225 Effect of increased hemicellulose-bound nitrogen and extended incubation time on in vitro fiber digestibility in horses.

Abstract Previous work has demonstrated that hindgut microbes in the horse can utilize various nitrogen sources for microbial metabolism. While microbes prefer soluble non-protein nitrogen, forages have hemicellulose-bound nitrogen (HBN) that improves fiber digestion for ruminants via hemicellulose (HEMI) and cellulose (CELL) digestion. However, it is unknown if increases in HBN-to-HEMI ratios (HBN/HEMI) improve fiber digestion in horses. Our objective was to determine if titrating HBN/HEMI increased in vitro HEMI digestibility (IVHD) and in vitro cellulose digestibility (IVCD) in horses and if fiber digestibility increased with extended incubation time (48 vs. 72 h). Four Quarter Horse geldings were fed 1.5% BW DM/d of coastal Bermudagrass hay (CBH) and 0.5 kg/d of a commercial concentrate for 14 d. On Day 14, fresh feces were collected for microbial inoculum. Feed mixtures [alfalfa hay (ALF), CBH, 60-40 ALF-CBH (A60), and 60-40 CBH-ALF (C60)] were formulated to have different HBN/HEMI. Dried, ground feed mixtures (0.5 g) sealed in F57 filter bags (n = 8/mixture; n = 16/incubation time) were pre-washed with neutral detergent solution and placed in duplicate into 8 jars between 2 DaisyII incubators (48- vs. 72-h incubation; 4 jars/incubator) in a split-split-plot batch-culture design. After incubation, samples were washed with water, dried, and weighed for residual neutral detergent fiber (NDF) then washed with acid detergent solution and 72% sulfuric acid to analyze for residual acid detergent fiber (ADF) and acid detergent lignin (ADL). A duplicate set of samples were analyzed for pre-fermentation NDF, ADF, ADL, neutral detergent insoluble crude protein (NDICP) and acid detergent insoluble crude protein (ADICP). Fiber fractions were determined as HEMI = NDF – ADF, and CELL = ADF – ADL, with HBN calculated as HBN = NDICP – ADICP. Digestibility measures were expressed as IVHD and IVCD. Data were analyzed within R Statistical Program (v4.4.0) using an ANOVA and a Pearson’s correlation with significance set at P ≤ 0.05. Mixture IVHD decreased in ALF versus all other mixtures (P < 0.001) with A60 similar to C60 (P = 0.12) but less than CBH (P < 0.01). No mixture effect occured for IVCD (P = 0.74). The 72-h incubation did not affect IVHD (P = 0.28), but increased IVCD (P < 0.001) compared to 48-h. Mixture IVHD was only moderately correlated to HBN/HEMI (r = -0.57; P < 0.01) and HBN (r = 0.61; P = 0.001), but more correlated to the ADL-to-HEMI ratio (ADL/HEMI; r = -0.88; P < 0.001). Overall, increasing HBN/HEMI did not improve IVHD or IVCD, likely due to increases in ADL/HEMI. However, increased retention time improved IVCD, indicating a shift in microbial fermentation from HEMI to CELL over time due to the enzymatic lag time required for CELL degradation in the hindgut of the horse.