Water’s Effect on Litter: Water activity (Aw) or Equilibrium relative humidity (ERH)

By August 28, 2017PLT, PWT

All organisms require water; its availability is essential to the growth of microorganisms, which means wet litter can
promote both pathogen growth and ammonia release. Moisture content measurements include both unavailable (bound) and available (unbound) water. Therefore, knowledge of moisture content alone is not sufficient evidence to determine whether an environment is compatible for microorganism growth or survival since a proportion of the
total moisture content is bound and unavailable to the microorganism. To control for pathogenic growth, producers
must make a closer determination about water availability.

Water activity measures how tightly water is bound structurally or chemically to a substrate (ie. bedding material). It is equivalent to the relative humidity of the surrounding air—the equilibrium relative humidity (ERH). Both of these indicate water availability. Water availability (Aw) is crucial for microorganism survival on the surface within poultry litter.

Research has shown that when the ERH of the air at the litter surface increases above 84%, conditions are favorable
for pathogen growth. Controlling relative humidity inside the poultry house, ideally at 50-70%, well below the 84% threshold, is an optimal way to control litter water availability and prevent microbial growth. Controlling ERH is a vital management strategy for controlling pathogens.

Strategies for Controlling ERH

Proper ventilation is a key management tool for removing excess moisture from the broiler house. Combatting the challenge of excess moisture can be largely overcome by ensuring proper directional airflow through sidewall inlets
within the house, allowing cooler, outside air to mix with warmer air at the house ceiling. Proper mixing increases the moisture-holding capacity of incoming air, which allows increased moisture removal from the litter. This is possible because the saturated air can then be circulated out of the house leaving a drier litter. Cold air that enters through leaks in the house or through sidewall inlets at a low velocity will drop downward along the sidewalls resulting in cold drafts, chilled birds, wet litter, increased ammonia levels and increased fuel usage.

  • Preventing leaks from water lines is another control measure to ensure dry litter. Wet litter can be caused by water lines that are too low, water pressure that is too high, or leaking nipples. Unleveled drinker lines can also be problematic especially in houses that have been windrowed. Air pockets can form in the line causing unequal water pressure. Even if the litter conditions are perfect in the rest of the house, donuts under drinker lines are sufficient to downgrade paws and create litter conditions favorable for pathogen development.
  • Proper decaking of wet litter between flocks also helps to control moisture levels. Only caked litter from high moisture areas of the house should be removed. Decaking equipment should be adjusted to remove only the top-caked portion of the litter, leaving the dry litter underneath. Adding new bedding at a proper depth of 4 to 6 inches helps further. When adding new bedding, it’s ideal to select a clean, dry material with desirable absorption properties.