I think you need to be concerned about three things in the runoff: pesticides/herbicides, fertilizers, and sediment. A couple of threads which have discussed these aspects of field tiles running into ponds are
Using farm tiles as a water source and
Best cap for tile overflow. I have only hay and pasture fields in my watershed, no row crops, so I am not very familiar with what exact problems you might have from the 'cides. Fertilizer runoff could cause too much plant growth in the pond, with the worst case being eutrophication and fishkill. There is info on this kind of stuff in the threads above and I am sure some more will be forthcoming.
One thing that will impact the effects is how much of the watershed of the pond will come from the field tile draining the row-crop fields. Obviously, if that's the sole source of water, it will have a much larger impact than if it is just a small % of the total water your pond will receive. If you are in the latter situation, perhaps you want to consider NOT running the field tile into the pond, but rather ditching around the pond to detour the ag runoff.