Read:

www.ajcn.org/cgi/content/full/78/3/660S

It seems to suggest that cows don't just eat grass; they get varying suppliments of feed:

"The 2 livestock systems depending most heavily on forage but ALSO USING SIGNIFICANT AMOUNTS OF GRAIN ARE THE BEEF AND LAMB PRODUCTION SYSTEMS (Table 3). The beef system has a ratio of 40:1, while the lamb has the highest, with a ratio of 57:1 (Table 2). If these animals were fed on only good-quality pasture, the energy inputs could be reduced by about half." (caps are added by me for emphasis)

So it seems there would be no problem...if the farmers were actually just feeding grass to the cows, which it seems they aren't doing.
(Musuko, Fri 20 Feb 2009, 12:21, 1 reply)

Our cattle only get grass
and then hay in the winter - which is pretty much long dry grass.

I live on a mini cattle farm and I'm vego. I feel bad.
(Poppet some assembly required., Thu 26 Feb 2009, 6:17, closed)