Yeah, just simply cultivating the wheat (assuming you're talking about wheat when you say "grain") generates a lot less product for the amount of work you have to do. For cows, you just put in a friggin grass pasture, no needing to cultivate special crops, and you get to use ALL of the meat, rather than just a portion of it.
Also, with wheat, to make it edible, you have to grind it, mix it with egg (chicken embryo, oh my!) and form it into dough, then roll it and shape it, yeast it, and bake it to make a loaf of bread. Wow, that's a shitload of work for something that will last you, what, a week or two at most? Whereas a whole cow, which takes a lot less work to cultivate, can last several months, if properly rationed.