E - Parturient paresis The two most common causes of polioencephalomalacia (PEM) are low thiamine (due to thiaminase activity from plants like bracken fern or low thiamine diets) and high sulfur in the diet (which can come from a high molasses-urea diet, corn or sugar cane byproducts, water, or other plants, including alfalfa, Canada thistle (Cirsium arvense), kochia, (Kocchia scoparia), and lambs