A Republican rancher is challenging the incumbent Democratic candidate in November’s race for Colorado’s Senate District 8, which covers 10 counties in the northwest corner of the state. Corey Marshall, a fourth-generation Coloradan who owns Rudolph Ranch in Gilpin County, is challenging Dylan Roberts, a Frisco resident who is seeking reelection to his seat. “I’ve watched the state change over my lifetime, through the history of our family, and …