well there are a couple things that could have burned them. residuals from when they sprayed the first time...chemicals like atrazine can carry over for years. also, if the farmer had problems with weed control...which this last year LOTS of farmers had problems with weed control due to the hot weather and frequent rain in some parts of the country washing the herbicide away...so they had to go back in and spray a second and in some cases a third time. another thing with corn, is lots of farmers side dress it, which means to fertilize after the corn is up, the most common side dressing fertilizers are extremely concentrated and high in nitrogen, which would easily burn your plant to a crisp if it hit the leaves or roots.
my suggestion is, dont plant it in corn fields. chemical companies are releasing more and more fertilizers and additives that can be used in combination with herbicides, pesticides and fungicides and that can be sprayed all at once later in the plants life. the corn varieties we use are genetically engineered to withstand that...marijuana however, is not.
how tall was the corn when you noticed it was burnt? you really cant spray or side dress corn once it is waist high...but if they get severe problems they may come in and spray with an airplane or helicopter.
also, lots of seed companies put signs in the fields of what variety corn the farmer used...if you take that info and look it up online and you can see if its an early or late day corn. so if you pay attention to when the corn sprouts, you'll know about when its going to be harvested.