Understanding how hay dries in the field
Having a basic understanding of plant response after mowing can help hay and haylage makers produce higher quality forage.
According to University of Wisconsin forage specialist Dan Undersander, mown hay dries in three distinct phases. Understanding what happens in each phase can lead us to management practices to improve hay quality and reduce losses. Dry hay requires the removal of about three tons of moisture for every ton of hay produced. The energy required is the equivalent of 70 gallons of fuel oil. Warm air temperature and low humidity also aids drying, but the sun is the primary driving force. Wet soil under the swath also slows drying by allowing moisture to move up into the swath. Using the right field process at the right time is important. When forage is cut, it has 75 to 80 percent moisture and must be dried down to 60 to 65 percent moisture content for haylage and down to 14 to 18 percent moisture content for hay (lower figures for larger bales).
Phase I. The first 15 percent of moisture loss is primarily through stomates, openings in the leaf surface that allow water vapor to leave the plant. Stomates open in daylight and close when in dark and when moisture stress is severe. Cut forage laid in a wide swath maximizes the amount of forage that is exposed to sunlight. This keeps the stomates open and encourages rapid initial drying. This will reduce loss of starches and sugars and preserve more total digestible nutrients in the harvested forage. This initial moisture loss is not affected by conditioning.
Phase II. Moisture is loss from the leaf surface and stem. At this point, stomates have closed, but the plant tissue is still alive and respiring. Conditioning can help increase the drying rate during Phase II, especially as the hay approaches the lower end – around 30 percent moisture.
Phase III. The final phase of drying is the loss of more tightly held water, particularly from the stems. Conditioning is critical to enhance drying during this phase. Conditioning to break stems every two inches allows more opportunities for water loss since little water loss will occur through the waxy cuticle of the stem.
For a good graphic representation of this concept, read Field Drying Forage for Hay and Haylage, an article written by the University of Wisconsin Extension.
In haymaking, the best recommendation is to dry hay rapidly. Mechanical conditioning should be used and high yielding crops should be spread in wide swaths. Tedding may be useful in drying grass crops, but it should be avoided with alfalfa and other legumes, particularly after the crop has partially dried. In silage making, drying is a little less critical. Wilting in narrow swaths can reduce raking loss, particularly for low yielding harvests. Raking can be used to improve harvest capacity. A substantial economic benefit can often be obtained by rolling swaths together to allow large balers or forage harvesters to operate more efficiently.