ABSTRACT:This study was conducted to evaluate the effects of dietary supplementation of protected organic acid blends including medium chain fatty acids and different nutrient density diets on growth performance, nutrient digestibility and faecal noxious gas content in growing pigs. A total of 80 crossed [(Landrace × Yorkshire) × Duroc] growing pigs with an initial body weight (BW) of 22.61 ± 2.32 kg were used in a six-week trial. Pigs were randomly allocated into one of four treatment groups in a 2 × 2 factorial arrangement with two levels of nutrient density (high and low) and protected organic acid (0% and 0.1%) according to their sex and BW (five replicates with two gilts and two barrows per pen). Pigs fed high nutrient density diets had increased (P < 0.05) average daily gain (ADG0 and gain: feed (G : F) than those fed low nutrient density diets. Likewise, pigs fed protected organic acid diets exhibited increased (P < 0.05) ADG compared with pigs fed no additional protected organic acids. An interactive effect (P = 0.03) between organic acid and nutrient density was observed on feed conversion by pigs. Dry matter (DM) digestibility tended to improve (P = 0.08) in pigs fed high nutrient density diets compared with low nutrient density diets. However, nitrogen (N) and energy (E) digestibility was not influenced by the nutrient density. Likewise, protected organic acid supplementation did not influence (P > 0.05) DM, N or E digestibility. Organic acid supplementation reduced (P < 0.05) H 2 S content from faeces on Day 1, Day 3, Day 5 and Day 7 of incubation. Low nutrient density diets led to a reduction (P < 0.05) in H 2 S gas content on Day 1 of incubation. No interactive effect on faecal noxious gas content was observed between nutrient density and organic acid. In conclusion, dietary supplementation of protected organic acids with a high nutrient diet improved growth performance and reduced H 2 S acid emission.Keywords: digestibility; growing pig; growth performance; microflora; protected organic acid List of abbreviations ADG = average daily gain, ADFI = average daily feed intake, BW = body weight, DM = dry matter, E = energy, G : F = feed efficiency, H 2 S = hydrogen sulphide, MCFA = medium chain fatty acid, N = nitrogen