Frank,

I've noticed a definite fall off in the quantity of what the grass carp consumes as a function of age. The older they get, the less they seem to eat (unlike me \:\) ). Because of that and in order to avoid overstocking them, I add a few new grass carp to my ponds each year to maintain that consumption...but my approach has been somewhat negated by escaping grass carp going over the spillways in very heavy rains.

To try to answer your question, if you have older grass carp (and three years is older in my opinion) I doubt it will make much difference if you turn off the feeders. Get some new younger GC in there and I'd bet you see a difference.