Varying Corn Flour Inclusion Levels Modulate Fiber Fraction Degradation and Nutritional Value of Rice Straw via Co-Extrusion

Rice straw, one of the most abundant agricultural residues worldwide, remains significantly underutilized as a ruminant feed source owing to its intrinsic lignocellulosic recalcitrance. This study investigated the effects of co-extruding rice straw with varying proportions of corn flour on nutritional composition and in vitro digestibility for ruminant nutrition. Extrusion was conducted using a twin-screw extruder at 180 °C barrel temperature, 5 MPa pressure, and 50% feed moisture content. Five corn levels were formulated on a dry matter basis: pure rice straw (RS100); three blends with increasing corn flour inclusion: RS75:C25 (75% straw + 25% corn flour), RS67:C33 (67% straw + 33% corn flour), and RS60:C40 (60% straw + 40% corn flour); and pure corn flour (C100) as a control. Chemical composition including neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), cellulose, hemicellulose, water-soluble carbohydrates (WSC), and starch was analyzed. In vitro dry matter digestibility (IVDMD) was determined using a pepsin-cellulase assay. Regression analysis within the practical 0–40% corn flour inclusion range revealed a significant quadratic relationship with IVDMD (R2 = 0.999, p < 0.001). The optimal corn flour proportion was calculated to be approximately 37.5%, which closely matched the RS60:C40 formulation (40% corn flour). Among the tested formulations, RS60:C40 exhibited the greatest extrusion-induced nutritional improvements. Relative to its pre-extrusion values, cellulose decreased by 55.7% (p < 0.05), followed by ADF (16.1%), NDF (12.8%), and hemicellulose (10.2%); IVDMD increased by 34.2% (p < 0.01) and WSC by 56.7% (p < 0.05). Compared with RS100 after extrusion, RS60:C40 raised IVDMD by 49.5% and lowered cellulose by 60.6%. Its IVDMD also surpassed those of RS75:C25 and RS67:C33 (p < 0.05), whereas RS75:C25 showed only marginal improvements. ADL content showed no extrusion-induced change (p > 0.05). Scanning electron microscopy (SEM) of the RS60:C40 formulation revealed that, unlike the intact fibrous structures observed prior to extrusion, post-extrusion samples exhibited extensive disruption of the fibrous matrix. Pearson correlation analysis further supported these findings, showing strong positive correlations between IVDMD and WSC (r = 0.96, p < 0.001) and strong negative correlations between IVDMD and NDF (r = −0.95, p < 0.001). In conclusion, extrusion generally increased IVDMD and WSC while reducing fiber fractions, with the effect depending on corn level. Co-extrusion with 40% corn flour effectively enhanced the nutritional value of rice straw, offering a viable strategy for producing a more digestible ruminant feed.