There was a time when Vermont’s landscape was dotted with weathered red barns full of dairy cows, and every country store was chockablock with local maple syrup and candies. The barns are there still, as are their fading illustrations of cows, and the sugar maples still draw leaf peepers in the fall. But because of industry shifts and climate change, many of the cows are gone and the state’s biggest agricultural products are imperiled.