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January 2008

Optimizing Use of Nutrients for Crops:
Dairy Manure Replaces Commercial Fertilizer

As the cost of fertilizer rises land application of manure becomes an increasingly attractive alternative. While expensive itself, land application of manure can provide soil with nutrients needed for crop production. Developing effective and responsible land application methods is important, though. The guidelines in this article can be the basis for manure land application strategies that increase profits, minimize environmental risks, and ease tensions with neighbors.

Natalie Rector
Extension Nutrient Management Educator

Jon Rausch
Extension Program Director
Environmental Management
The Ohio State University

As commercial fertilizer prices continue to increase, use of livestock manure as a nutrient resource for crop production is becoming increasingly more important. Table 1 shows the expected nutrients available from dairy manure when it is injected in the spring compared with being broadcast during hot, dry conditions of summer. Under hot and dry conditions, surface applications of manure result in volatilized ammonium-nitrogen reducing the quantity of nitrogen (N) available for the following crop. Each manure type and source will have different nutrient content. The decision to replace fertilizer with manure should only be made by utilizing a representative manure sample. For this article, the sample composition and nutrient values in Table 1 would be typical of manure from the milking herd. Producers are encouraged strongly to use their own values from actual analysis in decision-making for their specific situations.

Table 2, Table 3, and Table 4 highlight the expected commercial fertilizer costs and manure nutrient application costs for three crop rotations. The cost of each fertilization program is different based upon the out-of-pocket (direct) costs associated with the purchase of commercial fertilizer and the associated cost(s) of applying the respective nutrient. Each producer’s cost of manure and fertilizer application depends on the equipment they own and or rent. There is a wide cost range depending on the size and type of equipment and the distance that must be traveled to land-apply the manure. Farmers are encouraged to calculate their application costs for both fertilizer and manure to make this example more relevant to their operations. Custom application rates also are variable and should be the actual costs for each specific operation.

A successful manure nutrient management program maximizes the value of manure nutrients by retaining N, giving proper credit for the phosphorus (P), and potassium (K), and minimizes purchased fertilizer and environmental risks.

Application practices that conserve ammonium-N and apply manure where P and K are needed and fully utilized by growing crops are important steps to a manure nutrient management program that minimizes the cost to producers. Typically, the most profitable strategy also minimizes environmental impacts because manure nutrients are recycled to meet the needs of a growing crop and are not lost to air or water in the environment.

Corn-soybean Rotation

Table 2 compares the nutrient input costs for a corn-soybean rotation when a soil test indicates a likely yield response from additional nutrient inputs over a 4-year rotation. This assumes P and K are needed and fully utilized by the growing crops. In this strategy, the N-P-K needs are met by commercial fertilizer or by manure supplemented with commercial fertilizer. The commercial fertilizer in Option 2A is expected to cost $298 over 4 years. In Option 2B, the cost of the animal manure plus fertilizer N is expected to be $205 over 4 years. A total of 160 pounds of N is needed for a 140 bushel per acre corn crop and is supplied by 6,250 gallons per acre of dairy manure injected in the spring. The remaining 60 pounds of N per acres is applied as side dress, which should be enough to justify the side dress application while taking some of the risk out of relying on a perfect manure analysis and application. At this manure application rate, excess P and K are supplied and will be banked in the soil for the next soybean crop. No additional nutrients will be needed for the following soybean crop. Under these assumptions, manure plus commercial fertilizer (Option 2B) is expected to save $93 per acre compared with a 4 year rotation that utilized all commercial fertilizer (Option 2A).
Over the life of this rotation, the potash levels may need to be supplemented with commercial fertilizer. Phosphate (P2O5) levels may increase slowly, but because this field needed P, building it with manure is less expensive than building it with purchased fertilizer.

Corn-soybean-wheat Rotation

Table 3 compares the nutrient input costs for a 3-year corn-soybean-wheat rotation when a soil test indicates a likely yield response from additional nutrient inputs. This assumes P and K are needed and fully utilized by the growing crops. In this example, the N-P-K needs are met by commercial fertilizer (Option 3A) or manure supplemented with commercial fertilizer (Option 3B). The commercial fertilizer option over the 3-year rotation is expected to cost $243 per acre. Alternatively, the cost of applying animal manure plus the value of commercial fertilizer is expected to cost $160 per acre over this same rotation. There is $83 per acre advantage of using manure compared with commercial fertilizer alone over a 3-year period.

In the above example, 10,000 gallons per acre of dairy manure are injected in the spring. This should supply all the N and excess P and K for the corn crop. Excess P and K will be banked and utilized over the next soybean and wheat crops. In this strategy, $83 per acre was saved by utilizing manure and valuing the nutrients in the total crop rotation. Over the rotation, the potash may need to be supplied with commercial fertilizer. Phosphate (P2O5) levels may increase steadily, but because this field needed P, building it with manure is less expensive than building it with purchased commercial fertilizer. This strategy assumes that the N from the manure is not being lost and is being applied evenly and consistently across the field.

Alternate Corn-Soybean-Wheat

The final strategy in Table 4 is a 3-year corn-soybean-wheat rotation, but manure is broadcast on wheat stubble the summer prior to corn and soybeans. In this strategy, dairy manure is applied at the rate of 6,250 gallons per acre (Option 4B). Only 31 lb/acre of N is expected to be available for the corn crop, a loss of nearly 70 lb/acre or $34/acre due to N volatilization. This strategy just breaks even when manure is compared with purchased fertilizer (Option 4A). Nearly 70 lb of potential N was lost into the air due to the timing and application method. All the P2O5 and K2O will be available for future crops.

In the above strategies, only the nutrient value of commercial fertilizer and the hauling/application costs are considered. The value of utilizing manure should be discounted by the P and K values when manure is applied on fields that do not need the nutrients. Therefore, current soil tests are important to directing where manure nutrients will best be utilized. There are other agronomic benefits to manure for crop production including improved soil tilth and organic matter, which improves water infiltration and water holding capacity. Manure also supplies micro nutrients. There also must be some value to a nutrient management plan that is less dependent on commercial (synthetic) fertilizers that are produced from fossil fuels. Negative aspects of manure application may include soil compaction, especially if manure is applied when soils are wet. There is also the risk of complaints from neighbors not accustomed to manure in their area. Fortunately, the same practices that help retain N (injection, rapid incorporation) also reduce complaints and reduce the risk of manure leaving the field.





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