Effects of dietary protein and threonine levels on growth performance and immunocompetence in broiler chickens

Abstract

The present work was conducted to study the effects of feeding low crude protein diets (LCP-diets) with supplementation of threonine (Thr) on growth performance and immunocompetence in broiler chickens. One day old 264 Cobb chicks were divided into 6 equal groups (each of 4 replicates and each replicate was of 11 chicks). The 1^st group is considered the control one and fed normal basal control starter, grower and finisher diets containing the recommended levels of CP, threonine, methionine, lysine, tryptophan, valine, arginine and metabolizable energy as specified for Cobb500 broiler performance. The 2^nd experimental group was fed ideal protein diets which were formulated to contain the minimum nutrient specifications based upon supplementing digestible essential amino acids (lysine, methionine, tryptophan, threonine, arginine, and valine). The other experimental chick groups (3^rd, 4^th, 5^th and 6^th groups) were fed isocaloric diets containing low levels of total dietary CP% (1 and 2% less than the minmum recommended CP%) with supplementation of Thr to 120 or 140% of the minimum recommended levels. The diets were isocaloric (3035, 3108 and 3180 Kcal/kg for starter, grower and finisher diets respectively). All chicks were vaccinated against Newcastle disease virus (NDV) at day 14 of age through eye drops. Growth performance indices (body weight, feed intake, feed conversion ratio and protein efficiency ratio) were measured. Blood samples were collected from the broilers at 14, 21, 28 and 35 days of age to determine the serum titre of antibodies against NDV using haemagglutination inhibition test (HI). At 42 days of age, blood samples were collected for determination of serum metabolites and 12 chickens from each group were slaughtered to obtain some of the carcass traits. The results revealed that BW and FCR of the broilers fed 2% LCP- diets with 120 or 140% Thr supplementation were significantly lowered than those fed control or ideal protein diets, as well as those fed 1% LCP- diets with Thr supplementation. There were no differences between the groups in dressed carcass percentages. The abdominal fat % in the carcass of the broiler chickens was not significantly affected by decreasing the dietary CP or supplementation of threonine. Liver weight% significantly increased with feeding the 2% LCP- diets. Feeding 2% LCP- diets also resulted in significant increase in serum uric acid and decreased serum total protein, albumin and globulin. The highest HI titer values and lowest serum levels of uric acid were observed in chickens fed ideal protein diets. While, chickens fed LCP- diets (-2%) had reduced HI titer values. From the present study, it could be concluded that the dietary protein level could be decreased by 1% of the recommended levels with threonine supplementation to 120 % of requirements with no adverse effects on body weight development, FCR, serum metabolites and immune competence in broiler chickens. Formulating broiler diets based upon digestible essential amino acids achieved optimal growth performance and maintained immunocompetence.