|Abstract:||Three experiments were conducted to evaluate the effects of isoquinoline alkaloids (IQ) in corn-soybean meal (SBM) diets fed to young growing pigs on growth performance, gut health, energy and nutrient digestibility, and digestible energy (DE) and metabolizable energy (ME) concentrations in the diets. In the first experiment, 32 ileal cannulated barrows (12.19 ± 1.38 kg) were allotted to 4 diets with 8 replicate pigs per diet to determine effects of dietary inclusion of IQ on apparent ileal digestibility (AID) of amino acids (AA), crude protein (CP), starch, and acid hydrolyzed ether extract (AEE). The corn-soybean meal basal diet was supplemented with 0, 90, 180, or 360 mg IQ/kg complete feed. Diets were fed for 27 d and ileal digesta were collected on d 13 and 14 (period 1) and d 26 and 27 (period 2). Results indicated that dietary inclusion of IQ resulted in a quadratic increase (P < 0.05) in the AID of starch, Thr, Trp, Val, Pro, and Tyr in period 1. Additionally, the AID of starch was greater (P < 0.05) in period 1 than in period 2. The AID of CP, Arg, His, Ile, Leu, Met, Phe, Thr, Trp, Val, Pro, and Tyr in period 2 was greater (P < 0.05) than in period 1. There were no differences among treatments or periods observed for AID of AEE. The second experiment tested the hypothesis that including IQ in diets fed to young growing pigs increases the apparent total tract digestibility (ATTD) of gross energy (GE) and concentrations of DE and ME of the diets. Twenty-four gilts and 24 barrows (13.67 ± 1.35 kg) were allotted to 8 diets and 6 replicate pigs per diet. A basal diet consisting of corn and SBM and a second basal diet consisting of corn, SBM, and distiller’s dried grains with solubles (DDGS) were prepared. Six additional diets were prepared by adding 0, 90, 180, or 360 mg/kg IQ to each of the 2 basal diets. Pigs were housed in individual metabolism crates for the 12 d experimental period, which included a 5 d adaptation period and a collection period from d 6 to 12. Dietary inclusion of IQ in corn-SBM and corn-SBM-DDGS diets had no effect on overall energy digestibility. Addition of IQ to corn-SBM-DDGS diets quadratically increased (P < 0.05) average daily feed intake (ADFI), dry feces output, and fecal GE output of pigs with the greatest values observed in the 90 and 180 mg/kg diets. Additionally, IQ linearly decreased (P < 0.05) the ME of corn-SBM diets, with the highest ME value in the 90 mg/kg diet. The third experiment tested the hypothesis that dietary inclusion of IQ improves growth performance and gut health of weanling pigs. A total of 160 pigs (6.33 ± 0.61 kg) were allotted to 4 corn-SBM based diets with 4 pigs per pen and 10 replicate pens per treatment. A 3-phase feeding program was used with d 0 to 8 as phase 1, d 8 to 21 as phase 2, and d 21 to 34 as phase 3. Within each phase, the 4 diets were supplemented with 0, 90, 180, or 360 mg IQ/kg complete diet. There were no effects of IQ on overall growth performance of weanling pigs; however, ADFI quadratically (P < 0.05) decreased in phase 1 and linearly decreased (P < 0.05) in phase 2, G:F quadratically increased in phase 3 (P < 0.05), and ADG decreased (quadratic; P < 0.05) during phase 1. Plasma urea nitrogen tended to increase in phases 2 and 3 (linear; P < 0.10) if IQ was added to the diet and total plasma protein quadratically increased (P < 0.05) in phase 1. In the ileum, crypt depth and lamina propia thickness decreased (quadratic; P < 0.05) and there was a tendency for the villus height: crypt depth ratio to increase (linear; P < 0.10). Neutrophil infiltration tended to increase in the jejunum and decrease in the ileum (quadratic; P < 0.10) with the greatest response in the tissues of pigs fed the diet with 180 mg IQ/kg diet. Results indicate that dietary inclusion of IQ may improve gastrointestinal health and increase apparent ileal digestibility of AA and starch, with the greatest response observed at inclusions between 90 and 180 mg IQ/kg complete diet.