There are several deworming and other medications given to sheep, cattle, horses and other livestock that can make canines very ill, if not kill them. Please be very aware of where your dogs are when you are using these medications as well as keeping an eye out that they do not consume the livestock manure.
Research the medication withdrawal time to see how long to keep your dogs away from manure. For example, if the medication withdrawal time is 21 days, keep your dogs off pasture and away from livestock manure the same period of time: 21 days.
Helpful link:
Safe Use of Ivermectin: by Dr. Jennifer Coates, www.petmd.com