Photo from USDA
So, you may have heard that a bunch of cattle died in Texas as a result of eating GM Bermuda Grass. Well that was not entirely accurate. Yes, the cattle did die and yes they died from eating Bermuda Grass, but the Bermuda Grass was a traditional hybrid Tifton 85, not a GM crop. Tifton 85 was bred to be cold tolerant and have high forage production.
What went wrong when the rancher opened the gate to that beautiful field of fresh grass? Why did 15 of Abel's 18 cattle die? They died of "Prussic Acid" aka HCN poisoning. The field of Tifton 85 was producing Cyanide. Tifton 85 is a polyploid offspring; one of the parental species is Cynodon nlemfuensis African Bermuda Grass or Stargrass. Stargrass is a known cyanide producer, though it does so rarely.
Why do plants produce Cyanide? Plants produce all sorts of Secondary Metabolites under stress. Plant Stress Response occurs as a result of either biotic or abiotic stressors. Of course a major biotic stressor is grazing, so production of toxins is not surprising. Different plants produce different secondary metabolites but many produce cyanogenic glycosides that can result in cyanide poisoning if consumed. The Bermuda Grass in question has just survived several years of a severe drought- that stress may have been the cause of this poisoning.
If you read the review article on Plant Stress Response, you will see that many of the plant secondary compounds that are produced in response to stress are the very phytochemicals that we are learning are so important in our diet because they act as antioxidants. Many modern varieties of crop plants have lower levels of these phytochemicals than varieties that were common just a few decades ago. Furthermore, conventionally grown crops tend to have lower levels than organically grown. One possible explanation for this difference is that conventionally grown plants are subject to less nutrient and grazing stress, therefore they do not produce as many secondary metabolites as organically grown crops.