Why are women temporarily separated from their husbands called “grass widows”?

The expression grass widow originated hundreds of years ago in
Europe where summers were unbearably hot. Because grass was
scarce in the lowlands, husbands would send their wives and chil-
dren, along with their resting workhorses, up into the cooler grassy
uplands while they stayed in the heat to till the land. It was said that
both the wives and horses had been “sent to grass,” which gave us
the expression grass widows.