11.4: Manure Handling Systems

Solid, Liquid or Composted

The type of barn on the dairy farm farmstead frequently determines how manure is handled. There are generally three kinds of dairy manure. 1) Solid from regular stall barns, 2) liquid, from large CAFO cow 'sheds', with >200 cows per shed, 3) semi-solid from cows out grazing pasture.

Dairy cow manure containing a fair amount of bedding (straw or sawdust), 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. In dairy country, you might see a farmer pulling a manure spreader out to the field full of cow manure. On some farms, this meant a 'daily haul' chore to keep up with the pile of poop. A cow eats about 40 pounds of feedstock in a day and excretes as much as about 18 people. Winter spreading of manure has become less common as farmers began, in the early 2000s, to value the fertilizer contained in the manure that was lost to winter weather.

Liquid manure-handling systems are common where animals are kept in a free-stall barn. Minimal bedding is added to the manure. Most often the cows lie on sand-filled or rubber mats in stalls that they can freely get up, leave, and walk over to a bunk feeder. Thus the name free-stall barn. Liquid manure is usually very wet, in the range of 2–12% dry matter (88% or more water!). Most large dairy farms automatically flush the concrete barn floors with water every fifteen or twenty minutes. This is followed by a slow-moving scraper that pushes all the cow poop into a receiving trough. Then the liquefied manure is conveyed along the trough to pass through a liquid-solid separator and then sent to a storage lagoon. The lagoons usually have several million gallons of capacity; enough for six months of storage. Sometimes plumbing fails and the liquid spills causing great ecological damage. Liquid manure is trucked by the tanker-load to farm fields miles from the cow barn. Some liquid manure is pumped via pipes or hoses to nearby fields. Once crops start growing, no more manure is applied.

Figure \(\PageIndex{1}\): A lagoon holding millions of gallons of liquid manure. Photo credit: Dan Bertalan.

Pasture cow manure is unmixed with water or bedding and simply falls onto the farm field in its semi-solid form. In well-managed grazing paddocks, the active soil life like dung beetles and earthworms will pull the cow pie into the earth in a matter of a week or two. A natural 'crust' forms on top of the cow pie to keep nitrogen from volatilizing and escaping into the air. Cows have four legs, let them walk out to the pastures, get exercise, eat, and spread their own manure on the fields and return to the ground the valuable nutrients. You have never heard of a manure spill from a pasture, since they do not happen. Manure spills only happen when manure is collected in a big hole in the ground and the man-made system fails.

Figure \(\PageIndex{2}\): A pastured cow herd at Full Circle Farm. Photo by: Valerie Dantoin

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. Composting manures is becoming an increasingly popular option for farmers. Compost stabilizes nutrients (although some Nitrogen as ammonium is usually lost in the process). Other benefits of composting manure are that there is a smaller amount of solid material to spread, and it stinks less than liquid manure 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. We have a composting barn at Full Circle Farm. We shred a large round bale of straw onto the area where the cows are free to come in and lie down. This 'bedded pack' stays warm all winter because it is actively composting. New straw keeps the animals clean and comfortable. The mass of manure and straw is left in place after the cows leave the open barn in spring to spend the next six months completely on pasture. We then compost the manure straw mixture by windrowing it and turning it to meet organic and produce safety standards. We reduce the volume of an entire winter's worth of manure for 100 animals by about two-thirds. When active composting on the cowyard is complete, we work it into the beds of the commercial organic vegetable garden on the farm, or we spread it back on a pasture that needs a boost. Our manure handling method uses about 1/3 of the fossil fuel energy required to start tractors, pump flushing water, run barn cleaner/scrapers, operate conveyors, load and haul tankers, and apply and incorporate the manure. In short, a compost barn uses microbes to handle manure rather than all the energy-intensive, man-made handling activities of CAFO manure systems

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 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 the storage method, it is important to understand how potential losses occur in order to select a storage method and location that minimizes environmental impact.

Anaerobic bio-digesters are sometimes used to process manure on large livestock farms. In these systems, specific bacteria 'eat' the manure and produce methane (and a few other gases) that are captured. The 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 bio-digesters can reduce greenhouse gas emissions, odors, and pathogens, and improve air and water quality. They are a major capital investment for a farm costing millions of dollars. To make them profitable, farmers typically need to make full use of the energy, carbon credits, tipping fees from external organic wastes, and co-products 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. Some farms re-use the solids from manure as bedding for their cattle.

Table \(\PageIndex{1}\): 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