Author: Maegan Diep, Class of 2029

Figure 1. An image of a cow.

Fescue toxicosis (FT) is a common condition brought about by endophyte bacterium-infected varieties of tall fescue plants. While beneficial to the plant, it is harmful to the cattle that consume the plant. In consequence, vasoconstriction, or the narrowing of blood vessels, is a common symptom associated with FT, harming their cardiovascular systems. Should these conditions remain untreated, the animal is put at an increased risk of right-sided heart failure and death. These detrimental effects, alongside heat stress induced by warmer temperatures, can ultimately impact cattle welfare and have larger implications in the beef industry. 

In order to further investigate the impact of FT and thermal stress, researchers at the Upper Piedmont Research Station in North Carolina conducted a study measuring the changes in the pulmonary arterial pressure (PAP) and other physiological factors. Thirty-six purebred Angus heifers between 9-11 months old were randomly assigned to one of two groups: an endophyte-infected (EI) or endophyte-free (EF) ration. All heifers were exposed to an acclimation period before the 49-day experimental period. On day 1, iButton temperature data loggers were inserted to record vaginal temperatures (VT) throughout the entire study. Beginning on day 2, PAP measurements were collected every 20 days by inserting a catheter in the heifer’s neck. Additionally, the cattle comfort index (CCI) was utilized to assess comfort under exposure to various levels of heat stress. 

The PAP results revealed that consumption of the EI ration did not necessarily produce vasoconstriction, but demonstrated that PAP may be affected by heat stress instead. Researchers observed symptoms of FT, such as increased VT, but no differences in mean PAP values between the two groups. 

The results of VT monitoring displayed how cattle body temperature and thermoregulatory abilities are influenced by FT and the fluctuations of temperature throughout the day. Though EI heifers had greater body temperatures compared to their EF counterparts, both groups still exceeded the established fever threshold during the study. The thermoregulation imbalance is consequently due to the given timeline of the study, in which all heifers were exposed to heat stress during the summer months. To further reinforce conclusions, there was also a reduction reported via the CCI in the last PAP evaluation, compared to the first and second evaluations.

Heat stress, when paired with FT, can amplify symptoms, as vasoconstriction and retention of winter hair coats—another FT symptom—can reduce cattle’s abilities for heat exchange. It is crucial for future research to explore ways to mitigate the adverse effects of both FT and heat stress to ensure greater animal welfare and productivity. 

Works Cited:

[1] Rajo-Gomez, E. B., Pickworth, C. L., Weaver, A. R., Foster, D. M., & Poole, D. H. (2025). Influence of heat stress and fescue toxicosis on the pulmonary arterial pressure of beef heifers. Journal of Animal Science, 103. https://doi.org/10.1093/jas/skaf059. 
[2] Image retrieved from: https://itoldya420.getarchive.net/amp/media/cow-on-a-field-1ac5bd