Keeping Litter Moisture in Balance

It’s a cool, damp morning (40oF / 80 % RH) and you step in to your 40' x 500' broiler house where 20,000 chicks were placed ten days ago. Things are looking pretty good. The young birds are spread out evenly and actively eating and drinking. You check the controller and the house temperature is 80oF and the relative humidity is 55%. You check yesterday’s water usage and find out the birds drank 550 gallons of water yesterday, which is right on target. The litter appears in good condition wit...h a moisture content that looks to be about 20%. Two 36" exhaust fans are operating one minute out of every five, and you can only smell a slight bit of ammonia. Since conditions look pretty good, you see little need to increase the minimum ventilation fan runtime time. However, less than a week later the litter is caking over, and ammonia levels are approaching 35 PPM. How did things get out of hand so quickly?



When the birds were ten days old and drinking approximately 550 gallons of water each day, they were adding 550 gallons of moisture to the air and litter in the house. You might expect the birds to add less moisture to the house than they consume, since some of the water is retained as new muscle tissue, blood, and other growth. While it is true, birds retain about 20% of the water they consume, the digestion of feed actually produces water, which is roughly equal to the amount they retain. As a result, whatever the birds drink each day is effectively added to the litter and the air.

The primary goal of a house’s minimum ventilation system is to simply remove the same amount of water the birds are adding each day. If this balance is maintained, litter moisture remains stable.



At a temperature of 80°F and a relative humidity of 55%, 1,000 cubic feet of air is holding about 14 ounces of water (Figure 2). If the aforementioned house has a volume of 190,000 cubic feet (assuming an average ceiling height is 9.5') that means the air inside the house was holding roughly 2,660 ounces (190 X 14 ounces), or about 21 gallons, of water.



What about the outside air? At 40°F and 80% relative humidity, the air contains only about 5 ounces of water per 1,000 cubic feet (Figure 3). This may sound counterintuitive, but even though the relative humidity is higher outside, the outside air is actually much “drier” because 40°F air can hold only about 25% as much moisture as 80°F air. So yes, it’s 80% “full” of moisture—just 80% of a much smaller amount.



If the two minimum ventilation fans were each moving 8,000 cubic feet of air per minute, then during each timer cycle they would exchange 16,000 cubic feet of air. With 12, 5 minute cycles per hour, the minimum ventilation fans were moving roughly 190,000 cubic feet per hour (12 X 16,000 ft3) — essentially one complete air exchange per hour. That means each hour the fans were replacing 190,000 cubic feet of inside air containing about 21 gallons of water with 190,000 cubic feet of outside air containing only about 7.5 gallons. In other words, the fans were removing approximately 13.5 gallons of water from the house each hour (Figure 4).



 

The problem in this case was that the birds were adding about 23 gallons of water per hour (550 gallons per day ÷ 24 hours). This means the house was accumulating roughly 9.5 gallons of water per hour (23 gallons added - 13.5 gallons removed = 9.5 gallons surplus (Figure 5)). As a result, both house relative humidity and litter moisture levels were destined to increase. For example, if each day all of the excess moisture went directly into the litter, moisture levels in the top 2" of litter would rise about one percent. If this happens for a day or two it’s not necessarily a problem, but the fact is that it won’t take long for the litter to begin to cake if minimum ventilation rates are not continually and significantly increased (Figure 6). This is because early in a flock the amount of water the birds are consuming—and ultimately adding to the litter—is increasing by approximately 10% per day. This means minimum ventilation rates must also be increased by roughly 10% per day just to keep up with the additional moisture being added to the house.


One simple way to know if your are in “balance” is to simply monitor house relative humidity. The ideal relative humidity is between 40 and 60%. If it is increasing day after day that typically means the birds are adding more moisture than the exhaust fans are removing and minimum ventilation rates should be increased. If it remains relatively constant, then moisture added by the birds is roughly equal to that removed by the exhaust fans.

Effective litter moisture control simply comes down to maintaining a balance between how much water the birds are adding and how much the minimum ventilation fans are removing. Even when litter and air quality appear acceptable, moisture can quickly accumulate in the litter if ventilation rates do not keep pace with the rapidly increasing amount of water the birds are consuming. When this balance is not maintained, litter moisture levels will rise and bird performance, health, and welfare will tend to suffer