Good question. Does starter fert help with germination? If not germination--early growth(say 21 days) ? Or does it mainly help after grass is 2 inches tall? Is the nitrogen or phosphorus more important?
Does potash have any role at all?
When establishing new grass what percentage improvement can one expect? How do you measure early success? I suspect weighing clipping yield is the only practical method.
Can starter fert overstimulate the old grass and thereby slow-down your new variety?
Does starter affect equally all grasses? Sod or seed? Blue? Rye? Fescue F? Fescue T? Bermuda? At Aug? Zoysia? Centipede?
How soon should you add additional fertilizer, and what type?