In search of novel phenotypes and biomarkers associated with tall fescue and heat tolerance in crossbred beef cattle

Dawn A Koltes^1,2, Sarah K Chewning¹, Jeremy G Powell¹, Laura M Meyer¹, John Tucker^1,3, Donald S Hubbell, III^1,3, and James E Koltes¹

¹ Department of Animal Science, University of Arkansas, Fayetteville, AR USA 72701; ² Department of Poultry Science, University of Arkansas, Fayetteville, AR USA 72701; ³ Livestock and Forestry Research Station, Division of Agriculture, Batesville, AR USA 72501

Tall fescue grass is grazed by >8.5 million cattle in the southern US. A fungal endophyte in fescue confers heat and drought tolerance, but produces ergot alkaloid mycotoxins (i.e. ergot valine) that increase susceptibility to heat stress in cattle. Ergot alkaloid toxicity causes vasoconstriction, reduced serum prolactin concentrations, increased body temperature and reduced animal growth and reproductive performance. The objective of this study is to identify new biomarkers of heat and toxic fescue stress. A total of 100 fall-calving cows were selected for allocation to pasture treatments (N=50 toxic; N=50 non-toxic), with ergot valine levels monitored monthly. Within pasture, two different sire breeds (Hereford and Charolais) were phenotyped during cold (March) and hot (August) ambient temperatures. Phenotypes collected included: hair shedding, body weight, body condition score (BCS), respiration rate, caudal vein blood pressure, and rectal temperature. Blood samples were collected to measure prolactin, metabolites, minerals, general immune cells and RNA. Samples were also collected to assess microbiomes. During the final week of the study, iButton thermosensors recorded vaginal temperatures every 5 minutes during greatest heat stress. Significant differences due to pasture treatment were observed for body weight (p<0.02), BCS (p<0.05), respiration rate (p<0.002) and blood pressure (p<0.02). Phenotypic differences were observed due to ambient temperature for body weight, BCS, respiration rate, and rectal temperature (p<0.0001) with significant pasture by ambient temperature interactions for rectal temperature and respiration rate (p<0.0001) and BCS (p<0.007). Additional analyses to identify biomarkers associated with heat and fescue toxin stress are underway.