Effect of ash content on protein quality of meat and bone meal

The effect of ash concentration on amino acid (AA) composition, true AA digestibility, and protein efficiency ratio (PER; weight gain per unit of protein intake) of meat and bone meal (MBM) was evaluated. Commercially rendered MBM samples containing 16 to 44% ash were obtained from two sources. Additional samples of MBM varying in ash from 9 to 63% were obtained by chloroform floatation or lab screening of a beef crax sample. Protein quality of selected MBM samples was assessed by determining true AA digestibility using the precision-fed cecectomized rooster assay and by a PER chick growth assay wherein chicks were fed 10% CP diets containing a MBM as the only source of dietary protein from 8 to 18 d of age. Increases in Ala, Pro, Gly, and Arg as a percentage of CP were observed in all MBM samples as ash percentage increased, with Pro and Gly accounting for most of the increase. In contrast, the levels (% of CP) of all essential AA, other than Arg, decreased as ash level increased. For example, Lys concentrations per unit of CP decreased from 5.7 to 4.0% as ash increased from 9 to 63%. There was little or no effect of ash content on AA digestibility of MBM varying in ash from 9 to 44%. The PER of MBM markedly decreased from 3.34 to 0.72 as ash increased from 16 to 44%, and most of the effects of ash on PER were not due to differences in dietary Ca and P levels. The results indicate that the reduction in protein quality of MBM as ash content increases is almost entirely due to a decrease in analyzed essential AA per unit of CP, not a decrease in digestibility of AA.