If it was me, I would avoid that area for at least another 7 months. Some of those ferts and weed killers can hangout for a while. Plus the grasses they would be consuming would have that stuff in them. If you still have snow then there is a good chance most of it is still there. That is just me. From now on if you just use your chickens as fert and let the grasses grow a little taller then most like for their lawns 4-6" and maintain it there you should never really have a weed problem. Weeds only invade when they are allowed to by unhealthy grasses or bare grounds. But anyways my answer would be no,keep them off that grass that the stuff is on and let it go away about 10-12 months. And never put it on again use more natural methods of control.
