One thing that would help is ensuring that all your sheep are disease free.
Debilitation and Disease are mentioned on page 180 ArM 05.
You could give all the sheep in an area a bonus to Recovery Rolls.
This can be from +01 at level 01 to +21 at level 20 (Base Effect).
Clear the fields of all plants and critters harmful to sheep.
If the Covenant has the Resources , buy a Prize Ram.
Use a Rego spell to artificially inseminate all the ewes in a given area.
I think rego's the way to go, as well... though I think you'd probably need to have a good idea of the husbandry involved, but a rego animal ritual would probably be better I think. They talk a bit about this sort of thing in the Hermes Portal on the Isle of Man, either #14 or #15. Plus, you could consider a the herbam ritual as a guideline...I'd figure you want it to last the year-- impregnation to gestation to birth. Interesting idea, wish I had my book handy to punch something up.