Abstract:
Two experiments were conducted to investigate the effect of low crude protein (CP) diets
with increased or constant metabolizable energy to crude protein (ME:CP) ratios supplemented
with limiting amino acids (AA) on the performance of broilers. Experiment 1 was conducted to
study the effect of lowering dietary CP with optimum limiting AA levels on the performance of
broilers from 1 to 35 days of age. Four experimental diets were formulated to have four levels of
CP as 23, 22, 21 & 20% in starter phase; 22, 21, 20 & 19% in grower phase and 20, 19, 18 &
17% in finisher phase, respectively. All diets were isocaloric within a phase (2925, 3075 & 3125
kcal/kg ME for starter, grower and finisher periods, respectively). Digestible lysine was
maintained at 1.10, 1.02 and 0.90% of the diet in starter, grower and finisher periods,
respectively. Results showed that the dietary CP level could be reduced without harmful effects
on the growth performance, body composition and carcass characteristics when supplemented
with limiting AA. Experiment 2 was conducted to determine the effect of low CP diets with
constant ME:CP on the performance of broilers from 1 to 35 days of age. Four experimental
diets were formulated to have four levels of CP and ME, respectively, in each phase: 23, 22, 21
and 20% CP, with 3036, 2904, 2772 and 2640 kcal/kg in starter phase; 22, 21, 20 and 19% CP,
with 3146, 3003, 2860 and 2717 kcal/kg in grower phase; and 20, 19, 18 and 17% CP, with
3100, 2945, 2790 and 2635 kcal/kg in finisher phase. Digestible lysine was maintained at 1.10,
1.02 and 0.90% of the diet in starter, grower and finisher periods, respectively. Results showed
that chicks fed low CP diets with constant energy to protein ratio grew slower, used feed less
efficiently and retained more fat than chicks fed the control diets even when standard levels of
critical AA were maintained in the diets. In conclusion, reducing the dietary CP level provided
substantial economic returns without affecting the performance of broilers. However, reduction
of dietary CP with constant ME:CP ratio resulted in reduced broiler performance and carcass
composition.
