Mixed and applied at label rates, 2,4-D poses little problem in and of itself. It has a relatively low order of toxicity for mammals, and has a short half-life in the environment, degrading within just a few days.
However, it can cause some plants, as they are damaged, to take up/accumulate nitrates, which can be toxic to cattle, and there are multiple documented instances of cattle being poisoned by consuming toxic plants that they would normally avoid - something about plant damage by 2,4-D seems to make them more palatable.
I don't remove cattle when I'm doing spot-spraying for thistles or blackberries, etc. in fencerows, but if I were doing wholesale spraying, say, to knock back meadow buttercup, then I would pull the cows off the pasture for the recommended time frame.