One objection I get to the Paleo diet from people unacquainted with it is
that the wild predecessors of today's domestic cattle, such as the
aurochs, likely ate some wild grains amidst the grasses they grazed on.
That is likely true, but my guess is the proportion of the diet that
grains made up was quite small. Is there any data on the estimated
proportion of the wild grazers' diet that was grains? I am guessing less
than 1%.

The only thing I found on the net is the following:

"Food In spring and summertime, the aurochs’ diet consisted probably
mostly from grasses, grass-like plants, completed with herbs and leaves of
trees and bushes. In autumn they ate somewhat less grass and more from
trees and bushes. This was completed with tree fruits, like acorns. During
wintertime the aurochs diet consisted beside grass, grass-like plants and
herbs for an important part from branches and even some bark from trees
and bushes. The last population of aurochs in Poland in the forests of
Jaktorów were extra fed in the winter with hay." (Van Vuure, 2003)
http://home.hetnet.nl/~harrie.maas/speciesinfo/aurochs.htm

Interestingly, the article says that aurochs might be rebred from Spanish
Fighting cattle and also discusses an attempt to bring back the aurochs,
the Heck cattle.