PSIV-14 Fecal Microbiome of Horses Grazing Integrated Warm- and Cool-Season Grass Rotational Pasture Systems

Abstract

Abstract The objective of this study was to characterize shifts in the fecal microbiota of horses grazing different forage types within integrated cool- and warm-season grass (CSG; WSG) pasture systems and to explore relationships between forage nutrients and microbial composition. Eight mares grazed integrated rotational systems containing mixed CSG and one of two WSG: bermudagrass or crabgrass. Fecal samples were collected after 2–3 weeks grazing WSG, CSG, and following an orchardgrass hay diet (HAY). Forage nutrients were determined by near-infrared spectroscopy and analyzed by two-way ANOVA in SAS (v.9.4). Following DNA extraction, 16S rRNA gene sequence analysis was conducted in QIIME 2 (v.2020.2) with Kruskall-Wallis tests for alpha diversity, Spearman correlation with forage nutrients, and taxonomic assignment with Greengenes. A random forest classifier and regressor determined ability to predict forage type and nutrients based upon bacterial composition. Statistical significance was set at p ≤ 0.05. Forage water-soluble carbohydrates (WSC) were greatest in CSG and lowest in WSG; neutral detergent fiber (NDF) was greater in HAY than CSG or WSG. Species richness and evenness (Shannon Index) was greater in horses adapted to WSG vs. CSG or HAY and was correlated with WSC (rs = -0.49) and ethanol-soluble carbohydrates (ESCrs = -0.62). The random forest classifier resulted in model accuracy of 1.0 and identified amplicon sequence variants (ASV) most important in prediction of forage type. Sixteen ASV were from the order Clostridiales including Lachnospiraceae, Ruminococcacceae, and Veillonellaceae families and the genus Coprococcus. Other important taxa included Prevotella spp., Streptococcus luteciae, and Fibrobacter succinogenes. The regressor accurately predicted WSC (r2 = 0.95) and ESC (r2 = 0.84), but not NDF (r2 = 0.09). These results suggest that the equine hindgut microbiome is impacted by forage type and soluble carbohydrate content; however, further research is required to determine functional and physiological significance in grazing horses.