A 35-day study on 300 Cobb 500 and Ross 308 broilers tested whether a fructooligosaccharide and multi-species probiotic synbiotic (0.5 g/kg) could improve growth, gut health, tibia strength, and behavior. Strain choice had the biggest effect. Cobb 500 birds outperformed Ross 308 on weight and feed intake regardless of diet. The synbiotic improved duodenal villi height and added some tibia weight, but did not move body weight, FCR, leg strength, or lameness. Bottom line for producers: strain selection drives performance. Synbiotics may help gut and bone quality, but dose and formulation need work before they pay off at the scale.