Estpr13, as suggested, you really need to find out which herbicide was used. Then, a phone call to the company may be enuf to provide you with the necessary information.
I don't think that we here on BYC can do much more than commiserate with you over this sorry event.
Herbicides are fairly nasty things. But, we need to realize that most of our farm soils are treated with these chemicals each year. Indeed, many of our food crops are directly sprayed with herbicides. More of this sort of thing is occurring each year with the advent of Roundup-Ready varieties.
I pass by an alfalfa field often. It was sprayed last Spring and again this year with an herbicide to kill the weeds invading the field. The stand of alfalfa looks terrible after this treatment. The 1st time, I was convinced that the farmer had deliberately killed all of the plants - then the alfalfa recovered and, for the most part, the invading weeds did not. That alfalfa was, of course, fed to livestock.
Manure from livestock fed plants that has been treated with herbicides can kill or stunt crops. Applying manure can sometimes turn out to be a real problem for a gardener or farmer who thought that the manure would benefit a crop. The result, dead and stunted plants, was not expected.
Herbicides can cause an increase in the nitrates in plants and that may result in poisoning. There's even the problem of herbicide-treated plants that are naturally toxic becoming more palatable to grazing animals. Yes, that's right. There's a plant that your animals shouldn't eat but don't like to eat. So you spray it and as it wilts, the animal decides that it tastes okay now. The result is that not only has the critter eaten herbicide but it has also eaten plants that are naturally toxic.
These events tell me a couple of things. One is that we better be a little more careful with these chemicals. Another is that herbicides are now everywhere in our food-growing environment. As to what this may mean for your hens and their eggs . . . I don't know. But, I'm wishing you the very best of luck.
Steve