Use of melatonin plus citrulline or arginine to mitigate ergot alkaloid-induced vasoconstriction in sheep

Fescue toxicosis, a form of ergotism, causes chronic vasoconstriction in livestock that negatively impact livestock performance. Melatonin, citrulline, and arginine have all been previously associated with vasodilation. The objective of this experiment was to evaluate dosing of melatonin, melatonin + citrulline, and melatonin + arginine in sheep as potential supplements to offset the vasoconstriction caused by toxic tall fescue seed (E + ). Eighteen Suffolk lambs were randomly divided across 3 treatments: melatonin (MEL), MEL+citrulline drench (MEL-CIT) and MEL+arginine drench (MEL-ARG). Jugular blood samples and Doppler ultrasound measurements of carotid artery were taken daily for 12 days. Lambs only received a total mixed ration (TMR) on d 1–3, on d 4–6, lambs received TMR plus E + seed and on d 7–12 lambs received MEL, MEL+CIT, or MEL+ARG. For all 3 treatments, supplementation of MEL resulted in large increases in plasma melatonin on d 7–12 compared to d 1–6 (P < 0.05). The MEL+CIT resulted in 2- to 3-fold increase in plasma citrulline compared to the other 2 treatment groups (P < 0.05). Plasma arginine was greater in MEL+CIT and MET+ARG lambs on d 7 and 8 than MEL (P < 0.05). There was a steady decline in heart rate and luminal area associated across days with initiation of E + seed dosing (P < 0.01) and did not differ across treatment groups. Conversely, velocity increased with E + seed dosing (P < 0.01) and did not differ when treatments were administered. There was no significant effect of treatment on carotid artery blood flow. Increases in plasma melatonin, arginine, or citrulline did not offset vasoconstriction associated with ergot alkaloids.