Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Poultry Science (PhD)

Degree Level



Poultry Science


Samuel J. Rochell

Committee Member

Billy M. Hargis

Second Committee Member

Guillermo Tellez


Direct fed microbial, enzyme, Poultry, Reduced energy, Soybean meal


Bacillus spp. are ubiquitous, Gram-positive, spore forming bacteria that are commonly recovered from the environment and gastrointestinal tract (GIT) of poultry. These spores are capable of withstanding harsh condition such as feed pelletization, which facilitates inclusion in poultry feeds. Once ingested by the bird, spores germinate into metabolically active vegetative cells which can produce extracellular enzymes which can hydrolyze otherwise indigestible components of the feed. Soybean meal (SBM) is the gold standard vegetable protein source for non-ruminant animals worldwide and is included in practically all poultry diets in the United States at levels ranging from 10 to more than 30%. However, SBM also contains various indigestible carbohydrate fractions that can exert antinutritive effects, limiting some of the available energy and nutritional value of SBM for poultry. These anti-nutritive effects can primarily be attributed to soluble non-starch polysaccharides (NSP; e.g., pectins and β-mannans) and oligosaccharides (raffinose, stachyose, and verbascose). Soluble NSP increase digesta viscosity, leading to changes in intestinal physiology that not only inhibit digestibility of dietary energy and nutrients, but can also compromise gastrointestinal health. Thus, many nutritionists attempt to restrict SBM to inclusion levels lower than those that would be realized if relying solely on least-cost feed formulation. More than 50% of the U.S. commercial broiler industry has adopted antibiotic-free production methods, with as many as one-third of broilers fed all vegetable-based diets to align with growing consumer demand for these products. As such, demand for all-vegetable based diets presents a clear opportunity for increased SBM demand, but the aforementioned concerns related to high-SBM broiler diets are heightened under these production systems. The objective of the present dissertation was to evaluate the effect of Bacillus isolates ability to increase performance and welfare conditions of broilers fed diets with elevated levels of SBM. In the first experiment, 6 trials were conducted to evaluate performance at 21 d post hatch in broilers fed a mash, corn-high SBM diet with either control, CTL (3,050 kcal/kg) or reduced energy, RED (2,925 kcal/kg) or RED supplemented with select Bacillus isolates. Consistent positive responses of increased body weight gain (BWG) and lower feed conversion ratios (FCR) were observed for birds fed RED + Bacillus isolate 46 (BI-46) compared to birds fed RED alone. Pooled data from trials 1-3 showed RED + BI-46 fed birds to be intermediate compared to CTL and RED for BWG and FCR. Data pooled for trials 4-6 showed birds fed RED + BI-46 had higher BWG and lower FCR compared to birds fed RED alone and were similar in performance to CTL group. This experiment confirmed not all isolates that performed well in vitro yielded performance benefits in vivo. In experiment 2, birds were fed similar diets as in experiment 1, in either mash or pelleted form, with a 125 kcal/kg difference in AMEn between the CTL and RED diets throughout the starter, grower, and finisher feeding phases. Birds fed pelleted diets had higher BWG and lower FCR in the starter and overall phases compared with birds fed mash diets. The birds fed RED + BI-46 had lower FCR in the starter phase than birds fed RED alone. Birds fed the CTL diet had the lowest cumulative FCR among all treatment group. The birds fed RED + BI-46 had greater hot and chilled carcass yield compared to birds fed the CTL. Birds fed RED + BI-46 had higher breast yield than birds fed RED and greater tender yield than birds fed CTL. Inclusion of BI-46 also lowered foot pad dermatitis lesion scores at 36 d post-hatch compared to the CTL group. Overall, this research demonstrates that BI-46 could potentially increase performance of birds fed higher levels of SBM and enhance welfare conditions.