MONTPELIER: Vermont farmers are facing new rules to prevent runoff into Lake Champlain, which some call the biggest change to the industry in their lifetime. The new agricultural practices, with took effect Monday, include rules for small farm certification, storing and spreading of manure, planting cover crops to improve soil and prevent erosion, and expanding vegetated buffer zones on fields near water and ditches. The rules are part of Vermont’s commitment to reduce phosphorus runoff into Lake Champlain, which has been plagued by toxic algae blooms. The state says agriculture is the biggest phosphorus contributor at more than 40 percent. “A lot of farms, whether it’s small, medium or large haven’t had this type of regulation handed down with as many changes,” said Brian Kemp, president of the Champlain Valley Farmer Coalition. The rules are a big challenge to some dairy farmers already struggling with low milk prices, particularly those that may need to update manure storage or other facilities, he said.