How Rain Affects Hay Quality
Frequent rainfall during haying season can significantly reduce hay yield, quality, and marketability. Wet conditions lead to nutrient losses, increased microbial activity, and storage challenges. Below are key impacts and management considerations.
Effects of Rain on Hay Quality
  • Nutrient Loss: Rain leaches soluble carbohydrates and minerals, reducing energy content. Protein is generally less affected.
  • Respiration: Plants continue converting carbohydrates to CO₂ until moisture drops below 30%. Re-wetting restarts respiration, further reducing energy.
  • Leaf Loss: Repeated handling and wet-dry cycles increase leaf shatter, reducing quality.
  • Microbial Breakdown: Wet hay supports mold growth and nutrient degradation.
  • Color Bleaching: Prolonged exposure to sun and rain reduces green color, lowering market appeal.
Storage Risks with Wet Hay
  • Hay baled above 18% moisture can heat rapidly due to microbial activity and respiration, increasing the risk of spontaneous combustion.
  • Use a temperature probe regularly to monitor heat production inside the bale (2-3 feet into the bale)
  • Temperature Guidelines:
  • Up to 490 C: Caused as fungi and bacteria carry out normal respiration. The process is referred to as normal sweating during hay curing. This temperature rise occurs when hay is baled at 15-20% moisture. These temperatures generally do not cause serious concerns in forage quality loss. However, mold may develop at this temperature range.
  • 43-650 C: Caused by fungi able to grow at this temperature range. Chemical re-actions during heating will denature some protein and cause some fiber to be less digestible.
  • 57-710 C: Caused by fungi respiration. At 650 C, check temperature every day. At temperatures above 710 C, chemical reactions dominate the heating process. If the temperature continues to rise, check it every four hours. At this stage, the situation may become dangerous.
  • 790 C: Continue to check the temperature every few hours and notify your local fire department. Do not attempt to move the hay without fire department assistance.
  • 900 C or more: Spontaneous combustion is possible. 
Management Recommendations
  • Leave wet bales (when baled at 18% moisture or more) unstacked for 3 weeks to dissipate heat.
  • Use temperature probes to monitor stacks daily; check bale centers.
  • Consider enzyme- or acid-based additives for hay baled above 20% moisture.
  • If temperatures rise, pull stacks apart to cool and prevent fire risk.