Herbivores
Most plants have leaves that contain prickles or spines to help deter animals from eating them. Xanthorrhoeas or Grasstrees do not have this defence mechanism. Instead the leaves produce toxic chemicals that have anaesthetising effects to help younger plants survive.
Xanthorrhoea johnsonii along with X. fulva has been reported to poison cattle that graze on its leaves, especially young leaves, according to a study done by Cock and Kalt (2010) with reports that cattle become uncoordinated and lose condition, become dehydrated, and in some cases die. We have not observed this with our neighbour's cattle. During dry periods we have observed cattle eating the centre out of the plants as a food source which then causes the plant to die. At times we've observed cows eating one plant within a minute or two, then moving on to the next plant, often destroying dozens and dozens of plants within a day. We actually think the cows may like it due to its drug-like affect, evidenced by the smiles on their faces and glassy eyes :).
But according to this study the methanolic extracts of the leaves seem to be responsible for the anesthetic effects which acts as a neuromuscular blocker and muscle relaxant by blocking neuronal nicotinic acetylcholine receptors (aAChr). Other chemical compounds were not tested here.
On a positive note the study also found that these compounds may have anti-tumor activity although we are certainly not advocating for people to start chewing grasstree leaves.
