Prickly pear bear spines and glochids (stiff bristles on the surface of the cladode) that are responsible for mechanical irritations in humans (Greenberg, 2003). Spines cause injuries when eaten by livestock and must be removed by burning or by chopping off the most thorny parts of the cladodes.
Opuntia contains high amounts of oxalate crystals that cause mechanical trauma, make Ca less available and may have a laxative effect on animals (Barceloux, 2008; Nefzaoui et al., 2001). Cases of diarrhoea caused by excess Opuntia fodder can be successfully controlled by providing a source of fibre in the diet (Dubeux, 2011). Low Ca levels lead to unsuitable Ca:P ratios for livestock and must be counterbalanced by P and N licks (Nobel, 2002).