Though some ranchers might be tempted to speed up the process and put up some questionably cured hay, it is important to keep in mind the consequences of baling hay that is too moist.

According to Joe Brummer, A. Wayne Cooley, and our very own Mark Volt, in the Intermountain Grass and Legume Forage Production Manual Chapter 9 (Irrigated Hay Meadow),

Small bales should be baled with no more than 20% moisture content.  Large bales should have between 15-18% moisture content.  

Baling hay with less than 12% moisture content will result in leaf loss and stemmy hay. At moisture levels above 20%, heating and molding will cause dry matter loss and a decrease in forage quality.

Brummer et al. warns that "If mold activity raises the temperature to 104°F or more, "browning" can occur which reduces digestibility of protein and carbohydrates.  In a worst case scenario, if the temperature rises above 150°F, spontaneous combustion of the hay can occur."

Read the entire chapter by Brummer, Cooley, and Volt here (Chapter 9, starting on page 81).