Cows give milk. Milk has calcium. But where do the cows get the calcium? Is it in their diets? Do grasses contain a lot of calcium?
It would seem so. They (herbivores) also get a significant amount of the calcium that their own bodies need that way.
http://www.amazinggrass.com/nutritional-breakdown-organic-wheat-grass.html
Green leafy veggies are decent sources of calcium, and considering the quantity of greens and the quality of digestion for a cow, they absorb a good amount of it.
Most people would have a hard time getting most of the calcium from greens, but you can get a fair amount.
I thought it might also be interesting to note that cows often become deficient in calcium during calving.
It’s an amazing thing to watch a cow with milk fever after receiving a shot of calcium, they usually get right up like nothing was wrong to start with!