It’s tough to get through every forage cutting during the season without coming up against Mother Nature. Some wilting forage crops will be damaged by rainfall during the season, which can impact silage quality and fermentation.

The problem is more complex than damage to wilting forage crops via leaching, extended or reactivated plant respiration, or leaf shatter. Common consequences of uncooperative weather also may include:

  • spontaneous heating or combustion that occurs when producers try to complete baling operations of incompletely wilted hays prior to an oncoming rainfall event.
  • poor silage fermentation.
  • excessively mature forage that results from delaying haying or silage harvesting operations until weather is more favorable. Maturation effects on forage quality can be as severe as spontaneous heating and/or rain damage.

What happens when rain falls on wilting forages?

Plant sugars are assumed to be 100% digestible. Therefore, any loss of sugar has a direct effect on the energy density of the harvested forage. During the normal wilting process, respiration of plant sugars continues to occur at moistures suitable for ensiling; numerous research studies have shown that this process slows considerably by the time forages reach 50% moisture, but may persist at a low level until the forage is nearly dry enough to bale as hay.

These factors explain why rapid drying to the desired forage moisture concentration is important, regardless of whether the forage is to be preserved as silage or hay. Unfortunately, rainfall events can reactivate respiration within dry forages and promote the growth of microorganisms on the forage.

Rain falling on wilting forages also directly leaches soluble nutrients (primarily sugars) from the forage. Leaching losses are affected by forage species; the moisture concentration of the forage when the rainfall event occurs; concentrations of soluble sugars within the forage; and the number, amount, intensity and duration of the rainfall event(s). Plant sugars also serve as the primary substrate for the formation of lactic acid during the silage fermentation process; therefore rain-damaged forages can be problematic to ensile.

Significant losses of dry matter (DM) also can occur directly because of leaf shatter, particularly if the forage is a legume. In addition, any rainfall during the wilting process may lead to additional tedding and raking operations that result in even more leaf shatter before the forage is dry enough to bale.

To Delay Harvest or Not?

Decisions to delay harvests of hay or silage while waiting for favorable weather are never made without costs. Forage plants continue to mature during harvest delays. This results in hays and silages with greater concentrations of fiber and lower energy densities. These forages often will be consumed less readily by livestock.

Rain damage or poor drying conditions normally extend wilting times, which also causes losses of sugars through extended or reactivated respiration. When ensiling rain-damaged alfalfa, producers should consider wilting forages to less than 60% moisture, and using a silage inoculant formulated to support production of lactic acid. It also is advisable to ensile rain-damaged forages separately from forages without rain damage, and to feed rain-damaged silages first.

These safeguards will likely decrease the risk of unsatisfactory secondary fermentations dominated by undesirable end products, such as ammonia and butyric acid.

For the full story on managing the effects of rain damage on wilting forages, click here.