Right now it's actually Plant Harvest Success Chance - below 1 it gives a chance to make pawns fail the harvest, but above 1 it does not increase the yield. I'm looking for a small mod that changes this stat's behavior so it actually does what one thinks, if you have a low PlantHarvestYield stat you get less and above 1.0 you get more. The harvest chance should always be 100%, just the yield should be affected.