The Effects of Fermenting Psophocarpus tetragonolobus Tubers with Candida tropicalis KKU20 as a Soybean Meal Replacement Using an In Vitro Gas Technique

This study evaluated the effects of replacing soybean meal (SBM) with winged bean tuber (<i>Psophocarpus tetragonolobus</i>) fermented using ruminal <i>Candida tropicalis</i> KKU20 on gas kinetics, ruminal fermentation, and degradability using the in vitro gas production technique. A 3 × 4 factorial arrangement in a completely randomized design was used. Factor A included three roughage-to-concentrate (R:C) ratios: 60:40, 50:50, and 40:60. Factor B consisted of four levels of SBM replacement with yeast-fermented winged bean tuber (YFWBT): 0%, 33%, 66%, and 100%. Fermentation with <i>C. tropicalis</i> KKU20 increased the crude protein content of winged bean tuber by 13.32%. No significant interaction was found between the R:C ratio and YFWBT level for cumulative gas production at 24, 48, or 96 h (<i>p</i> > 0.05). Cumulative gas production at 96 h was not affected by either factor. However, at 24 and 48 h, gas production increased with higher proportions of concentrate (<i>p</i> < 0.05). Both the R:C ratio and YFWBT level significantly influenced pH and ammonia-nitrogen (NH₃-N) concentrations (<i>p</i> < 0.01). After 24 h, NH₃-N ranged from 7.66 to 13.76 mg/dL, rising to 16.44-16.63 mg/dL after 48 h. A significant interaction (<i>p</i> < 0.01) was observed for in vitro dry matter degradability (IVDMD) and in vitro organic matter degradability (IVOMD). Increasing concentrate levels along with YFWBT inclusion improved degradability at both incubation times. The highest IVDMD (64.49%) and IVOMD (65.81%) were recorded at 48 h in the 40:60 R:C ratio with 33% YFWBT. At 48 h, a significant interaction effect (<i>p</i> < 0.05) was also found for total volatile fatty acid (VFA) and propionic acid (C3) concentrations. Total VFA peaked in the 40:60 group with 33% YFWBT (104.31 mM), while the highest C3 concentration (26.22%) was observed in the same R:C group with 66% YFWBT. At 24 h, total VFA was significantly affected by the R:C ratio (<i>p</i> < 0.05), with the lowest values in the 60:40 group and increasing in response to higher concentrate and YFWBT levels. Incorporating YFWBT at 33% in diets with an R:C ratio of 40:60 optimized degradability, indicating its potential as a sustainable alternative to SBM in ruminant nutrition.