the effect of different levels of crude protein and threonine on performance and immune system of broiler chickens during starter and grower periods

this experiment was conducted to evaluate the effect of different levels of crude protein and threonine on performance, carcass characteristics and immune system of broiler chickens during starter (7-24) and grower (25-42) periods. a total of 288 male broiler chicks of ross 308 strain were used in a completely randomized design with factorial arrangement (2×4) with three replicates in each treatment. experimental diets were included two levels of protein (ross 308 guideline recommendation and 5% lower than the recommendation) and four levels of threonine (90, 100, 110, 120 of nrc recommendation). at starter period, threonine levels had a significant effect on feed conversion ratio (p<0.05), and dietary threonine levels higher than 90 percent of nrc recommendation improved feed efficiency. in addition, the effects of interactions between protein and threonine levels on feed intake were significant. gastrointestinal tract weight was also affected by dietary protein levels during starter period. during growth period, increasing levels of threonine above 90% of nrc recommendation improved body weight gain and feed conversion ratio. dietary threonine levels had a significant effect on breast yield percentage/live weight gain at 24 d of age. also, the effects of interactions between protein and threonine levels on the abdominal fat were significant. in both periods, decreasing the dietary protein levels and increasing dietary threonine levels decreased feed cost per kg of weight gain and increased the profit. best performance was observed by 110 percent threonine above the nrc recommendation. at 41 d of age, the effect of threonine on antibody response to srbc injection was significant (p<0.01), and increasing levels of threonine higher than 90 percent of nrc recommendation increased the antibody titer in response to srbc injection. in conclusion, results of current experiment showed that threonine supplementation up to 110 percent of nrc recommendation improved performance, carcass characteristics and immune system of broiler chickens.