The following is an excerpt from an interesting article by a sheep
specialist working for Ministry of Agriculture, Ontario and says some
interesting thiings about grassfeeeding.

I don't know if this is policy or not but it helps explain another reason
why grass-fed is superior to grain-fed feed (in sheep).

Perhaps it's possible the resulting better digestion from grass-feeding
enables the animals to harvest greater amounts of essential oils from their
feed?

Marilyn

http://www.gov.on.ca/OMAFRA/english/livestock/sheep/facts/hay.htm

Bill McCutcheon, Ontario Sheep Specialist, OMAFRA
March, 1991

[...]

"When hay is eaten by the ewe, large amounts of saliva are secreted. This
saliva is basic (high pH) and creates an environment in the rumen to
encourage the growth of microbial populations that will digest the fibre in
the hay. The saliva also acts as a buffer to control the pH in the rumen
from dropping too low. When the rumen pH becomes acidic, the microbes needed
for efficient digestion of the forage are killed. If the pH in the rumen
falls too low, the ewe will become ill and suffer from acidosis, commonly
known as grain or rumen overload.

When grain is fed to the ewe, it creates an environment in the rumen that is
acid (low pH). If too much grain is fed at one time or if grain is consumed
by the ewe into a rumen that has not been buffered with the ewe's saliva,
grain overload or acidosis can occur. When large amounts of grain are being
fed, it is important that the rumen is properly buffered. A large amount of
grain would be in excess of one pound per feeding. The rumen can be properly
buffered by feeding part of the total hay consumed by the ewe to her before
feeding grain. I would recommend the ewes consume hay for about 30 to 45
minutes before feeding grain. By allowing the ewe to eat hay before feeding
grain a drop in rumen pH can be avoided. By feeding hay first, the forage in
the rumen will break down more efficiently because the pH is maintained at
the proper level to promote regeneration of microbial populations.
Carbohydrates supplied by grain provide energy for microbial regeneration.
"
[...]