Managing manure for economic and environmental benefit is based, in part, upon our ability to efficiently recycle manure nitrogen (N) between animals and crops. This article introduces University of Nebraska-Lincoln (UNL) proposed changes in recommendations for crediting manure nitrogen in a crop’s fertility program.

We propose an organic-nitrogen availability factor of 40% in year 1, 20% in year 2, and 10% in year 3 following application (40-20-10) for animal manures except for composted manures. We invite industry representatives to share their feedback to these proposed recommendations and possible implications before updating our Extension recommendations.

What the Research Shows

A UNL research team led by Dr. Charles Wortmann conducted research at six sites for three years and two other sites for two years in which the organic N availability of materials was assessed. The study involved a comparison of four livestock manures, three municipal biosolid (treated sewage) products and an industrial by-product. All were solids. The conclusions of this research (see links at end of article) suggested:

• The average fertilizer-N equivalence of applied organic N was 44% for the first crop, 21% for the second crop, and 10% for the third crop after manure application (Org-N equivalence of 44-21-10)[1];
• There is no basis for differentiation of the “fertilizer nitrogen equivalency” for pre-plant applied organic nitrogen sources; 
• On average the fertilizer-N equivalence of applied organic N was similar for rainfed and irrigated fields but greater for lower (< 3%) soil organic matter sites compared with higher (> 6%) soil organic matter sites;
• The ammonium-N credit from sprinkler-irrigation application of manures and holding pond water is 90% of total ammonium-N applied;
• Composted organic products produced similar fertilizer equivalency rates as other organic materials that were not composted.  However, these results are inconsistent with previous USDA ARS research in Nebraska for composted feedlot manure; and
• For corn fields receiving manure, a recommended fertility program should include decision criteria for supplemental pre-plant and in-season inorganic fertilizer.

Our Proposal for Manure Nitrogen Availability

Our current UNL recommended equivalences for organic-N are illustrated in Figure 1 (right hand box). For example, current recommendations for feedlot manure suggests the fertilizer-N equivalence for organic-N to be 25-15-7.

We propose to adjust UNL recommendations for nitrogen availability from animal manures.  Figure 2 summarizes these adjustments. The proposed Organic N rates are slightly lower than the averages observed by the Wortmann and others research.

Additional research (planned for publishing in 2020) suggests that nitrogen loss from the ammonium-N fraction in liquid manure when applied by pivot irrigation is low, far less than the 50% loss suggested by current recommendations. Our data supports increasing the availability factor of ammonium-N from sprinkler irrigation applied manures or runoff water to be 90%.

After a review of the results for organic-N availability from compost and our observation that these results were not supported by previous research with feedlot manure compost in Nebraska, our team agreed to not change recommendations for compost at this time pending a more complete review of the literature.

Benefits/Risks for This Proposal

Our recommendations for nitrogen availability were known to be conservative due, in part, to limited previous research from Nebraska. They regularly under-estimate the N credit available for manure. Farmers following the new recommendations will further validate their stewardship of our natural resources by:

• Increasing manure’s nitrogen credit, thus demonstrating greater economic value (lower inorganic fertilizer costs) for animal manures; and
• Reducing the total application of nitrogen (including manure) resulting in reduced leaching loss of nitrate-N and reduced nitrous oxide loss as a greenhouse gas; and
• Applying our same manure resource on more crop acres (N based manure applications will use lower manure rates) will result in more acres gaining from the soil quality and erosion reduction benefits of manure.

The primary risk is an occasional nitrogen deficit for crop development, most likely occurring when summer temperatures are cooler than normal. Cooler soils experience slower soil biological activity and less organic-N release. However, the yield response to nitrogen when applied near optimal rates is relatively flat (Figure 3). If manure nitrogen rate is applied near the peak net return (preferred) or yield, then an over-estimate of available N due to a cool summer is likely to maintain a yield near the optimum. The yield penalty from a cooler than normal summer is likely to be small and generally insufficient to offset the higher fertilizer costs associated with a more common manure nitrogen fertilizer equivalency during more typical years. The example shown in Figure 3 indicates that in a cool year when yields might be reduced by 5 bushels due to N deficiency of 40 lbs./ac, the net income loss fromthe N deficiency would be $4.35/acre. These conditions are infrequent and it would not be economical to over-fertilizer every year for this infrequent return.

