Analysis of Fecal Microbiome of Crossbred Beef Cows Grazing Toxic or Novel Fescue.

Tall fescue is the primary forage for grazing beef cows in the southeast United States. It is responsible for a variety of adverse physiological effects, including: hyperthermia, poor conception rates, decreased weight gain, and vasoconstriction that can lead to loss of extremities. The objective of this study was to determine if variation in fecal bacterial structure and content can indicate exposure to toxic fescue or the severity of its impact on host phenotype. A total of 100 fall-calving cows were blocked by sire breed (Charolais or Hereford) and parity (first, second, or third) on either toxic (n=50) or novel (non-toxic; n=50) endophyte infected tall fescue pastures for five months (March to August). Pasture toxin levels were monitored monthly. Pastures were rotated every other week. One novel fescue pasture was identified as contaminated with toxic fescue (ergovaline > 300ppb), leading to exposure of 21 animals to toxic fescue for two weeks each month, alternated by novel fescue. This group was analyzed as the alternating treatment condition (Alt: n=21). Traditional measures of fescue stress were recorded monthly, including hair score, and weight gain. Rectal temperatures were recorded on the first (pre-exposure) and last day of the trial. Fecal samples were collected in both March and August. Bacterial DNA was isolated for 16s rRNA sequencing using the Illumina Miseq platform. Data was processed using mothur v.1.39.5. For August samples, Bray-Curtis distance-based comparison indicated little to no difference in community structure among treatment conditions (ANOSIM, R =0.069376, p=0.037). Shannon index, observed operational taxonomic units (OTUs) and selected OTU abundance were analyzed with Proc Mixed of SAS, where the class variables included: treatment, breed, parity, pregnancy, and breed*treatment interaction. Shannon index measure of diversity was greater (p=0.0176) in the Alt. than toxic but was not different (p=0.326) from novel. Shannon diversity did not differ (p=0.1871) between toxic and novel, however, number of observed OTU’s was greater (p=0.0241) for toxic than novel. Initial random forest analysis in R 3.2.4 indicated that an OTU from the family Coriobacteriaceae was most predictive of toxin exposure. Animals grazing toxic fescue exhibited an 8.4-fold increase in abundance compared to novel animals (p <0.0001). Herefords hosted 2-fold more of this OTUs than Charolais regardless of treatment (p= 0.0309). Hereford sired individuals exhibiting a greater increase in the abundance of the Coriobacteriaceae OTU on toxic pasture. Further analyses are underway to track community changes overtime.