Date of Graduation


Document Type


Degree Name

Master of Science in Animal Science (MS)

Degree Level



Animal Science


James Koltes

Committee Member

Jiangchao Zhao

Second Committee Member

Jason Apple

Third Committee Member

Jiangchao Zhao

Fourth Committee Member

James Koltes

Fifth Committee Member

Steven Ricke


Bacteria, Breeding, Cattle, Fescue, Genetics, Microbiome


Fescue toxicosis in grazing beef cattle is caused by consumption of Ergot alkaloids associated with tall fescue (toxic fescue) and is responsible for substantial economic loss to the U.S. beef industry. Cattle consuming toxic fescue suffer adverse physiological responses, such as: hyperthermia, increased respiration rate (RR), poor reproduction and growth performance. Other adverse responses to fescue toxins include the retention of a winter hair coat and vasoconstriction in the extremities, which can lead to tissue necrosis. Identifying cattle with reduced susceptibility to fescue toxins would allow for efficient use of fescue pastures; thus, the objectives of this thesis was to: 1) document breed differences in heat and fescue stress phenotypes; and 2) identify fecal bacteria taxa associated with performance on toxic fescue. One hundred crossbred beef cow sired by Charolais or Hereford, and of parities 1, 2, or 3, were allocated to graze toxic tall fescue paddocks (Toxic; n=54) or novel endophyte infected fescue paddocks (Novel; n=46) for 5 months (March to August 2016). Heat and fescue stress phenotypes and fecal samples were collected pre-treatment (March) and post-treatment (August). Cows on toxic fescue had higher (P <0.0001) rectal temperatures (RT), greater systolic blood pressures, and slower hair coat reduction rates (HRR) than cows on novel fescue. Hereford-sired cows had higher (P < 0.0002) RT, higher (P <0.0001) HCS, and slower (P < 0.0001) HRR than Charolais-sired cows. There were tendencies for sire breed × fescue-type interactions for ADG (P = 0.06) and heat accumulation measured by trapezoidal area under of the curve (P=0.07). These results indicated Charolais-sired cows had superior tolerance to fescue toxins. Increased abundance of bacterial taxa from Ruminococcaceae, Lachnospiracceae, and Bacteroides in post-treatment fecal samples were associated with fescue toxin exposure. OTUs from Rikenellaceae, Clostridiales, Odoribacter, and Lachnospiraceae in pre-treatment samples were correlated with of hair reduction rate and growth performance while grazing toxic fescue. Breed difference exist in bacterial taxa and may serve as indicators of toxin exposure and performance potential on tall fescue.