Public awareness of nitrogen (N) and phosphorus (P) and how they affect water quality continues to increase, especially with the growing number of large livestock operations. However, a significant number of livestock farms are still considered small animal feeding operations (up to 267 dairy cows, 1,333 market hogs or 640 feeder cattle) by the Iowa Legislature, and these operations are not subject to writing manure management plans and the resulting regulations.

Information from the Iowa Statistics Service shows that in 1998, the average dairy farm in Fayette County and Clayton County had 67 and 58 cows, respectively. The smaller farms are not required to produce detailed manure management plans. Each year, a 75-cow milking herd and replacements produce approximately 2,500 tons of manure containing 30,000 pounds of N and 15,000 pounds of P. As producers of a nutrient rich by-product, small livestock operations need to manage on-farm nutrients in an environmentally and economically sound manner.

With spreader calibration and a nutrient management plan, an economic analysis can be established for manure produced on an average 75-cow dairy herd in northeastern Iowa. A typical dairy herd produces 32.77 tons of solid manure per milk cow per year. (See Iowa State University Extension publication Pm 1811, Managing manure nutrients for crop production.) The 75 cows and replacements create 2,458 tons of manure per year. For this example, assume that manure (85 percent) is collected and surface-applied on the available cropland. The 2,089 tons of manure to be spread has an analysis of 12-6-12 (N-P-K [potassium]), and 70 percent of the N will be available to the crop after in-field volatilization. (See Pm 1811.) If 50 percent of the N will be available to the crop the first year, there will be 8,774 pounds of N; 12,534 pounds of P; and 25,068 pounds of K credited for crop use if soils are testing in the optimum range or lower.

The farm in this example will produce 150 bushels of corn and 50 bushels of beans per acre in a corn-bean rotation. There are 130 acres of corn per year following soybeans. A credit of 50 pounds N per acre is used in the crop rotation. To

supply the rest of the required N needs, the farmer typically applies N as anhydrous ammonia (NH3) and has the local cooperative spread a crop removal rate of potash and phosphate following beans. The total commercial N applied is 120 pounds per acre.

When developing a manure management plan, there are two different methods to determine the desired application rate. The first, and most widely used, method is to apply manure based on the desired N rate for the crop. The second method is to apply manure at the crop removal rate of P. This latter method may be required in the future for manure management planning. The economic benefit of each method can be compared in the table.

1Nutrient prices and spreader rate supplied by a northeastern Iowa fertilizer supplier.
  2 Iowa State University Custom Rate Survey.

Commercial N for the crop is from two sources: 102 units are from the anhydrous application at $0.14 per unit, and 18 units are from the plowdown fertilizer at $0.20 per unit. The unit rates for the P and K are $0.211 and $0.137, respectively. Planning for application at the N rate would require spreading manure on 73 acres (120 pounds N per acre) and commercial fertilizer on 57 acres each year. The costs associated with manure hauling are not included because it would be hauled to some fields anyway.

Applying manure at 28.6 tons per acre would provide the nutrients for this scenario. Results from Northeast Iowa Demonstration Project spreader calibrations show the average 280-bushel spreader applies 21.4 tons per acre. A nutrient management plan with the N rate application of manure would save $4,528.19 per year in input costs compared with the current plan where no manure credits are taken.

Using the P rate for manure application would require 130 acres each year. Fifty-five units of N per acre would need to be purchased under this plan; however, the P and K would not be overapplied and would maintain the soil test values.

The use of the P rate plan would result in spreading manure at a rate of 16 tons per acre. Profit would be increased by $6,243.90 per year when using the manure at the P rate application. This is an increase of $1,715.71 above the savings with the N rate plan.

To feel comfortable with taking manure credits, it is essential to calibrate the spreader to achieve the desired application rate of nutrients. With proper calibration, it is easy to acknowledge the nutrient credits that are available and apply the cost savings directly to the farm’s bottom line.

For more information about this topic and the nutrient demonstrations in the watershed, call the Maquoketa Watershed Project staff at (319) 425-3233. Staff are also available to help producers calibrate their manure spreaders.