Spontaneous heating is not the only factor that can affect the nutritional value of stored hay.
Over the last two decades, large round bales generally have replaced small rectangular bales as the preferred type of hay package largely because of the reduced requirement for labor. Many of these round bales are stored outside without any protection against the weather. The weathering of the outside layer can have a major impact on the nutritional characteristics and DM recovery of hay. It also may result in greater refusal and reduced intake by livestock.
Weathering is partially a physical process caused by the leaching of soluble forage nutrients during rainfall. Since most soluble compounds in forages are highly digestible, it is desirable to limit these losses during storage. A second type of weathering is the result of microbial activity that increases under moist, warm conditions. Infrequent heavy rains are likely to have less impact on weathering hay bales than smaller, more frequent, rainfall events. Losses are generally reduced in arid climates and in northern climates with severe winters because the environmental conditions are less favorable for microbial activity. Within any specific environment, DM losses are nearly proportional to storage time.
Some crops are naturally more resistant to weathering. Generally, fine stemmed, leafy, weed-free crops, such as bermudagrass or tall fescue, form an excellent thatch that sheds water. Other crops with large, coarse stems do a poorer job of shedding water. Good examples of these types of forages include sudan grass, pearl millet, sorghum-sudangrass hybrids and johnsongrass. Water can easily penetrate bales made from these forages and accelerate the weathering process. Hays with coarse-stemmed weeds also do a poor job of shedding water and weather quicker than weed-free hays.
Bale size and density
Dense, uniform hay packages will limit weathering losses compared to loosely baled hay packages. Bales that have 10 pounds of hay per cubic foot in the outer layer will help to reduce penetration by rain. The density of the inner core is less important than the outer layer. Bale density can be increased by raking hay swaths into smaller windrows and reducing the ground speed of the baling tractor. These practices will result in more layers per bale and a greater overall bale density. Unfortunately, this also will increase leaf shatter in legume hays. While baling dense hay packages will help to limit weathering effects, it also will increase the likelihood of spontaneous heating. Therefore, every effort should be made to reduce the forage moisture content to 18 percent or less before baling. It should also be noted that larger hay packages have lower percentages of weathered forage than smaller hay packages; however, larger and more expensive tractors are often required to handle larger hay packages.
Limiting hay and soil contact
It is easy to overlook the importance of the bottom of the bale when discussing weathering losses. Some reports suggest that approximately 50 percent of the storage losses in hays stored outside occur at the hay/soil interface. This occurs because the dry hay acts as a wick, drawing moisture from the soil. Depending on the site, air movement may not be as great around the hay/soil interface as it is around the top of the bale. These factors combine to provide a moist environment at the bottom of the bale that is more favourable to microbial activity.
There are many ways to limit contact between hay and soil. Wooden pallets, railroad ties, pipe, tires and telephone poles can all be used to support hay bales and prevent contact with the soil. Ideally, any base should allow some air movement under the bales to facilitate drying. Crushed rock can be used as a base to limit contact with the soil. Crushed rock that is 1 to 3 inches in diameter and piled 4 to 8 inches deep should not trap water but should channel it away from the bales. Crushed rock also has the added advantage of lasting many seasons and repair of the storage site is simple. If bales must be placed directly on the ground, select a well-drained site with a sandy soil type.
Any site selected for the storage of hay bales should be in a sunny, breezy, well-drained area, possibly near the top of a slope. Bales should be oriented in rows that run up and down the sloping area, preferably with a southern exposure. Rows of bales oriented perpendicular to a sloping surface will trap moisture following rainfall. Rows of bales should be positioned with the flat ends of each bale butted together. The rounded sides of adjacent rows should not touch each other. There should be about 3 feet between adjacent rows to insure good air circulation and penetration of sunlight. Bales should not be stored under trees or ever rest in standing water. It is best to select a site that has no objects that will attract lightening, and the posting of no smoking signs may remind others that a hay crop represents a serious investment of time and money. It is also a good idea to have multiple storage sites. This will reduce the risk of a fire destroying an entire hay supply at one time.