Differences between the faecal and ileal amino acid digestibility values of soybean meal, determined with broiler chicks at different ages

There is only little information how the age of chicks and the microbial activity in the hind gut of young broilers can affect the digestibility coefficients of amino acids. Ross 308 broilers were kept in battery cages and fed an experimental diet containing N-free components, soybean meal as the only protein source and TiO2 as a marker. Digestibility trials were carried out weekly from the 2 till the 6 week of age. A three day long adaptation period was followed by a three day long collection period. On the 5 and 6 days representative excreta samples were taken. On day 7, chicks were killed and the ileal content was removed immediately. From the soybean meal, dropping and digesta samples amino acid and TiO2 contents were determined. Before the experiments, chicks were fed commercial broiler diets. Except for lysine, methionine and glycine, faecal digestibilities exceeded the ileal values. It means, that bacterial proteases in the caecum of broiler chicks may release amino acids that are absorbed or hind gut bacteria destroy some part of the incompletely digested amino acids. According to our results microflora in the caecum can also convert some amino acids into bacterial protein. It could be the reason for the higher ileal digestibilities for lysine and methionine. Due to the microbial activity faecal digestibility of amino acids was significantly (p<0.05) higher in the 4, 5 and 6 week, compared to the values of the 2 and 3 week. However, ileal digestibilities decreased as the chicks became older and increased again in the 6 week. In the 2 week ileal digestibilities exceeded the faecal coefficient. No differences were detected in the 3 week. On the other hand, higher faecal coefficients were measured in the 4 and 5 week. These results prove that the caecum of broiler chicks can influence the results of amino acid balance trials, which depends on the age of chicks. From the ileal digestibility values it can be concluded, that amino acid digestion of broiler chicks decrease in the first few weeks of fattening.