Several wild and domestic ruminant species and horses apparently will
not consume leafy spurge (Euphorbia esula) while grazing range and pas
ture lands. It has been demonstrated that leafy spurge can elicit cond
itioned food aversions in cattle and sheep, and the aversion-eliciting
capacity of leafy spurge may account for why cattle seldom graze this
nutritious plant and why sheep may not readily consume it at some loc
ations. The identity of the aversive compound(s) in leafy spurge is un
known, but several different diterpenoid ingenol esters have been isol
ated from its tissues, and we suspect that one or more ingenol esters
may be aversion-eliciting compounds in leafy spurge. The objectives of
this study were to determine whether or not leafy spurge is aversive
to laboratory rats and if a crude acetone extract of leafy spurge, pre
sumably containing ingenol esters and other phytochemicals, could gene
rate an aversive response in sheep and laboratory rats. An additional
objective was to determine whether or not a particular ingenol monoben
zoate, which may be similar to ingenol esters in leafy spurge, might a
lso elicit an aversive response from rats. Rats exhibited food aversio
ns associated with leafy spurge (P < 0.05). An acetone extract of leaf
y spurge induced conditioned food aversions in both sheep and rats (P
< 0.01). The ingenol 3-monobenzoate also induced conditioned food aver
sions in rats (P < 0.01). Our interpretation of these data is that mts
can be used as a model for cattle and sheep with respect to their ave
rsion to leafy spurge ingestion. Additionally, we suggest that one or
more ingenol esters may be aversion-inducing agents in leafy spurge. H
owever, others may exist in leafy spurge that are also aversive or are
the only or prime aversive chemicals.