Mortalities among sheep grazing Phalaris tuberosa

Abstract

A description is given of the circumstances in which sudden deaths, now considered to be due to poisoning by tryptamine alkaloids, occurred among sheep grazing Phalaris faberosa L. in the Australian Capital Territory between 1960 and 1965. Mortalities usually occurred in autumn and early winter on pastures that, at the time, were pronouncedly Phalaris dominant. Concentrations of nitrate nitrogen were high in toxic pastures indicating the possibility of a positive relationship between levels of nitrate nitrogen and tryptamine alkaloids in Phalaris. There was a higher proportion of deaths among sheep grazing at low than at high stocking rates. Field evidence indicated that deaths were more likely on deferred and on rotationally grazed than on continuously grazed pastures. Effects of dosing sheep with cobalt chloride and of fasting them for 48 hours before access to a toxic Phalaris pasture were investigated in a short term experiment in 1965. Nearly twenty per cent of the sheep died during the five days that they were on Phalaris pastures. Deaths occurred among fed and fasting sheep. Mortality rates were higher among the hungry sheep but differences due to prior feeding were not statistically significant. Dosing with cobalt chloride at rates higher than those reported effective for chronic Phalaris staggers did not prevent sudden deaths.