I have addressed this subject in previous posts from other perspectives.
However, I have been wondering if large herbivores have entirely intrinsic regulation of their uptake of PSM rates (or uptake of other types of chemicals contained in specific food resource) or the regulation is reactive/responsive (herbivory is stopped when, e.g., the taste or smell prevents further tolerance).
Some publications (e.g., Iason, G., 2005) suggest that mammalian herbivores might not always know when to stop or which plants caused upset in their organism, especially, in species with a broader dietary niche.
It is possible that the ancient predatory-prey mechanisms also limited herbivore exposure to vegetation communities by encouraging rotational grazing/browsing and a more frequent movement from an area to an area.
Therefore, I propose a theory that without large predators (landscape of fear and other activity adaptations), mammalian herbivores might have lost an environmental mechanism that regulates not merely overbrowsing/overgrazing on plants as such but also overconsuming plant chemicals that might cause health problems but that the herbivore might not be easily capable of self-diagnosing as the cause of the health issue.
I believe this would make for an interesting research topic.
Notes by Ieva 