Effect of dietary protein content and threonine to lysine ratio on intestinal morphology and mucin secretion in broiler chickens

In order to evaluate effects of dietary protein content and threonine to lysine ration on intestinal morphology and mucin secretion, 480 male broiler chicks (Ross 308) were used in a completely randomized design (seven treatments of six replicates) with 2×4 factorial arrangement. Eight dietary treatments were formulated using four threonine to lysine ratio (0.55, 0.6, 0.7, and 0.8) and two standardized ileal digestible protein levels (17 and 18 percent) and fed for 15 to 35 days of age. Sampling was conducted on day 35 with two birds from each replicate. The ratio of 0.8 for threonine to lysine caused an increase in villus height in all parts of the small intestine and villus surface in the ileum, compared to other ratios (P<0.05). Reduction of dietary digestible protein level decreased crypt depth in the duodenum and jejunum (P<0.05). Increasing the threonine to lysine ratio up to 0.7, increased mucin secretion in the jejunum (P<0.05). Decreasing the threonine to lysine ratio to 0.55, increased digesta viscosity in all three parts of the small intestine (P<0.05). The ratio of 0.7 for threonine to lysine caused an increase in serum alkaline phosphatase (ALP). On the other hand, reduction of dietary digestible protein level decreased the serum ALP (P<0.05). Regarding to the results, it seems that the optimum digestible threonine to lysine ratio for wheat-based diets to be around 0.7 to 0.8; and that with reduction in dietary protein level, higher ratios would be of more importance.