I feed my ducks Dumor poultry layer feed. I've read that niacin needs to be added. How much and how do I add it? It there anything I can buy from my local feed store to add to it? Or should I just buy them from the grocery store? Is there anything else I should add to their diet? Like grit, calcium, oyster shell etc.? Also would it be a good idea to add a flock block? This is for laying hens.