Low crude protein (CP) diets and exogenous protease have shown benefits in reducing costs and minimizing environmental impact. However, previous studies with low CP diets generally add crystalline amino acids (AA) with higher digestibility, and consequently, the effects of enzymes on ileal amino acid digestibility are affected. The objective of this experiment was to evaluate the impact of an exogenous protease on ileal amino acid digestibility of broiler grower diets with three crude protein levels. A total of 294 Ross 308 AP male broilers were placed in 42 battery cages. Up to 7 days of age, all chickens were fed one common starter diet in crumbles. Six treatments resulted from a 3 x 2 factorial arrangement of three crude protein levels (17, 19, and 21% CP) and two protease levels (0 or 300 g/ton), with seven replicates per treatment and seven chicks per replicate. Data were analyzed in a completely randomized design and mean separation with Tukey’s test using JMP. Three isocaloric (3,100 kcal/kg ME) basal diets were formulated to reduce CP, but all diets maintained digestible Lys in 5.47% of CP and the same ideal protein profile. Crystalline Lys, Met, and Thr were used to obtain desired AA levels of balanced protein but not to keep similar AA concentrations across the three diets. The Lys:calorie ratios were 0.30, 0.34, and 0.37 %/Mcal ME. All diets contained titanium oxide as an indigestible marker. The exogenous protease or sand was added on top of these basal diets. On day 24, all birds were sacrificed with CO2 to collect the digesta of the 2/3 distal portion of the ileum. Ileal contents were frozen at – 20 oC and were analyzed for DM, titanium, and AA acid content by HPLC. Diets were analyzed for protein, AA, titanium, and DM. The parameters determined were ileal DM, apparent AA, CP digestibility, and Ly’s retention. No interaction effects were observed (P>0.05) in BW and BW gain. Interaction effects (P<0.05) were observed in FCR and for almost all ileal AA digestibility coefficients, except for Cys and Trp. Live performance worsened as CP decreased. In this two AA, the main effect of protease was observed (P<0.05). The protease improved the ileal digestibility of all AA only in diets with 21% CP. The protease improved the digestibility of Cys and Trp (P<0.05) independently of CP level. No differences in ileal AA digestibility due to dietary CP level (P>0.05) were detected, except for Cys digestibility (P<0.01), which was better in 19% diets than in 21% CP, while 17% CP was intermediate. A similar balance of AA across all diets may cause similar ileal AA digestibility.
The protease positively affected ileal digestibility when AA levels exceeded chicken requirements.