12.5: Manure Handling Systems

Solid, Liquid or Composted

The type of barn on the dairy farm farmstead frequently determines how manure is handled. Dairy cow manure containing a fair amount of bedding, usually around 20% dry matter or higher, is spread as a solid. This is most common on farms where cows are kept in individual stanchions or tie-stalls. Liquid manure-handling systems are common where animals are kept in a “free stall” barn and minimal bedding is added to the manure. Liquid manure is usually in the range of 2–12% dry matter (88% or more water), with the lower dry matter if water is flushed from alleys and passed through a liquid-solid separator or if large amounts of runoff enter the storage lagoon. Manures with characteristics between solid and liquid, with dry matter of 12–20%, are usually referred to as semi-solid. Pasture cow manure is unmixed with water or bedding and falls into this category.

Composting manures is becoming an increasingly popular option for farmers. With this, you help stabilize nutrients (although considerable ammonium is usually lost in the process), have a smaller amount of material to spread, and have a more pleasant material to spread, which is a big plus if neighbors have complained about manure odors. Although it’s easier to compost manure that has been handled as a solid, it does take a lot of bedding to get fresh manure to a 20% solid level. Some farmers separate the solids from liquid manure and then irrigate with the liquid and compost the solids. Some separate solids following digestion for methane production and burn the gas to produce electricity or heat. Separating the liquid allows for direct composting of the solids without any added materials. It also allows for easier transport of the solid portion of the manure for sale or to apply to remote fields. For a more detailed discussion of composting, see Chapter 13.

Some dairy farmers have built what are called “compost barns.” No, the barns don’t compost, but they are set up similar to a free-stall barn where bedding and manure just build up over the winter and the pack is cleaned out in the fall or spring. However, with composting barns, the manure is stirred or turned twice daily with a modified cultivator on a skid steer loader or with a small tractor to a depth of 8–10 inches; ceiling fans are sometimes used to help aerate and dry the pack during each milking. Some farmers add a little new bedding each day, some do it weekly, and others do it every two to five weeks. In the spring and fall some or all of the bedding can be removed and spread directly or can be built into a traditional compost pile for finishing. Although farmers using this system tend to be satisfied with it, there is a concern about the continued availability of wood shavings and sawdust for bedding. More recently, vermicomposting has been introduced as a way to process dairy manure. In this case, worms digest the manure, and the castings provide a high-quality soil amendment (see Chapter 13).

Manure from hogs can also be handled in different ways. Farmers raising hogs on a relatively small scale sometimes use hoop houses, frequently placed in fields, with bedding on the floor. The manure mixed with bedding can be spread as a solid manure or can be composted first. The larger, more industrial-scale farms mainly use little to no bedding, with slatted floors over the manure pit, and they keep the animals clean by frequently washing the floors. The liquid manure is held in ponds for spreading, mostly in the spring before crops are planted, and in the fall after crops have been harvested. Poultry manure is handled with bedding (especially for broiler production) or with little to no bedding (industrial-scale egg production).

Storage of Manure

Researchers have been investigating how best to handle, store and treat manure to reduce the problems that come with year-round manure spreading. Storage allows the farmer the opportunity to apply manure when it’s best for the crop and during appropriate weather conditions. This reduces nutrient loss from the manure caused by water runoff from the field, or leaching and gaseous losses. However, significant losses of nutrients from stored manure also may occur. One study found that during the year dairy manure stored in uncovered piles lost 3% of the solids, 10% of the nitrogen, 3% of the phosphorus and 20% of the potassium. Covered piles or well-contained, bottom-loading liquid systems, which tend to form a crust on the surface, do a better job of conserving the nutrients and solids than unprotected piles. Poultry manure, with its high amount of ammonium, may lose 50% of its nitrogen during storage as ammonia gas volatilizes, unless precautions are taken to conserve nitrogen. Regardless of storage method, it is important to understand how potential losses occur in order to select a storage method and location that minimize environmental impact.

Anaerobic digesters are sometimes used to process manure on large livestock farms and to generate biogas, mostly methane. This gas is used on the farm for heat and electricity generation, or possibly off the farm as a fuel for commercial or municipal vehicles. In addition, anaerobic digesters can reduce greenhouse gas emissions, odors and pathogens, and improve air and water quality. They are a major capital investment for a farm, and to make them profitable, farmers typically need to make full use of the energy, carbon credits, tipping fees from external organic wastes, and coproducts of the manure solids. Many digesters may not be economical without subsidies. Digesters separate liquid and solid manure, allowing for separate land application but generally do not change the overall nutrient content. Therefore, they provide some benefits but generally don’t solve problems involving excess nutrients or runoff.

Table 12.1 Typical Manure Characteristics

	Dairy Cow	Beef Cow	Chicken	Swine
Dry Matter Content (%)
Solid	26	23	55	9
Liquid (fresh, diluted)	7	8	17	6
Total Nutrient Content (Approximate)
Nitrogen
Pounds per ton	10	14	25	10
Pounds per 1,000 gallons	25	39	70	28
Phosphate, as P2O5
Pounds per ton	6	9	25	6
Pounds per 1,000 gallons	9	25	70	9
Potash, as K2O
Pounds per ton	7	11	12	9
Pounds per 1,000 gallons	20	31	33	34
Approximate amounts of solid and liquid manure to supply 100 pounds N for a given species of animal
Solid manure (tons)	10	7	4	10
Liquid manure (gallons)	4,000	2,500	1,500	3,600
Source: Modified from various sources