Influence of heat stress and fescue toxicosis on the pulmonary arterial pressure of beef heifers.

Peripheral vasoconstriction is a major consequence of fescue toxicosis (FT). However, it remains unclear if major blood vessels, such as the pulmonary artery, are impacted by this disease. This study aimed to evaluate changes in pulmonary arterial pressure (PAP) and different physiological parameters in replacement beef heifers consuming an endophyte-infected (EI) ration. Purebred Angus heifers (216.3 ± 6.1 kg; 9 to 11 mo of age; n = 36) were randomly separated and fed either an EI or endophyte-free (EF) total mixed ration for 49 d. On day 1, iButton temperature data loggers were inserted intravaginally to record temperature every 20 min throughout the entire study. The cattle comfort index (CCI) was used to assess comfort and define exposure to heat stress as thermoneutral, mild heat stress (MILD-HS), or moderate heat stress (MOD-HS). PAP was measured every 20 d to assess its response to ergot alkaloid and heat stress exposure. Weekly measurements were recorded to monitor growth and physiological responses of both treatment groups. Data were analyzed using the MIXED procedure of SAS with repeated measures. Both groups were exposed to MILD- and MOD-HS on several occasions. An interaction between PAP results and heat stress exposure was identified, as both groups reported increased PAP values as CCI values decreased. The EF heifers ended the study with greater body weight (P < 0.05), average daily weight gain (P < 0.01), body condition score (P < 0.01), and lower hair shedding score (P < 0.01) than EI heifers. In contrast, EI heifers maintained greater vaginal temperatures throughout the study (P < 0.01). Additionally, EI heifers reported lower caudal artery diameters (P < 0.01) than the EF heifers, but no differences were observed in caudal vein diameters (P > 0.05). Lastly, no differences were observed in the packed cell volume and prolactin concentrations (P > 0.05). While the EI group displayed classical symptoms of FT, including caudal artery vasoconstriction, the PAP results indicate that ergot alkaloid consumption does not induce vasoconstriction on the pulmonary artery, but rather reflect that PAP may be altered by the exposure to heat stress. Additional research should be conducted in regions where changes in PAP are of interest to fully understand the effects of heat stress exposure on the cardiopulmonary system of cattle. Moreover, future investigations in EI fescue-dominant environments should focus on developing strategies to mitigate the negative consequences of FT and heat stress, in order to enhance animal welfare, productivity, and profitability in the beef industry.