Hog farmers are on high alert across Minnesota. Last month, thieves stole nearly 750 hogs from two farms - one in Kandiyohi County, the other in Nicollet County. David Preisler is executive director of the Minnesota Pork Producers Association and says whoever is responsible has to know the business. He says most farmers have a close working relationship with a buyer.   Kandiyohi County Sheriff Dan Hartog says 594 hogs were taken from a farm near Lake Lillian last month.  There have been similar hog thefts in northern Iowa. Preisler encourages neighbors to be on alert and contact the farmer or law enforcement if you notice something suspicious.