ABSTRACT

Background and Aim: Oil palm by-products are abundant lignocellulosic resources with considerable potential as sustainable ruminant feed but are constrained by poor digestibility and limited rumen fermentability. White-rot fungi (WRF) produce ligninolytic enzymes capable of modifying lignocellulosic biomass, whereas fungal consortia may enhance substrate bioconversion through complementary enzymatic activities. This study evaluated the effects of single and consortium WRF on the nutritional quality, in vitro rumen fermentation, digestibility, methane (CH4) production, and rumen microbial populations of mixed oil palm by-products.

Materials and Methods: A substrate comprising oil palm fronds, palm kernel cake, and oil palm decanter cake was subjected to 28-day solid-state fermentation using 6% (w/w) inoculum. Five treatments were evaluated: Unfermented control, fermented control without fungal inoculation, Phanerochaete chrysosporium, Trametes versicolor, and their consortium, each with five biological replicates. Nutrient composition was determined after fermentation. In vitro gas production kinetics, rumen fermentation characteristics, CH4 production, digestibility, and metabolizable energy (ME) were evaluated over 72 h using buffered rumen fluid. Rumen microbial populations were quantified by quantitative polymerase chain reaction.

Results: Fermentation significantly increased crude protein (CP) by 24.8%–34.8% while decreasing ether extract, hemicellulose, and non-fiber carbohydrates (p < 0.05). Conversely, relative lignin concentration and structural fiber fractions increased, indicating preferential carbohydrate utilization during fungal growth. All fermented treatments reduced cumulative gas production, total volatile fatty acids, CH4 production, digestibility, and ME compared with the unfermented control (p < 0.05). CH4 production declined by 26.6%–33.6%, although this reduction was accompanied by lower fermentation efficiency rather than improved feed utilization. Ruminal pH and ammonia nitrogen remained within acceptable ranges and were unaffected by fungal inoculation. Total bacterial abundance differed among treatments, whereas the populations of Fibrobacter succinogenes and Ruminococcus albus remained unchanged. The consortium treatment did not provide additional benefits over single-fungus fermentations for nutrient utilization or rumen fermentation responses.

Conclusion: Bioprocessing mixed oil palm by-products with single or consortium WRF altered substrate composition by increasing CP while reducing CH4 production. However, these changes were accompanied by reduced rumen fermentability, digestibility, and energy availability, indicating that the tested fermentation conditions did not improve the overall feeding value of the mixed substrate. Consortium fermentation offered no clear advantage over single fungal cultures, highlighting the need to optimize fungal species combinations and fermentation conditions before practical application as a biological pretreatment for ruminant feeds.

Keywords: biological pretreatment, fungal consortium, lignocellulosic biomass, methane mitigation, oil palm by-products, Phanerochaete chrysosporium, Trametes versicolor, white-rot fungi.