Normally, a layer formula is 16%. That formula has been developed through lots of Agricultural studies. Some folks like to feed a higher ratio of scratch or other grains, which reduces the birds total protein intake. When such grains reach a higher percentage, many people then like to feed a slightly higher protein layer formula to compensate or to balance things out. It is common for 20% layer formulas to be higher priced.
It is important to remember that if some protein is good, more isn't always better. There are difficulties associated with a hen's diet being too high in protein, a factor not always recognized here. In my experience, there is little benefit to feeding protein in a layer's total diet that exceeds 18%.
