It is important to know the value of farm manure in order that more care may be exercised in conserving and applying it to the land. Too often it is regarded as a waste material to be disposed of in the easiest way possible. Frequently, no care is taken to prevent losses before it is applied, and on many farms, fields at some distance from the barns and feed yards are sometimes not manured
at all.”

The quote comes from Iowa State Experiment Station Bulletin No. 236 (1926). It illustrates the confusion that has existed for many years surrounding the value of manure. Eighty-five years after the bulletin was published, many pork producers still wrestle with the challenge of gaining maximum value from manure.

Surface-Applied Manure Value

Manure value is not determined solely by the quantity of nitrogen (N), phosphorus (P) and potassium (K) it contains. Other factors that affect value include the nutrient content of the manure, crop needs, cropping system, soil fertility, the nutrient-limit rule and the application method.

Let’s look at a few examples of how 500,000 gallons of swine pit manure is applied to corn and soybeans to illustrate how various factors affect manure value.

Table 1 indicates that a corn field yielding 160 bu./acre would remove 145 lb. of nitrogen, 58 lb. of phosphorus and 43 lb. of potassium. If the producer wanted to meet all of the nitrogen needs for that field (145 lb.), he/she could surface-apply pit manure at the rate of 5,100 gal./acre. But this would over-apply phosphorus and potassium. Unless the field is low in phosphorus and potassium, the over-applied nutrients do nothing to increase yield and therefore are not valued.

The value of the manure applied at 5,100 gal./acre is $125.90/acre, using the values shown for each nutrient in the table. With this application rate, the 500,000 gallons of manure would cover 98 acres and have a total value of $12,343.

Injected Manure Value

If we change the method of application from surface-applied to soil-injected, the economics change (Table 2).

Injection conserves more of the nitrogen, so to meet the 145-lb. nitrogen requirement, only 3,700 gal./acre needs to be applied vs. 5,100 gal./acre when manure was surface applied. The value per acre remains at $125.90 because the cropping system has not changed.

Remember, manure value is determined by what the crop needs, so if production is the same, the value of the manure is the same. But in this example, more acres are fertilized with the 500,000 gallons of pit manure available, so the total value of the manure is $17,014. More nitrogen reaches the corn, thus less phosphorus and potassium are wasted from over-application.

The excess phosphorus and potassium can be used, and valued, if the crop following corn is soybeans (Table 3). A 50-bu./acre soybean crop removes 44 lb. of phosphorus and 68 lb. of potassium.

When manure is applied every other year to corn in a corn/soybean rotation, all of the nitrogen and all of the potassium are valued and only the phosphorus is over-applied. The value of the manure is $184.10/acre when it meets the corn and soybean nutrient removal rates. Because more of the phosphorus and all of the potassium are valued, the total value of the 500,000 gallons of pit manure rises to $24,878 on the same 135 acres as in the previous example (Table 1).

Capturing Full Value

In these examples, it was assumed that manure is only needed to replace nutrients removed by crops. However, if the soil is low on phosphorus and potassium, the full value of the manure can be obtained because none of the nutrients are over-applied. Table 4 shows the value of the manure if it is applied to a corn/soybean rotation on land currently short on phosphorus and potassium. The full value of the 500,000 gallons applied to the 135 acres is $28,818. The producer gains a minimum of $3,940 by choosing land that needs all three nutrients.

Table 5 provides a quick comparison of the management decisions facing the producer. The lowest value is for surface-applied manure to a continuous corn cropping system. By simply injecting the manure, the value increases almost $5,000. The question becomes: “How many years would it take to pay for new equipment that could inject rather than surface apply?”

Assuming manure is applied to a corn-soybean rotation, finding land that is short of phosphorus and potassium and can use all of the nutrients supplied can increase the value of the manure by almost $4,000. The question then becomes: “How far can I haul the manure to find land that needs the phosphorus and potassium available in my manure?”

Any management option that increases the value of the manure beyond the cost of making that management change is extra money in the pocket. Increasing value can frequently cover the increased costs of properly managing manure-supplied crop nutrients.