How much should I be feeding my new garden doves?

I have recently become the proud caretaker of four young white garden doves. I feed them each morning with a small pot of mixed pigeon grains as advised by the breeder, they eat like they are starving and it is gone in five minutes. Is it possible to feed them from a hopper so that they can eat what they like or would they just gorge themselves. They are still under a homing net and will be for the next five weeks. They are outside in a Dove Cote and look very healthy apart from giving the impression of being starving first thing in the morning. Hope someone can help?