There's no free N...  

One of the rather blatant errors made by the organic community is the idea nitrogen in manure is somehow placed there by totally natural processes. Hardly.

A further source of confusion is the misconception that animals make nutrients. Animals do not fix nitrogen or (aside from one known exception, the shipworm⁷) acquire significant amounts of nitrogen directly from nitrogen-fixing bacteria. Nor do they make other nutrients, like phosphorous and potassium, so their manure contains no more nutrients than their feed. This is a point which Dan Barber, the influential New York eco-chef, seems to miss when he urges environmentally-conscious New Englanders to eat “a lot of meat” in part to avoid dependence on synthetic fertilizers. According to Barber, it’s important to raise a lot of animals so that we’ll have enough manure, which he terms a “free ecological resource,” to fertilize our vegetable crops. This advice stands in stark contrast to the recommendations of the UN fertilizer report, which explains that “inclusion of livestock in the food chain substantially reduces overall nutrient use efficiency, leading to large pollution releases to the environment” and identifies as a “Key Action” the reduction of animal protein consumption in affluent regions.
The discrepancy is explained by considering the Stone Barns Center for Food & Agriculture, the small non-profit farm which raises meats for Barber’s Blue Hill restaurants and is, in Barber’s words, ”a replicable model for the future of good food.” At Stone Barns, the manure of pigs, chickens, geese and turkeys is largely derived from feed of corn, soy, sunflower, and flax. While sheep, the farm’s ruminants, may take in some “free” nutrients like nitrogen fixed in the roots of clover on the pasture, that pasture is fertilized in part with the manure of grain-fed animals, so even their manure is made possible by the grain inputs.
That’s important because the nutrients in grains need to come from somewhere. Manure from grain-fed animals doesn’t solve the problem of soil fertility so much as transfer that problem to the grain farm. And although animal manures are natural, they aren’t entirely benign. Like synthetic fertilizers, nutrients in manure can run off in groundwater or escape into the atmosphere. This means that growing grains for animals to produce manure for vegetables tends to increase pollution by adding another opportunity (on the grain farm) for nutrients to escape. While recycling nutrients in manure is more efficient than discarding them, the possibility of doing this does not amount to a strong argument against reducing nutrient inputs, an end that would be achieved by eating grains and legumes in place of grain-fed meats. [More - interesting throughout]

I could not find a comparison of N content for grass/grain fed cows, although did see one source that showed a range of 3-20 lb/T. I think it's safe to say the less grain in, the less N out, however.

The key here is to remember to make the point that animals cannot create nutrients when rebutting unreasonable organic claims.