I don't know if rock salt would be safe for chickens or not, - but, I wouldn't use it.

The best way to get rid of poison ivy is to pull it, bag it, and put it in the garbage. Of course, use gloves - disposable gloves so you can throw them away too. Or sometimes I use the plastic bags that grocery stores use to pack your groceries. Insert your hand into the bag - grab and pull the poison ivy (the bag is like the glove) - then, invert the bag over the ivy and dispose it all in the trash. Do not burn it and do not compost it.

Hope this helps.