintained at minimized excess levels that were 15% (of ideal) above the various doses of Lys. After 24 h of feed deprivation, chicks were killed for whole-body protein and AA analysis. In Assay 1, protein accretion (r2 = 0.95) and Lys accretion (r2 = 0.98) were linear (P < 0.01) functions of Lys intake. Slope of the Lys accretion regression line indicated that 75.8% of the crystalline Lys ingested (above maintenance) was retained. The Lys required for zero protein accretion was 12 mg/d or 45 mg/d per kg3/4, whereas the Lys required for zero Lys accretion was 30.3 mg/d or 114 mg/d per kg3/4. With Avian x Avian chicks, protein accretion (r2 = 0.99) and Lys accretion (r2 = 0.99) were linear (P < 0.01) functions of Lys intake. Slope of the Lys regression line indicated that 79.3% of the Lys ingested was retained. The Lys requirement for zero protein accretion was 2.5 mg/d or 6.9 mg/d per kg3/4. The Lys maintenance requirement for zero Lys accretion, however, was 32.3 mg/d or 89.1 mg/d per kg3/4. The data demonstrated that at nitrogen equilibrium, chicks are in negative Lys balance but are in positive balance of glycine and proline.
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