To determine the validity of in vitro feed evaluation method, 20 day-old broilers were divided into four (4) groups and assigned to four (4) treatments, specifically: a basal diet, basal diet + 10 ppm avilamycin, basal diet + 150 ppm plant extract containing thymol (Thymus vulgaris), eugenol (Cinnamomum spp.) and piperine (Piper spp.), and basal diet + 300 ppm plant extract, following Completely Randomized Design (CRD). All growth parameters were recorded. After slaughter, duodenal digesta samples were collected and subjected to α-amylase and total proteolytic activities assays using potato starch and bovine serum albumin as substrates, respectively. Subsequently, the protein profiles of the crude duodenal digesta were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Results showed no significant differences in all growth performance parameters, in duodenal α-amylase and total proteolytic activities of the broilers fed with the different dietary treatments (P>0.05). The SDS-PAGE electrophoretogram showed qualitative differences in band intensity of the different proteases and the α-amylase present in the duodenal digesta samples. Results on the growth trial and enzyme activity assays are in agreement indicating that duodenal enzyme activity assays may predict feed quality and digestibility. This study suggests that duodenal in vitro enzyme activity assays coupled with digesta protein profiling can be used as a tool in feed evaluation.