Why might Gilead have shipped most of its older women to the colonies in The Handmaid's Tale?

In The Handmaid's Tale, most of the older women would have been shipped to the Colonies because these older women would no longer be fertile. In the novel's Republic of Gilead, handmaids are women of child-bearing age who are tasked with birthing the next generation. Besides nuns and lesbians, older women who are sterile or who find themselves politically opposed to the new order of government are usually shipped off to the colonies. So, you can see that a woman's worth in the Republic of Gilead is defined by her ability to bear children for the commander she has been assigned to. Without this ability, she is considered an Unwoman, fit only for hard labor in the colonies until she dies. 

The only way an older woman can preserve her place in the new society would be to become Aunts of the handmaids. Aunts are the overseers of the Handmaid class. They police the actions and sometimes, even the thoughts of the handmaids. Their job is to keep the handmaid class under tight control so that the handmaids can perform their jobs to the satisfaction of the republic.