"Sheep's Parsley" for Forage

I'm looking for forage herbs for my horse pasture (in Washington state). I have seen a mixture in the United Kingdom of 40% chicory, 5% yarrow, 10% ribgrass, 20% burnet, and 20% sheep's parsley. Do you know what sheep's parsley is?

I do not know what "sheep's parsley" is. It is not listed in any of our references in the Richters library.
From searches on the Internet, the name appears to be local to the United Kingdom, and I suspect that the same plant goes under another more common name in North America. An agriculturist, Newman Turner, apparently favoured a forage mix that included "sheep's parsley" and the same herbs that you list above.
One source suggests that "sheep's parsley" is actually a type of the true parsley, Petroselinum crispum. The wild form of parsley is available and I suppose that it is possible that this type is used for forage crops. But I could only find one mention of true parsley, so I am not yet convinced that this is indeed the plant used in forage mixtures.

Also, are there specific varieties of these herbs that I should use?

Generally, it is preferable to use the wild forms for pasture and forage mixtures. So, for example, you should not use the cultivated forms of chicory and yarrow; instead, use the wild forms that are available from Richters. For yarrow use Achillea millifolium, item no. S7000 in our catalogue. Ribgrass is listed under "plantain" in our catalogue, item no. S4495. For burnet use the salad burnet variety, item no. S1570.
If you are interested in experimenting with wild parsley, we can special order in that in larger quantities, at least 1 kilogram, or more. For a quote contact our commercial department at commercial@richters.com.