Federal programs designed to help farmers set aside more habitat for bees and wildlife have produced a dangerous offshoot this year: Some mixes used to plant native grasses and flowers were contaminated with harmful weed seeds that have now been introduced onto hundreds of farms in several Midwestern states, including Minnesota. The weeds include Palmer amaranth, one of the most prolific and devastating weeds in the country for corn, soybeans and other row crops. There’s reason for concern, said University of Minnesota Extension weed scientist Jeff Gunsolus, because a single female Palmer amaranth plant produces more than 250,000 seeds, grows...