Aim: To determine the effects of feeding ryegrass seed containing ergovaline to sheep selected for resistance or susceptibility to ryegrass staggers on concentration of lysergol (a metabolite of ergovaline) in urine, prolactin in plasma, rectal temperature and respiration rate.
Methods: Two experiments were carried out using 12 Romney crossbred ewe lambs aged 9 months, comprising animals resistant (n=4), susceptible (n=4) or outcross (n=4) to ryegrass staggers. In Experiment 1, sheep were given either a single (Part A) or six (Part B) feed (s) of endophyte-infected seed containing ergovaline at 30 mg/kg dry matter (DM), at 42 μg ergovaline/kg bodyweight (BW), to simulate acute and chronic exposure to ergovaline, respectively. The concentration and excretion of lysergol in urine and concentration of prolactin in plasma were measured over 3 and 12 days, for Parts A and B respectively. In Experiment 2, after a recovery period of 7 days, the same sheep were fed with ergovaline at 67 μg/kg of BW for 7 days. Soon after the seventh feed the sheep were moved to a hothouse at 36.5°C and 60% humidity, and 3 h later their rectal temperatures and respiration rates were measured.
Results: The concentration of lysergol and excretion in urine increased to a peak between 6 and 9 h after exposure to ergovaline whereas the concentration of prolactin in plasma was markedly reduced. Differences in concentration and rate of excretion of lysergol in urine between animals resistant, outcross and susceptible to ryegrass staggers were not significant (p>0.1). The animals resistant to ryegrass staggers had a lower rectal temperature (p<0.05) and a faster respiration rate than the outcross and susceptible groups when exposed to high ambient temperature and high humidity.
Conclusions: This study showed that excretion of lysergol in urine increased with each exposure of sheep to ergovaline. Animals genetically resistant to ryegrass staggers exhibited a lower rectal temperature and a faster respiration rate than those susceptible, demonstrating the possible cross resistance of sheep to ergovaline in a population originally selected for resistance to ryegrass staggers. Hence potential exists to select animals resistant to ryegrass staggers that are also resistant to ergovaline.