Steven where are you located to have that in the feed? Sounds like it is not the US. If it is the US I suggest you call your county extension office and chat with them. If you are elsewhere I suggest calling your agricultural ministry, whatever you call them, and ask. Have the label from that feed handy so you can answer questions. Dosage amounts and such as that may be important.
Another option is to call the company that manufactures the feed and ask them.
From a quick internet search it seems that lasalocid sodium is fairly quickly eliminated from the body so the withdrawal period is likely days, not weeks, but I would not trust anything over the internet for that.
