226 In vitro digestibility of acid detergent soluble crude protein by equine fecal microbes is decreased with diet lignification.

Abstract Research in ruminants has demonstrated that rumen microbes use acid detergent soluble crude protein (ADSCP) as a nitrogen source. However, it is unknown whether the hindgut microbes in the horse can use ADSCP and whether increases in acid detergent lignin (ADL) affects the use of ADSCP compared to hemicellulose (HEMI). Thus, our objectives were to determine if hindgut microbes from equine feces can use ADSCP and if in vitro digestion of ADSCP is limited by increased ADL. Four Quarter Horse geldings were fed 1.5% BW DM/d of coastal Bermudagrass hay and 0.5 kg DM/d of a commercial concentrate for 14 d. On Day 14, fresh feces were collected for microbial inoculum. Four diets were formulated to be isonitrogenous using soybean meal, coastal Bermudagrass hay, alfalfa hay, and whole oats. Mixtures were formulated to increase ADL/HEMI ratio with increasing ADSCP/HEMI ratio (H-H), increasing ADL/HEMI and decreasing ADSCP/HEMI (H-L), decreasing ADL/HEMI and increasing ADSCP/HEMI (L-H), or decreasing ADL/HEMI and decreasing ADSCP/HEMI (L-L). The dried, ground feed mixtures (0.5 g), sealed in F57 filter bags, were pre-washed with neutral detergent solution and placed in triplicate into 8 jars between 2 DaisyII incubators in a split-split-plot batch-culture design for 48 h. One bag of the triplicate was used to assay residual neutral detergent fiber (NDF) and neutral detergent insoluble crude protein (NDICP) residue. The remaining two bags of the triplicate were analyzed for residual acid detergent fiber (ADF) and acid detergent insoluble crude protein (ADICP). Fiber fractions were determined as HEMI = NDF – ADF, and CELL = ADF – ADL with ADSCP calculated as ADSCP = NDICP – ADICP. Averaging of replicates within a jar resulted in n = 8/mixture. Data were analyzed within R Statistical Program (v4.4.0) using a 3-way ANOVA with significance set at P ≤ 0.05 and trends at P ≤ 0.10. In vitro HEMI digestibility (g/kg HEMI) was greater in L-H than all other treatments (P < 0.001) with L-L similar to H-H (P = 0.184) but greater than H-L (P = 0.015). However, ADSCP digestibility (g/kg ADSCP) decreased in L-L compared to L-H (P = 0.05) and tended to be less than H-H (P = 0.081). When ADSCP digestibility was expressed as g/kg HEMI, H-H and L-H were greater than H-L (P = 0.02; P = 0.05) and L-L (P = 0.007; P = 0.019), respectively. These results indicate that increasing lignification of HEMI decreases digestibility of HEMI and ADSCP, and that microbes may use ADSCP as a source of carbon skeleton when HEMI digestibility is limited by ADL. Overall, increasing the amount of both ADSCP and HEMI to hindgut microbes may improve fiber digestibility in the horse.