The objective of this study was to evaluate the performance effect of the addition of an enzyme- treated soy protein (ESP, Hamlet Protein A/S, Denmark), with a very low anti-nutritional factor content, in broiler starter diets (d0-14) in the frame of two different amino acid (AA) density feeding programs (100% and 88% of the 2019 Cobb 500 recommendations) at three commercially significant market ages (d35, 42, and 49). A total of 1104 day-of hatch Cobb 500 male broiler chickens were used to assess the effect of ESP inclusion in starter feed and AA density on growth performance. Dietary treatments consisted of a 2×2 factorial arrangement with 2 protein sources (SBM or SBM+ESP) x 2 AA densities (100% and 88%), resulting in four starter diets (12 replicates/diet; 23 chicks/replicate): T1 (33.36% SBM, 100% AA density); T2 (27.65% SBM, 5% ESP, 100% AA density); T3 (27.17% SBM, 88% AA density) and T4 (21.23% SBM, 5% ESP, 88% AA density). In the grower (d14-35) and finisher (d35-49) periods, each respective AA density group was fed the same corn-SBM based diet (T1 and T2: 100% AA density; T3 and T4: 88% AA density). Feed and water were provided ad libitum. Body weight and feed intake by pen were recorded at d0, 14, 28, 35, 42 and 49. Weight gain and feed conversion ratio (FCR) were calculated. FCR was adjusted for mortality and fixed BW (d35=2.25kg, d42=2.94kg, d49=3.40kg). Data were analyzed using the statistical software R (R Core Team 2020 version 4.0.0) by 2-way ANOVA. Differences were considered significant at p<0.05. No interaction between ESP inclusion in starter feed and AA density level was observed for weight gain or mortality- and BW adjusted FCR on days 35, 42 and 49. However, ESP inclusion in starter feed had a significant effect on day 49 on mortality adjusted (SBM=1.73, SBM+ESP=1.71; p=0.03) and BW adjusted FCR (SBM=1.75, SBM+ESP=1.70; p=0.04). AA density levels significantly affected weight gain at day 35 (p<0.05). At day 35, 42 and 49, mortality- and BW adjusted FCR were significantly improved by increased AA density level (p<0.05).
In conclusion, replacing part of the SBM in starter diets for broiler chickens with 5% ESP affected performance on day 49 by improving mortality- and BW adjusted FCR by 2 and 5 points. The 100% AA density recommendation improved FCR significantly at all three commercial market ages when using SBM only and SBM + ESP diets. No interaction was observed between ESP inclusion in starter diets and AA density level on performance of broiler chickens at the three commercially significant market ages (d35, 42 and 49).