Adequate and balanced levels of nutrients are critical for your yeast to grow and convert sugar to alcohol properly and completely. A lack of nutrients in your fermentation can cause the fermentation to become stuck or create off odors. Nitrogen nutrient compounds are regarded by many winemakers as a sort of "junk food for yeast" and are not added until the yeast have had adequate feedings of more complex, "healthier" nutrient preparations that include B-vitamins and nitrogen-rich amino acids.
Notes from the winemaker: What specific nutrients to use (Fermaid, Superfood, or yeast extract, yeast hulls and Cerevit) is less important than assuring that adequate nutrition is present to ensure a complete fermentation. The larger question is how much DAP to use in conjunction with one or more of these other foods.
More notes from the winemaker: It is my regular protocol to try to get all nutrient additions into the ferment before the sugar drops to 8°-10° Brix. This allows the yeast to take up the nutrients during log phase growth - at least before late log phase - when the cells have the ability to do something constructive with the nutrients. Things like making more cell mass, and membranes that are more resistant to high alcohol levels.
Diammonium Phosphate (DAP)