Odor and Nutrient Management Newsletter

Fall 2003

Calibrating Slurry Tank Manure Applicators

by Kapil Arora, Agricultural Engineering Field Specialist and Mark Licht, Department of Agronomy

Correct application rates for manure has become more critical over the last few years. Future manure plans may require appropriate management of phosphorus (P), which for some producers will mean much lower application rates. The responsibility of livestock producers is not limited to completing manure management plans. It also requires that the actual application rate the field is in compliance with the approved manure management plan MMP).

Producers or custom applicators must know their application equipment and manure characteristics to ensure accurate application rates in the field. Liquid manure applicators generally have a slurry tank with pump that feeds to the distribution chamber, which in turn distributes manure to different points on the tool bar. A slurry tank generally has a specific design capacity for the quantity manure it can hold.

However, due to manufacturing variations, manure foaming, and solids build-up the slurry
tank may not be filled up to the rated capacity. Therefore, knowledge of how many gallons a slurry tank may hold is important.

Inaccuracies may result in either under-application that may affect crop yields or over-
application, which is not allowed by the manure management plan and could potentially cause loss of nutrients to the environment.

Producers and custom applicators can easily calibrate their manure applicators after following instructions on how to weigh the slurry tank applicator (full and empty), measure the spread pattern (length and width of spread) and application area, measure manure density, and calculate manure application rates.

Weighing manure tank wagon for calibration.
Weighing manure tank wagon for calibration.

Weigh-pad used to weigh manure tank wagon.
Weigh-pad used to weigh manure tank wagon.

Weighing the slurry tank applicator. Manure applicators can be weighed at the local grain elevators but this will take two trips: one to weigh the applicator full and one to weigh it empty. An alternative is to use weigh pads in the field or at the confinement site. Weigh pads should have enough capacity to weigh each tire without the tires squatting over the pad and touching the ground. If using weigh pads, make sure to take into account the weight on the hitch. Record both the empty and full weights of the manure applicator. If you weigh on a scale that weighs the complete unit, record the total full weight and total empty weight. The weight of the manure is the full weight minus the empty weight of the manure applicator.

Measuring the spread pattern and application area. A spread pattern refers to how widely and how far an applicator covers before it empties the slurry tank. To calculate the spread pattern, measure (in feet) the number and spacing of knives. The spread pattern width is obtained by multiplying the row spacing by the number of knives. In the case of splash plate applicators, the spread pattern width is the actual splash width over the rows in the field. To measure the length of the spread pattern, use a measuring tape, a measuring wheel, or a range finder. Range finders generally require a reflective surface to provide a measurable reading. A measuring wheel may be the most practical, although it does require the producer to walk the whole length of application. The number of acres the manure was applied to equals the spread pattern width multiplied by the linear distance the manure was applied, divided by 43,560.

Measuring manure density. Water weighs 8.34 lbs. per gallon. However, liquid manuremay weigh more based on the presence of organic solids and other heavy solids such as sand and silt. As a rule of thumb, liquid swine manure ranges in density from 8.3 to 8.9 lbs per gallon. As different livestock operations are managed differently, swine manure density is likely to vary from one operation to another. To calculate manure density, perform a five-gallon bucket test. This test requires a five-gallon bucket, a one-gallon measuring flask, and a scale.

Fill the measuring flask up to the one-gallon mark with water and pour it into the five-gallon bucket. Repeat this process four times. Mark the five-gallon bucket where the five measured gallons of water leveled off. Now, empty the bucket and fill it with manure up to the five-gallon mark. Weigh the five-gallon bucket on a scale. Repeat the process to get at least three weight readings. Then, divide the average weight of the five-gallon bucket of manure by five, to get the pounds of manure per gallon.

Calculating manure application rates. To calculate the application rate, divide the net manure weight by the manure density, to get the number of gallons applied. The actual application rate is the number of gallons divided by the number of acres to which manure was applied.

Manure application rates can be altered by adjusting the speed of application, adjusting the diameter of the gate opening, or by other methods. For more information on manure calibration and application, contact your area extension agricultural engineering field specialist.

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Iowa Manure Matters: Odor and Nutrient Management is published by Iowa State University Extension, with funding support from the USDA Natural Resource Conservation Service through Cooperative Agreement No. 74-6114-8-22. To subscribe or change the address of a current subscription, write to Angela Rieck-Hinz, 2104 Agronomy Hall, Iowa State University, Ames, Iowa, 50011-1010 or call 515-294-9590, fax 515-294-9985 or email: amrieck@iastate.edu. Please indicate you are inquiring about the Odor and Nutrient Management Newsletter. The newsletter's coordinators are Angela Rieck-Hinz, extension program specialist, Department of Agronomy, Wendy Powers, environmental extension specialist, Department of Animal Science, and Robert Burns, Department of Agricultural and Biosystems Engineering; the editor is Jean McGuire, the subscription manager is Rachel Klein, the production designer is Beth Kroeschell, and the web page designer is Liisa Jarvinen.

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