What happened to Douglass before he was a year old, and why does Douglass think this was done?

From the text:

My mother and I were separated when I was but an infant—before I knew her as my mother. It is a common custom, in the part of Maryland from which I ran away, to part children from their mothers at a very early age. Frequently, before the child has reached its twelfth month, its mother is taken from it, and hired out on some farm a considerable distance off, and the child is placed under the care of an old woman, too old for field labor. For what this separation is done, I do not know, unless it be to hinder the development of the child's affection toward its mother, and to blunt and destroy the natural affection of the mother for the child. This is the inevitable result.