This dissertation documents efforts to evaluate metabolic variables and the fecal microbiome in adult horses fed grass hay. In the first study, eight Arabian geldings limit-fed an 18% vs. 12% non-structural carbohydrate (NSC) hays in a cross-over design during two 28-day periods were included to evaluate the influence of grass hay NSC on serum insulin and plasma glucose concentrations. Serum insulin concentrations was higher in geldings fed the 18% NSC hay; however, this difference was only detected on day 7 and none of the geldings developed hyperinsulinemia. Blood glucose concentrations did not differ between hay groups.

The second and third studies were extensions of the first and were conducted to use denaturing gradient gel electrophoresis (DGGE) and real-time PCR in evaluating the effect of forage carbohydrates on equine fecal bacteria diversity and abundance, respectively. Fecal microbiomes were similar (80.5-87.9%) between geldings. The abundance of bacteria belonging to the Firmicutes phylum increased (p = 0.02) in the feces of geldings fed 12% NSC hay (mean 8.06 range [8.03-8.11] log10 copies/g feces) compared to the feces of the same geldings when fed the 18% NSC hay (7.97 [7.97-7.98] log₁₀ copies/g feces). The Firmicutes (43.7%), Verrucomicrobia (4.1%), Proteobacteria (3.8%), and Bacteroidetes (3.7%) phyla dominated the fecal microbiomes. This work was the first to report the presence of the Actinobacteria, Cyanobacteria, and TM7 phyla in the equine fecal or gut microbiome. There was a high abundance (38%) of unclassified bacterial sequences in the gelding fecal microbiome.

In the fourth study, 5 overweight adult mixed-breed mares and 5 adult mixed-breed mares in moderate condition, limit-fed a grass hay, were used to evaluate the effect of body condition on diet digestibility, plasma and fecal volatile fatty acid (VFA) concentrations, and fecal bacterial abundance. Hay, fecal, and blood samples were taken daily for 4 days after a 10 day adaptation period. A difference in hay digestibility, fecal VFA concentration, or bacterial abundance was not detected between overweight mares and mares in moderate condition. Plasma acetate, a product of microbial fermentation of fiber, was higher in the overweight mare group.