Not sure about the US but here in Eastern Europe our peasant grandmothers were definitely working on the fields themselves, most of the times even longer hours than the men. It was the job of the very old women around the house to stay at home and take care of the eventual infants, but even then, that was an exception rather than the rule. I know that one of my grand-grand-mothers wanted to go and work the fields until her late 80s, together with everybody else from the extended family, staying at home was a sign of weakness and was seen as almost courting sickness/death.