By Wendy Powers

Given today’s fertilizer prices, it makes sense to keep as many manure nutrients in the manure as possible by minimizing opportunities for nutrients to be lost to the air. During manure storage, nitrogen is the primary nutrient that can volatilize. Carbon and sulfur may also volatilize but to a much lesser extent. Depending on how manure is handled (as dry manure, a slurry or a dilute liquid) and then stored, nitrogen losses are estimated to be as much as 85 percent from the time that manure is excreted by the animal to the time that manure is land applied as a source of crop nutrients. Retaining valuable nutrients in manure relies on implementation of strategies or practices that prevent nutrients from being converted to gases.

Manure storage covers are perhaps the best way to keep nutrients where they are most valuable—in the manure. Cover options include organic materials such as straw or cornstalks. These are often referred to as biocovers and are permeable—some gases escape through them. The most affordable in  up-front costs, they need to be replaced—typically every year—because the cover material sinks over time. Removing that material once it has sunk can pose challenges, too. Geotextile materials are another permeable cover option; though not biodegradable and often more expensive than biocover options, they offer the benefit of longevity while avoiding the issues associated with rain and snow load.

Impermeable covers, which allow virtually no gases through, are typically composed of plastics or petroleum-based material. But a cover that is impermeable to gases doesn’t let rain and snow pass through, either, so these must be dealt with in the cover design to avoid unwanted sinking or breaks in the cover. Wind can also present problems; in some cases, covers have blown completely off the storage or torn enough to be considered ineffective.

The usefulness of covers as a means of retaining manure nitrogen is not disputed. Odor reductions of 40 to 50 percent have been observed with biocovers. Odor reductions of 80 percent and greater have been observed with geotextile covers and impermeable covers. Reductions in nitrogen emissions are similar. Though the impermeable covers may cost $2 per square foot installed, retaining half of the nitrogen in your manure storage by installing a cover may be a bargain compared with the current cost for nitrogen fertilizers (80 cents to $1.03 per pound).

Surface area exposed to air is the route for loss of nitrogen. Manure storage covers reduce or eliminate that exposure. Any changes to practices that can reduce exposure of the manure to the air can help conserve nitrogen. For example, changing aeration practices should be considered if you want to leave more nitrogen in your manure. Depending on the type of aeration system, how much it runs and how much mixing occurs, your losses can be considerable. Increasing the frequency of manure removal from lanes and lots also reduces the surface area of manure exposed. When alleys, lanes and lots are cleaned more often, the manure is relocated to a storage tank—typically with less surface area than the sum of all alleys, lanes and lots provided.

Use of additives is another way to reduce nutrient losses. Application of aluminum sulfate (alum), sodium bisulfate (trade name PLT) or sulfuric acid (Poultry Guard) have been shown to reduce ammonia emissions. These acids reduce the pH of solid manures, causing ammonia to remain in the litter rather than volatilize. In addition to keeping valuable nutrients in the litter, such applications improve air quality in poultry houses. This translates to benefits for the animals, workers and farmers. Acidulants may be appropriate for liquid manures, but care must be taken in their use. The above-mentioned products all contain sulfur, which is liberated as hydrogen sulfide, so their use in liquid manures is not recommended.

If you compost manure, you may want to think about your objectives. High temperature composting (above 130 degrees F) has benefits for pathogen destruction but higher nitrogen losses during the process (more than 20 percent more). Depending on your situation, it may be of greater benefit to compost at a lower temperature (120 degrees F) to retain more nitrogen in the finished product.

For new operations planning to handle manure as a liquid, an unconventional option is to segregate urine and feces prior to and during storage. Nitrogen is liberated from manure as ammonia when urea (in urine) comes in contact with urease (an enzyme excreted in feces). If urine and feces don’t come in contact with each other, ammonia is not formed. Again, in addition to keeping the urine nitrogen-rich, this practice drastically reduces ammonia concentrations in swine housing—by as much as 90 percent compared with a conventional finishing unit. This option does not fit all operations and certainly costs more, but it represents the types of options one can consider for new construction.
All options have advantages and disadvantages for any individual operation. But most operations have options available to them. Consider the numerous advantages of retaining nitrogen in your manure and determine what practices will work best for your operation.

This article is from the 2009 Manure $ense guide. To download the entire guide, click here.