Nutritive value of poultry by-product meal. 2. Comparisons of methods of determining protein quality

Studies were conducted to compare several methods of determining the protein quality of poultry by-product meal (PBPM). A sample of PBPM was compared to a soybean meal (SBM) sample fortified with .5% of L-methionine. Each was incorporated into corn starch and poultry oil-based diets at 2.5% increments between 0 and 30% protein. Diets were fed for 7 days starting when the chicks were 8 days old. Slope-ratio and saturation kinetics models were fitted for the two protein sources. Protein efficiency ratio (PER), net protein ratio, and net protein utilization were calculated for each diet. The PER was the most discriminating method of estimating protein quality, especially at lower levels (2.5% protein: PBPM = .65, SBM + methionine (Met) = 4.23). At higher levels, no differences could be detected (30% protein: PBPM = 2.45, SBM + Met = 2.31). The 6% protein level was chosen for further studies because there were large differences in PER, and the chicks were in positive nitrogen balance. For eight samples of PBPM (from three different processing facilities), Kjeldahl N ranged from 8.64 to 10.22% gross energy (GE) from 4.49 to 5.36 kcal/g; Ca from 3.6 to 8.84%, and P from 2.04 to 2.55%. A Monday morning sample (fresh broiler waste) was compared to a Friday afternoon sample from the same processing facility (broiler and hatchery waste, and DAF sludge) in semipurified diets at 6% protein. Significant differences in gain were detected in two experiments, but PER in only one. For the five samples tested by all methods, similar rankings were found by PER, pepsin digestibility, total lysine and Carpenter's available lysine.