"As mowing and conditioning equipment have evolved, some of the basic drying principles of forage have slipped by the wayside," says Dan Undersander, Univ. of Wisconsin extension forage agronomist. "If we understand and use the biology and physics of forage drying properly, the hay dries faster and has less chance of being rained on and the total digestible nutrients (TDN) of the harvested forage is higher."
Forage is typically at 75-80% moisture content when it's cut and must be at 60-65% moisture content for haylage and 14-18% or lower for baling.
To reach the haylage harvest stage, the hay must go through Phase I curing, which can be accomplished with a wide-swath mower.
"In Phase I, drying takes place when the cut hay continues to respire and lose moisture through the plant's stomates in the leaves. Stomates open in daylight and close in darkness, and that's why hay laid in a wide swath will dry faster because the sunlight triggers the stomates to remain open," explains Undersander.
"Respiration rate is highest at cutting and gradually declines until plant moisture content falls below 60%. Rapid drying to lose the first 15% of moisture content will reduce loss of starches and sugars and preserve more TDN in the harvested forage," he says.
Once haylage has gone through Phase I, it's ready for the chopper. However, forages destined for baling must go through Phases II and III curing before harvest. Conditioning alfalfa and alfalfa-grass blends assures faster dry-down for baling.
Phase II of the drying process involves continued moisture loss from the leaf surface (although the stomates have closed) and the stem. "At this phase, conditioning increases drying rate, especially on the lower end," Undersander says.
In Phase III, tightly held water in the stems evaporates. "Conditioning breaks stems every two inches and allows more opportunities for water loss because little water loss will occur through the waxy cuticle of the stem.