That's actually and apples and oranges comparison.
the number of cattle alive on the planet today, is far, far, far, greater than would be supported, or produced, in a natural ecosystem. I think there are 100 million cows in the U.S. alone. That's way more than a natural ecosytem would ever produce. Ergo, cow populations that are essentially man-made, and far in excess of what a natural ecosystem would support is essentially a man-made problem.
Dinosaur populations were limited by an natual equilibrium within a natural ecosystem. I seriously doubt there were 100 million T-Rex's roaming north america in the cretaceous.