While this is not very toxic once placed on the ground it is different when it is ingested. The dog can do damage to the throat causing a voice change. It can also do damage to the stomach when eaten and to the lungs when inhaled. If you notice the dog acting lethargic, vomiting, having diarrhea, not eating or drinking then a vet check would be in order. If the dog acts normal but there is a voice change it is still worth a checkup to determine the damage.
I was in this same predicament with one of my own dogs. She still acted normal but her voice was changed forever.
Here is more on garden products. If you go to the vet bring the box or wrapping so they may look at the ingredients.