Much interest has been expressed in compounds to reduce field curing time of forages.
First, it is important to recognize that two totally different types of products with different modes of action are sold: one is a desiccant which is a compound applied to the hay at cutting to increase drying rate and the other is a preservative which is applied to hay as it is baled to allow baling of wetter than normal hay without spoilage during storage. Both products are usually applied through a spray system, which costs $600 to $1000, either on the mower (for dessicants) or on the harvesting equipment (for preservatives).
Moisture from alfalfa is lost rapidly at very high moisture contents (right after cutting) and less rapidly as the hay becomes more dry. This means that the initial drying of alfalfa is rapid (e.g. from 75 to 55% moisture) and loss of the final few moisture percentages before baling takes considerable time (e.g. from 30 to 18% moisture)
The desiccants that are effective contain potassium or sodium carbonate. These compounds disturb the waxy cuticle of the alfalfa stem to allow it to dry faster. Desiccants work only on legumes such as alfalfa, trefoil, and clovers. Effectiveness varies with climatic conditions. Desiccants reduce drying time most when drying conditions are good. Thus, they tend to work better on second and third cuttings in Wisconsin. They are recommended for hay making and are of less usefulness when forage is harvested as haylage.
Preservatives are applied to the hay as it is harvested and prevent heating and spoilage of hay baled at high moisture contents. Preservatives are cost effective if used only when needed to prevent rain damage to hay and if applied uniformly to windrow as it is entering the baler. The most effective preservatives for alfalfa are organic acids, primarily propionate (proprionic acid) and acetate (acetic acid).
Propionate (propionic acid) has been most commonly used and any product containing a high percentage of this compound will be effective. Use of ammonium propionate (also called buffered propionic acid) rather than propionic acid is recommended because the product is less caustic - therefore safer to handle and less corrosive to machinery. When purchasing preservatives, compare cost on a per pound of propionic acid basis. Other additives do little if anything to preserve hay. Some hay preservative products dilute the proponic acid and require greater product use rates.
Rates of propionic acid required to preserve hay vary with the moisture content of the hay. As indicated in the table above, the amount of propionic acid required varies from 8 lb/dm ton for hay with less than 18% moisture to 20 lb/dm ton for hay with over 25% moisture.Note that rates are for pounds of propionate not product. Therefore a product with 50% propionate would need to be applied at twice the above rates.
Acetic acid is about half as effective as a preservative and therefore requires twice as much product for equal preservation.
Use of preservatives for hay above 35% moisture is not recommended.
Anhydrous ammonia is an effective preservative for grasses. It can be injected into bales or released into a stack of bales covered and tightly sealed with plastic. Ammonia should be applied at the rate of 20-40 lbs/ton with higher rates used for hay near 35% moisture and lower rates used when moisture is near 20%. Anhydrous ammonia should not be used as a preservative on alfalfa because the additional nitrogen is of little benefit to animals and toxic chemicals can form in the hay.
Currently, evidence is not sufficient to indicate that microbial hay preservatives are effective in preserving hay.