Nutrient and Amino Acid Digestibility of Fermented Cottonseed Meal and Fermented Cottonseed Protein Produced with Different Microbial Combinations in Growing Pigs

High-fiber unbalanced amino acid (AA) profiles and toxic free gossypol restrict cottonseed by-products for swine diets, while solid-state microbial fermentation can upgrade their nutrition. We prepared 103 fermented batches with diverse microbes under aerobic or anaerobic conditions, selected 10 representative groups via multivariate statistics for scaled-up fermentation and chemical analysis, and conducted two Youden square pig trials to determine available energy and standardized ileal digestible (SID) AAs before establishing relevant prediction equations. The metabolism experiment used 24.10 kg pigs to detect total tract digestibility and energy values, and an ileal cannula trial on 60.88 kg barrows adopted a nitrogen-free diet to correct endogenous amino acid losses. Fermentation largely lowered free gossypol, and short-term aerobic Aspergillus niger fermentation yielded the highest digestibility of most nutrients and essential amino acids. Digestible and metabolizable energy was positively related to crude protein (CP) and ash but was negatively linked to fiber fractions. SID lysine was negatively correlated with CP and ash yet was positively associated with fiber, while SID methionine and tryptophan displayed identical positive correlations with CP and negative correlations with fiber and free gossypol. Stepwise regression models (R2 = 0.57–0.93) allow rapid nutritional assessment via routine chemical analysis to guide precise pig feed production.