Manure Management on Organic Farms
Manure Application Methods
Choosing a method of manure application depends on the physical characteristics of manure (liquid or solid), type of operation, manure handling and storage, type of spreader and cost. Typically, beef cattle and poultry manure are handled as solid manure, whereas dairy and swine manure are stored and handled as liquid manure. The application system used will have implications for nutrient management and environmental risk, in particular nutrient placement and nutrient retention.
Solid Manure Application
Most solid manure (20% or more solids) is spread using broadcasting equipment, followed by tillage to incorporate the material into the soil. It is possible to broadcast during any season; however, there are some limitations to this method. When applied to the surface of some soils, nutrient sources containing urea or ammonium-N can be lost as a gas through a process called volatilization. The volatilized (or vaporized) nitrogen is usually in the form of ammonia.
Solid Manure Application Equipment
Solid manure application equipment (commonly referred to as spreaders) comes in various types including box and side-discharge spreaders. Most tractor-drawn spreaders are PTO operated, but some are driven from the ground wheels. Some are hydraulically powered for greater speed variation, especially for the apron drive, to vary the application rate. Rear-delivery technologies may include a spinner spreader, single or double horizontal beaters, and vertical beaters. Manure spreaders may be tractor-drawn models or they may be mounted on a truck.
Side-Discharge Box Spreaders
Side-discharge spreaders are open-top spreaders that use augers within the hopper to move wet manure toward a discharge gate (See Figure 9.3). Manure is then discharged from the spreader by either a rotating paddle or set of spinning hammers. Side-discharge spreaders provide a uniform application of manure for many types of manure with the exception of dry poultry litter.
Box Spreaders with Vertical Beaters
These spreaders provide an excellent spread pattern and exceptional breakup of materials. These characteristics, combined with a fast unloading time, make this type of spreader an ideal choice for producers seeking to take maximum advantage of the nutrient value of their manure, from pen pack to dry yard manure.
Box Spreaders with Spinner Beaters
Box spreaders with spinner discharge can handle many dry, flowable, high-value materials such as compost and poultry litter. A heavy-duty metering end gate, effectively controls the movement of materials to the spinners.
Box Spreaders with Horizontal Beaters
A box spreader with horizontal beaters is a great choice for a basic manure spreader (See Figure 9.4). It has a relatively low power requirement and is simple to operate. In addition, the spread pattern of this type of machine is relatively narrow and covers just the width of the spreader to the outside of the tire tracks. Single-beater spreaders have difficulty spreading manure uniformly to take full advantages of nutrients in manure.
Liquid or Slurry Manure Application
Liquid or slurry manure (less than 12% solids) can be surface applied or directly injected using a number of different systems. Because of relatively low nutrient concentration, liquid livestock manure may be applied at relatively high volumes, but it is generally recommended that it not be applied at rates that exceed the soil infiltration rate, nor exceed the amount needed to bring the soil to field water holding capacity.
Broadcast application of liquid manure slurry has the highest rates of airborne nitrogen loss because there is greater exposure of the manure to wind and sun/heat, which increases ammonia volatilization potential. Tank spreader systems, equipped with splash plates, are very common systems used to apply liquid manure.
Surface banding of liquid manure uses coulters, chisels, or shanks for placing the manure in a series of narrow bands on the soil surface, increasing contact with and incorporating it into the soil. Surface banding can increase nitrogen availability and reduce ammonia emissions by up to 80 percent, compared to traditional surface broadcasting.
Liquid or Slurry Manure Application Equipment
Liquid or slurry manure application equipment comes in various types including broadcast spreaders, trailing hose spreaders, shallow disk injection, and chisel (knife) injection with sweeps.
These combine a tractor and a tanker with a liquid manure spreading device at the rear. The liquid manure is forced under pressure through a discharge nozzle, onto an inclined splash plate to increase the sideways spread.
Trailing Hose Spreaders
In a trailing hose system, manure is pumped from the tank through a chopper/distributor manifold to a series of hanging or trailing pipes attached to a boom. The hoses are dragged along the surface and lay manure in bands along the soil just below the crop canopy. The width of a band spreader is typically 40 feet (12 m), with about 12 inches (30 cm) between bands.
Shallow Disk Injection
Disk injectors typically include a set of coulters that cut crop residues and make furrows in the soil; drop hoses for placing manure in the furrows (See Figure 9.5). The manure is not covered, but is contained in the slot. The reduced exposure to the air lowers volatilization and surface runoff losses. The result is reduced odor, reduced ammonia loss, and more uniform distribution of manure.
Chisel (Knife) Injection with Sweeps
Injector-type applicators (either tanker mounted or tractor-mounted and hose-fed) usually have injector chisels placed on 20- to 25-inch (50 to 64 cm) centers, cut six- to eight-inch (15 to 20 cm) deep grooves in the soil into which the manure is placed (See Figure 10.8). The chisel can be adjusted to a desired depth. Typically, the injector chisels are mounted on a tool bar and a manifold directs manure streams close to the chisels, usually just behind them. Chisel injectors may have an advantage in heavy soils if subsoiling is needed to break up a clay pan.
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