Speech defect link with agrichemicals?

A friend who has worked on farms all his life has a strange (to a lay person like me) speech defect, seeming to have to spit out the words with an upward nod of the head. It does not greatly hinder conversation with acquaintances but it is enough to hinder his day-to-day dealings with members of the public and therefore ability to find a job.
Medical investigations some years ago found nothing and he seems to have given up on that.
It occurs to me that it might have been brought on by exposure to agricultural chemicals, especially in the past when controls were less rigorous than they are now.
Does this possibility ring a bell with any experts or is it just speculation on my part? I'd like to be able persuade my friend that something can be done to help.