Reading Time: 4 minutes Silos — tower-like structures on farms that hold fermented feed for livestock — have dotted the Midwestern landscape for 150 years. But they’re threatened by development and old age. One “silo hunter” in northern Illinois has been tracking down silos her grandfather built before they disappear. Marianne May drove around the backroads of McHenry County on a recent afternoon, scanning for those tall, round structures with a…