Exploring the impacts of fescue toxicosis on the pulmonary arterial pressure of angus cows

Abstract

Vasoconstriction of peripheral blood vessels is one of the hallmark symptoms of fescue toxicosis in cattle. Thus, it was hypothesized that exposure to ergot alkaloids would increase the pulmonary arterial pressure (PAP). The objectives of this study were to examine the relationship between PAP and different physiological parameters of cows grazing either endophyte-infected (EI) or novel-endophyte (EN) fescue, then evaluate changes in PAP and other physiological measurements in cows exposed to EI pastures and deemed as susceptible or tolerant based on animal performance. Pregnant Angus cows at 2 different locations grazed either EI or EN fescue pastures for 14 consecutive weeks starting in early April of 2022. Forage measurements were collected to assess ergot alkaloid exposure throughout the study. In addition to measuring PAP, weekly measurements and blood samples were collected to evaluate physiological responses to ergot alkaloid consumption. The Fescue Toxicosis Selection Method (FTSM) was used for a post hoc analysis to identify cattle as either tolerant (EI-TOL) or susceptible (EI-SUS) when challenged with ergot alkaloid exposure. Data were analyzed using a MIXED procedure of SAS with repeated measures. Cows grazing on EN pastures had greater mean PAP values than EI cows, (P < 0.01), whereas a location effect was identified when comparing both EI-TOL and EI-SUS groups (P < 0.01). Cows exposed to EN pastures had greater average daily gain (ADG) (P = 0.04) and progesterone (P4) concentrations (P < 0.01), and lower hair shedding scores (HSS; P < 0.01) than EI cows. The EI-TOL cows tended to have greater final body weight, ADG and had lower HSS (P < 0.01) than EI-SUS cows. While cattle consuming EI tall fescue exhibited classical physiological changes, the decrease in PAP of cattle consuming EI fescue was unexpected and contradicted the initial hypothesis. Furthermore, the FTSM provides a means to identify animals with superior performance in spite of the chronic exposure to ergot alkaloids. Continued investigations examining the interaction between ergot alkaloid exposure and cardiovascular parameters will lead to a fuller understanding of the disease and are pivotal for developing innovative strategies that enhance best management practices to help guarantee the sustainability of the U.S. beef industry.

Keywords: fescue toxicosis; pregnant cows; pulmonary arterial pressure.

Figure 1.

Experimental timeline used during phase of data collection from early April to mid-July 2022.

Figure 2.

Temperature-humidity Index (THI) by location over time (weeks): BBCFL cows were exposed to a greater THI when compared to UPRS cows (loc P < 0.01; loc × week P < 0.01).

Figure 3.

PAP by treatment (LINES) and ergovaline concentrations (BARS) over time (dates): Cows (n = 35) grazing on novel-endophyte fescue pastures (EN) had greater mean PAP values compared to cows (n = 37) grazing EI fescue pastures at Butner Beef Cattle Field Laboratory (BBCFL). In contrast, ergovaline concentrations were greater in EI pastures when compared to EN pastures. (trt P < 0.01; trt × date P < 0.01).

Figure 4.

(A) PAP by FTSM outcome: mean PAP values did not differ between EI-TOL and EI-SUS groups at BBCFL, but the EI-TOL group had a lower PAP mean than the EI-SUS group at UPRS. (Trt(Loc) P < 0.01). (B) Ergovaline concentrations by location over time: Ergovaline concentrations were greater in EI fescue pastures (EI) at UPRS than EI fescue pastures at BBCFL (loc P < 0.01; date P = 0.08).