Influence of Short Term Dietary Starch Inclusion on the Equine Cecal Microbiome

The objective of this study was to determine bacterial community profiles of the equine cecum in response to abrupt inclusion of varying levels of dietary starch. Seven cecally cannulated Quarter horse geldings (497 to 580 kg) were utilized in a crossover design with two 28 d periods and a 28 d washout between each. Horses were randomly assigned to dietary treatments consisting of a commercial concentrate offered as-fed at either 0.6% (LS) or 1.2% BW (HS) daily that was divided into 2 meals at 12 h intervals. Prior to the start of each period, horses were allowed ad libitum access to coastal bermudagrass (Cynodon dactylon) hay. Concentrate was fed on d 1 with no adaptation. Cecal fluid was collected on d 1 at h 0 and 3, 6, 9, and 12 h relative to the morning concentrate meal on d 1. Additional samples were collected 6 h post feeding on d 2, 3, and 7 of each period. Cecal contents were used to determine pH, VFA concentrations, and extract genomic DNA. The V4-V6 region of 16S rRNA gene was amplified by PCR and sequenced on the Roche 454 FLX platform. Sequence analysis was performed with QIIME and data were analyzed using MIXED procedure of SAS. Cecal pH tended to decrease (P ≤ 0.09) in HS horses for the first 12 h after the first concentrate meal and remained lower (P ≤ 0.05) the following 7 d in HS horses. Total VFA were greater (P < 0.05) in HS horses in the initial 12 h and 7 d after addition of concentrate. Although species richness determined by Chao1 was unchanged (P > 0.2) over the initial 12 h, it did decrease (P = 0.01) over 7 d for both treatments. Community diversity determined by the Shannon index tended to decrease (P = 0.06) over the 7 d with the greater decrease (P = 0.09) in HS horses. Relative abundances of Paraprevotellaceae increased (P < 0.01) and Streptococcaceae tended to increase (P ≤ 0.10) in HS horses in the first 12 h. Over 7 d, relative abundances of Paraprevotellaceae, Veillonellaceae, and Succinivibrionaceae increased (P ≤ 0.02) in HS horses compared with LS horses. Abrupt and short-term exposure to dietary starch does alter cecal fermentation and microbial community structure in horses; however, the contributions of specific bacterial communities to horse health needs further investigation.