Weather and pasture characteristics associated with outbreaks of perennial ryegrass toxicosis in southern Australia

Abstract

Severe outbreaks of toxicosis caused by the natural endophyte Neotyphodium lolii in perennial ryegrass (Lolium perenne L.) have occasionally resulted in heavy loss of livestock in Victoria and Tasmania. Meteorological records were examined from locations where acute toxicosis was recorded. High rainfall (>350 mm) in spring–summer was an obvious common feature; typically, it prolonged the period of high growth rate of pasture, increasing predicted spring–summer (September–March) growth (by ~36%) and increasing the predicted digestibility of organic matter of summer pasture (1 February) by 2.7% units (up to 55.6%). In years when severe toxicosis occurred, such rainfall was accompanied by dry, warm conditions in March and April, viz. mean maximum daily temperatures on the mainland of ≥21°C (March) and ≥18°C (April). During summer–autumn 2002, pastures from 10 Victorian farms on which acute perennial ryegrass toxicosis resulted in the loss of >7000 sheep/deer, were found to be dominated by perennial ryegrass. The ryegrass endophyte-produced toxins, ergovaline and lolitrem B, were detected in 74% and 100%, respectively, of composite faecal samples collected from acutely affected animals. The populations of perennial ryegrass all tested positive for Neotyphodium lolii endophyte, with a mean infection frequency of 90.5% (s.e.m. 1.81). The mean mycelial mass in basal tillers of perennial ryegrass was estimated at 66.00 (s.e.m. 3.40) from a sample of 20 plants, each of which was assessed on a scale of 1–5. Mycelial mass accounted for 19–20% of the variation in the concentration of ergovaline and lolitrem B in perennial ryegrass. Toxin concentrations in the plants where acute toxicosis was observed exceeded the tolerance levels for sheep (ergovaline 0.8 mg/kg; lolitrem B 1.8 mg/kg) in the minor (green) fraction of the plant in all but one affected pasture for ergovaline and all but two for lolitrem B. Maximum concentrations recorded were 4.3 and 4.6 mg/kg for ergovaline and lolitrem B respectively. Ergovaline increased in an asymptotic relationship with lolitrem B, which accounted for 45% of the variation in ergovaline. The concentrations of ergovaline and lolitrem B in the whole plant (green and dead fractions combined) were not greater than those recorded from randomly sampled pastures in earlier seasons, when only occasional mild cases of toxicosis were reported. Further examination of stored grass samples collected during the 2002 outbreak recently revealed unidentified peaks on the chromatograms for both ergovaline and lolitrem B; peaks that are not seen on chromatograms for toxic perennial ryegrass from New Zealand, where the expression of perennial ryegrass toxicosis is usually milder and rarely fatal.