The state says it has begun visiting farms suspected of having illegal pigs.
Michigan's Department of Natural Resources has banned certain species and it took full effect on Sunday. The DNR says it visited six farms Monday and found no feral swine.
The state has published a list of physical characteristics it uses to identify pigs. Wildlife officials say these kinds of pigs are an invasive species and a wild population would be very destructive in Michigan.
Farmers and ranchers have responded with a handful of lawsuits.
Last week Mark Baker, a farmer from McBain, testified before the state Legislature about the rule. He says it would put him out of business. He raises pigs called mangalitsas, which he sells to fine restaurants in Traverse City and elsewhere. He's one of the farmers suing the state.
State Senator Darwin Booher introduced a bill Thursday to delay enforcement of the ban by 90 days but he couldn't get the votes needed in the senate before the legislature went on break.
A letter to the governor signed by 16 lawmakers calls the rule one of the most significant injustices ever imposed by a state department on Michigan residents and businesses.