Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Animal Science

First Advisor

Dean R. Zimmerman


Four growth performance and nitrogen (N) retention trials were conducted to investigate potential reasons for the relatively poor performance of broiler chicks fed low crude protein (CP) diets. Male chicks (1-d-old) were fed a common corn-soybean (SBM) diet (23% CP) for 7 d and subsequently allotted to treatment diets in a completely randomized design (10 chicks per floor pen, 6 replications). Chicks had free access to the isoenergetic diets (3,200 kcal MEn/kg) for 2 wk, after which chicks were weighed, fasted for 24 h, and the whole-body N contents of two chicks per pen (and six baseline chicks) determined. In Trial 1, dietary treatments consisted of an intact-protein diet (corn-SBM, 24% CP) and a similar diet, in which half the intact protein was replaced on a true digestible basis with all essential and all nonessential amino acids (EAA and NEAA, respectively) in crystalline form. In subsequent trials, corn-SBM diets (23% CP) served as control diets and low-CP diets were formulated by partially replacing SBM with corn and crystalline arginine, isoleucine, lysine, methionine, threonine, and valine to meet 105% of NRC (1994) recommended EAA concentrations. In Trial 2, glutamine or asparagine replaced 1% triammonium citrate in the low-CP diet (19% CP). In Trials 3 and 4, dietary concentrations of crystalline EAA and NEAA, respectively, were increased incrementally in the low-CP diets (19 to 20% CP). Chicks fed the intact-protein diet (Trial 1) grew faster, utilized feed more efficiently, and retained more N (P < 0.05) than did chicks fed the free amino acid (AA) diet, notwithstanding equal dietary concentrations of true digestible AA. In subsequent trials, chicks fed low-CP diets grew slower, utilized feed less efficiently, and retained less N than chicks fed the control diets (P < 0.05), despite additions of crystalline glutamine or asparagine, and despite increased dietary concentrations of crystalline EAA and/or NEAA. The growth performance and N retention of chicks fed diets containing large amounts of free AA were inferior to those of chicks fed intact-protein diets. Consequently, dietary free AA may not be utilized as well for growth and protein deposition as AA originating from intact protein.



Digital Repository @ Iowa State University,

Copyright Owner

Kristjan Bregendahl



Proquest ID


File Format


File Size

96 pages