Performance, carcass variables and immune responses in commercial broiler chicks fed graded concentrations of threonine in diet containing sub-optimal levels of protein

The effect of supplementing five graded concentrations (0.64, 0.67, 0.70, 0.73 and 0.76 units of dietary lysine) of threonine (Thr) in low crude protein diets (LCPD) containing 25 g less crude protein (CP) kg−1 compared to the levels in control diet (CD) on performance, carcass traits, retention of nitrogen (N), calcium (Ca) and phosphorus (P), and immune responses (lymphocyte proliferation ratio - LPR, antibody titres against Newcastle disease (ND) vaccine) in broilers were investigated. Three CDs containing 220 g, 200 g and 180 g CP; 13.3, 11.4 and 10.4 g lysine (Lys) kg−1 were fed during pre-starter, starter and finisher phases, respectively. Ross 308 broiler female chicks were distributed to 6 treatments with 10 replications of 6 chicks in each. Experimental diets in mash form were fed ad libitum from day 1 to 42 d of age. Body weight gain, feed intake and feed efficiency were reduced (P<0.01) in broilers fed LCPD at 14 d of age. At 28 and 42 d of age, the performance of broilers fed LCPD with 0.64 Thr was similar to those fed the CD. Body weight gain and feed efficiency were not (P>0.05) influenced by Thr supplementation to LCPD. The carcass variables, retention of Ca and P, lymphoid organ (bursa, spleen, thymus) weights and immune responses were not affected (P>0.05) by either reduction in CP or supplementation of Thr to LCPD. Retention of N was significantly (P<0.01) higher in LCPDs compared to those fed the CD, while Thr supplementation to the LCPD did not influence (P>0.05) the N retention. The results suggested that broiler female chicks require more than 195 g CP kg−1 diet during pre-starter phase. The CP can be reduced by 25 g kg−1 in starter and finisher diets (175 and 155 g kg−1, respectively) without affecting the broiler performance, carcass yields and immune responses by maintaining 7.6, 6.4 and 5.7 g Thr kg−1 diet, respectively during pre-starter, starter and finisher phases.