Here are some useful links regarding swathing timing of various crops. At the bottom of this post is a chart for seed moisture and swathing timing. These moisture levels were developed through years of research by OSU.
Response of Seed Yield to Swathing Timing in Tall Fescue and Creeping Red Fescue
Using Seed Moisture as a Harvest Management Tool to Determine Swathing Time in Grass Seed Crops
Response of Seed Yield to Swathing Time in Annual and Perennial Ryegrass
Crop Optimum Moisture
Crimson Clover 35%
Annual Ryegrass 43-46%
Tall Fescue (forage) 40-43%
Tall Fescue (turf) 35-41%
Perennial Ryegrass 35-43%
Fine Fescue 20-35%
Fescues, perennial ryegrass, and annual ryegrass will lose about 2-3% moisture per day while orchardgrass loses about 1% per day. Meadowfoam loses 2% moisture per day.