Historical clue formulations

• A cow can produce about 200 pounds of it a year — 2026-02-21
• Dairy farm product — 2019-12-07
• Flammable gas represented in 18-Across and 9-Down — 2018-09-18
• CH4 — 2015-10-11
• Coal miner's concern — 2015-05-19
• Marsh gas — 2013-12-23
• Dangerous buildup in a mine — 2009-08-30
• Natural gas constituent — 2003-03-28
• Marsh gas, essentially — 1995-03-18