Endogenous Amino Acid Flows are Influenced by Age of Broiler Chickens

During the process of digestion and absorption of ingested feed in poultry, significant losses of endogenous amino acids (EAA) occur from various digestive secretions, mucoproteins and the epithelial cells lining the gastrointestinal tract (GIT). Several factors such as the development of GIT and changes in dry matter intake (DMI) with advancing age, type of bird, method of euthanasia and ileal digesta collection may influence EAA loss. Correction for these inevitable losses is necessary to standardise amino acid (AA) digestibility values. The present study was carried out to measure the basal EAA loss in male broilers (Ross 308) at different ages (d 7, 14, 21, 28, 35 and 42), using a nitrogen-free diet (NFD). The assumption is that, since no protein is fed, all nitrogen and AA in the ileal digesta are of endogenous origin and represent the basal losses. The NFD was composed of maize starch (842 g/kg), fibre source (cellulose; 50 g/kg), soybean oil (50 g/kg) and mineral and vitamin premix (53 g/kg). Titanium dioxide (5 g/kg) was also added to the NFD as an indigestible marker. The NFD diet was fed to six replicate cages housing 14 (d 7), 12 (d 14), 10 (d 21), 8 (d 28), 8 (d 35), and 6 (d 42) birds per cage for four days prior to digesta collection. Following euthanisation by intravenous injection of sodium pentobarbitone, the digesta was collected by gentle flushing with distilled water from the lower half of the ileum. The basal EAA flow was calculated as grams per kilogram of DMI (g/kg DMI). Data were analysed by using general linear models procedure of SAS (SAS Institute, Inc., Cary, NC) with cage means as the experimental unit. Orthogonal polynomial contrasts (linear and quadratic) were used to compare the treatment means. The flow of ileal endogenous N and all AA, on a DMI basis, decreased quadratically (P < 0.05 to 0.001) as birds grew older. The values of ileal endogenous loss of N and total endogenous loss of all AA (TAA), when expressed per kg of DMI, were higher (P < 0.01) at d 7 compared to other ages. The values for ileal endogenous N and total EAA losses at d 7 were 3.60 and 12.9 g/kg DMI, respectively. In agreement with the present findings, Adedokun et al. (2007) recorded approximately two times higher total EAA loss in broilers at d 5 compared to d 15 and 21. However, the endogenous flow values for N and TAA were similar at d 14, 21, 28 and 35. The lowest endogenous loss for TAA (4.48 g/kg DMI) was determined at 42 d. When an NFD is fed, the source of EAA is largely mucoproteins (Lemme et al., 2004). Decreased mucin secretion, increased endogenous protein digestion and absorption, and increased DMI with age may account for the lower EAA secretion in older birds (Ravindran and Bryden, 1999). Current data suggest that specific age-related values for EAA loss should be used to standardise AA digestibility coefficients in broilers.