Harvest has taken off in recent weeks as crops are quickly drying down. The open fields are tempting manure applicators to get started on their manure applications; however, it is worthwhile to pause and consider the fertilizer value that is given up when manure is applied to warm soils.
What is the value and cost of manure?
For crop production, consider manure’s nutrient content and set a baseline price per unit of each nutrient using prices for commercial fertilizer. Even if you aren’t “paying” for the manure nutrients, you are forfeiting real money if you don’t manage manure to maximize its value. This means carefully considering manure application rate, timing, and method. 
- A liquid swine finishing manure could be valued this fall at about $46 per one thousand gallons. Over half of that value, $25, is from the nitrogen alone.
- In this example, a manure nutrient analysis of 50-15-30 pounds N, P2O5, K2O, per one thousand gallons was assumed. Nutrient prices used were $0.50 per pound of N, $0.60 per pound of P2O5, and $0.40 per pound of K2O, based on current fall 2023 price trends for anhydrous ammonia, MAP, and potash fertilizers.
- A producer who manages manure application to maximize the manure’s value could offset as much as $100 per acre in commercial nitrogen fertilizer and $184 per acre total when also considering P2O5, and K2O.
- At an assumed application rate of 200 pounds of N, or 4,000 gallons of manure per acre.
- There is undoubtedly additional value from manure that has not been considered, such as sulfur, zinc, and other micronutrients which would further increase the manure’s worth.
- The cost to haul and apply manure might be $120 per acre or more with an application rate of 4,000 gallons per acre (assuming $0.03 per gallon or more, depending on how far it is hauled).
This means that manure users might be paying more to haul and apply this year’s manure than it is worth based on nutrient value if they aren’t managing the manure application rate, timing, and method to prevent nitrogen loss.
When is manure application “go-time”?
Injecting into very dry soils can be difficult, so watch and wait for soil conditions that are favorable for injection so that manure makes good contact with soil below the surface to limit ammonia volatilization.
Keep an eye on the ISU Soil Moisture Network County 4 inch Soil Temperature Maps and wait for soil temperatures to be cool enough for manure application, 50 degrees and cooling. This is the same as recommended for anhydrous ammonia applications. As of Oct. 8, 4-inch soil temperatures ranged from the mid to upper 50s across much of the state. Cool weather is in the forecast, but like most years, we aren’t likely to have soil temperatures consistently 50 degrees and falling until early November.
Applying manure to warm soils results in rapid conversion of ammonium to nitrate, which is prone to leaching loss if fall or early spring rain leads to excess soil moisture. This conversion process slows when soils are cooler than 50 degrees but doesn’t stop until soils are at the freezing point. Although conditions are generally dry across the state now, the risk for leaching and loss of useable nitrogen does become a reality most springs in Iowa and could still be a concern this fall if conditions get wet.
Research has consistently shown that delaying high-ammonium manure application improves crop yield, likely because more of the manure nitrogen is available for use by the crop.
- This research summary reports corn yield advantages of 19 bu./ac. for late fall manure applications (soil temperatures of 50 degrees and cooling) compared to early fall applications with warmer soils (12 site-year average) and 33 bu./ac. for spring applications compared to late fall with cool soils (four site-year average).
It makes economic sense to save some of your manure for application in the spring, even though it may not be practical for all of it considering the logistical challenges of spring manure applications (compaction concerns, relatively short time window, etc.).
Are you using a cover crop this fall?
Crop producers can limit manure nitrogen loss with a cover crop, like cereal rye, because that cover crop can utilize that nitrogen rather than letting it leach away. As indicated by recent research in Iowa, seeding a cover crop is especially important and beneficial when pits are full early in the fall and consequently some manure absolutely must be applied before the soils have appropriately cooled.
What about the P and K?
To maximize the value of the phosphorus and potassium in the manure, consider application to fields with below optimum soil test P and K levels using recommendations found in PM 1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. Compare the value of the P and K in the manure to the cost of hauling the manure. The value of those nutrients may offset the additional cost to haul manure farther away from the barn site if that would replace a commercial fertilizer application.
Additional Resources:
The Manure Scoop: Optimizing Crop Success: The Crucial Role of Manure Application Timing in Iowa
Iowa Manure Management Action Group (iastate.edu)
IEM :: ISU Soil Moisture County Temperature Estimates (iastate.edu)
Early Fall Applied Manure Can Lead to Corn Yield Loss | Integrated Crop Management (iastate.edu)
A General Guide for Crop Nutrient and Limestone Recommendations in Iowa (iastate.edu)
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on October 9, 2023. The information contained within may not be the most current and accurate depending on when it is accessed.