If sheep producers needed any further convincing that their industry is in a slump, the Agriculture Department has a few more discouraging figures for them.

Market prices for slaughter lambs have been below $50 per hundredweight, down from around $80 a few years ago. And USDA analysts say the outlook is pretty grim through 1991, at least.The U.S. mid-March average reported last week for lambs was $48.60 per hundredweight, compared with $66 a year ago.

Ron Gustafson of the department's Economic Research Service said lamb imports also have edged down somewhat. Part of that has been because U.S. domestic supplies have been "just a little bit larger" recently, and also because U.S. prices have been less attractive.