While I agree with feelnrite that attacking the root of the problem by trying to increase the selenium content of forage is a good long-term approach, I would be careful about relying on that method alone.

In SW WI I am in a selenium deficient area also. Selenium is absolutely critical to my cattle as well as you sheep.

I would suggest using the sheep version of the Crystalyx mineral tubs simillar to the cattle version I use.

Here is a link to the Sheep-lyx nutrient analysis sheet. I would call them and see where you can buy tubs in MA.

While you can give them selenium shots etc, this tub also includes other required trace elements for sheep as well as NOT including copper which the cattle tubs do but is evidently harmful to sheep.

Using these tubs would provide an almost immediate source of selenium with confirmed consumption while you try to build up the Se levels in your forage - which may or may not be successful.

Best of luck to you.

Jim at Dawn