Odor and Nutrient Management Newsletter

Fall 2001

Manure management demonstration at Thole-Humphrey Farm

by Eric Palas, Division of Soil Conservation, Iowa Department of Agriculture and Land Stewardship

What are all those orange stakes doing in your field? This question has been a common one for Don Thole this spring. Although the field in question may have looked like a housing development early in the season, it is actually the site of a manure management demonstration. The goal of the demonstration is to gain insight into the crop nutrient value of the dairy manure Don hauls and spreads.

Photo by Charles Wittman, Maquoketa River Watershed Project. Manure provides a good source of nitrogen (N), phosphorus, and potassium for crop production. Because manure is a concentrated source of these nutrients, proper management of this resource also has become an environmental concern. Refining commercial fertilizer and manure applications can have a major impact on a farm’s bottom line, as well as on the water quality of Ensign and Hewett creeks.

One of the first steps in the demonstration was to determine the amount of manure being spread. ISU Extension nutrient management specialist Chad Ingels used a set of portable scales to calibrate Don’s manure spreader in April. After determining the loaded weight of the spreader, Chad measured the area Don covered with manure and reweighed the empty spreader. A sample of the manure was then collected and submitted for laboratory analysis.

Based on the calibration, Don was spreading 17.8 tons of manure per acre. The laboratory analysis showed that for each ton of manure, 13 pounds of N, 6 pounds of phosphate, and 10 pounds of potash were applied. The significance of the nutrient value of the manure becomes apparent. A total of 231 pounds of N, 107 pounds of P2O5, and 178 pounds of K2O was being applied per acre from the manure.

Half of the plot area was spread with manure. A conservative first-year N credit of 88 pounds per acre was used. This number was determined by first multiplying the total of 231 pounds of N available in the manure by the 40 percent or the first year N availability for dairy manure (according to ISU Extension publication PM 1811, Managing Manure Nutrients for Crop Production) to equal 92 pounds of N. The next step was to determine the N availability based on the type of manure application. Because the manure was incorporated within 24 hours of application, approximately 95 percent of the total N would be available for crop growth, or 0.95 x 92 = 88 pounds of N. If the field had a history of manure applications in recent years, a larger N contribution from manure would have been estimated. The plot is approximately a half-acre. Aside from not applying any commercial N, Don has managed the corn exactly as he would normally treat his crop acres.

Various increments of N were hand applied after planting in April. Each plot treatment is roughly six rows in width by 40 feet in length and is replicated three times within the demonstration. In June, the late-spring test for soil nitrate was used to estimate the amount of available N within the plot treatments. This test is conducted when the corn plant is 6–12 inches in height at the whorl. Results of 20 parts per million (ppm) or more generally indicate sufficient amounts of available N for corn production. Results are shown in Table 1.

Table 1. Late spring soil nitrate test results from Thole-Humphrey demonstration for 2001.

 Plot Treatment


Check (no manure, no N)


Manure only


Manure plus 50 pounds of N


Manure plus 100 pounds of N


Commercial N at estimated manure credit value


100 pounds of commercial N


Based on the results of the test, all of the treatments except for the check area seemed to have adequate amounts of available N this spring. The end-of-season cornstalk test will help determine whether there was adequate N available to the crop throughout the growing season. Corn yields will provide another key source of information from the plot. Approximetely 20 feet of the middle two rows of each treatment will be hand-harvested and weighed after crop maturity this fall. Similar demonstrations were conducted at eight sites in the Maquoketa River Watershed during the 2000 crop season. Average corn yields for the eight sites are shown in Table 2.

Table 2. Corn yields from eight manure management demonstrations for 2000.

 Plot Treatment (8 sites)


Check (no manure, no N)


Manure only


Manure plus 50 pounds of N


Manure plus 100 pounds of N


Commercial N at estimated       manure credit value


100 pounds of commercial N


A good rule of thumb is that each cow and her replacement in a dairy herd produce enough N from manure to fertilize 1 acre of corn at a 150-pound N rate. Because Don and his family milk more than 100 cows, the manure that is produced by the dairy herd has the potential to meet the N needs of a significant portion of Don’s corn acres. It is just a matter of properly collecting, storing, and spreading the manure on the right acres, and making the necessary adjustments to commercial N applications once the manure is applied. The demonstration conducted this crop season provides a key source of information for this purpose.

 This article was adapted from the Ensign Hollow II Watershed Project Summer 2001 Newsletter. For more information on this demonstration call (563) 245-1048.

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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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