It is our team’s judgement that corn fertility decisions that follow the new recommendations will increase net income and protect our natural resources.

Manure and Fertilizer Recommendations for Corn

Previous research has demonstrated a complimentary benefit for co-applied manures and commercial fertilizer. The global analysis of data from 15 field studies comparing co-applied poultry litter and fertilizer demonstrated an average yield increase of 18% over inorganic fertilizer alone. A separate global review of 56 field trials suggested co-application of manures (50 to 75% of applied N) and fertilizer observed a 13% increase in yield when compared with fertilizer alone. The co-application benefit will often be less for highly productive fields in Nebraska.

Solid manures, often low in ammonium-N and often surface applied, are most likely to benefit from co-application. Slurry or liquid manures that are irrigated or directly injected into the soil often provide sufficient ammonium-N that will nitrify and be accessible for the young corn plant (check left hand box in Figure 2 above).   

Manure organic-N availability will vary year-to-year from predicted values. Sensor-directed, in-season application of supplemental nitrogen¹ to a field receiving animal manure provides an opportunity for more fully achieving the synergistic benefits of these two valuable nitrogen sources. This is especially true for sandy loam or sandier soils. Supplemental fertilizer is best done with:

1. 60 lb./ac fertilizer-N applied or crop available manure ammonium-N applied pre-plant,
2. A non-limiting high N reference strip,
3. Use of sensing canopy reflectance for the Normalized Difference Red Edge Index with the in-season N rate calculated by the equation:
4. If an in-season N application is done, the minimum N rate should be 30 lbs./ac even when the sensor information indicates adequate N availability.

The in-season N application can be variable rate, but the N need is commonly uniform enough that the application can be at a single rate for the field or for management zones within the field. Fields that go from sandy loam or sandier to silt loam or silty clay loam are likely to need different rates. A single uniform rate for the field or management zone can be determined using the average of five or six observations with a handheld canopy sensor or using an aerial (drone, plane, satellite) image. Sandy soils may also require more than one in-season N application.

If high clearance application equipment is not available, an alternative for in-season N application is to apply in early June with the ammonium-N (fertilizer or manure) need determined using the pre-sidedress soil nitrate test. The 0-12” soil sample should be taken before corn is one foot tall (see CropWatch article on PSNT Spring Testing). A weakness of this method is that it does not account for organic N mineralized in June and early July. This alternative should include #1 and #4 above for fields receiving animal manures.

Additional Information

Garcia-Montealegre, J.P., C. Wortmann, J. Schepers, R. Little. 2019. Applied Organic Nitrogen: Pre-plant and in-season estimation of corn nitrogen uptake. Field Crops Res. 241.

Garcia-Montealegre, J.P., C. Wortmann, R. Ferguson, T. Shaver, and J. Schepers. 2019. Nitrogen sidedress guided by corn canopy reflectance for manured fields. Agron. J. 111:2453-2461.

Garcia-Montealegre, J.P., C. Wortmann, R. Ferguson, T. Shaver, and J. Schepers. 2019. Fertilizer equivalence of organic nitrogen applied in beef cattle manure. Nutr. Cycl. Agroecosyst. 114:225-235.

Shapiro, C., L. Johnson, A. Schmidt, and R. Koelsch. 2015. Determining Crop Available Nutrients from Manure.  NebGuide G1335.

[1] Org-N equivalence of 44-21-10 suggests that 44%, 21%, and 10% of the applied organic nitrogen in manure is the fertilizer equivalence for the first, second, and third cropping seasons, respectively, following manure application.

[2] Crop available nitrogen from manure factor illustrated in Figures 1 and 2 indicate the fraction of ammonium or organic-N in manure that may be credited against the crop nitrogen requirement.  See NebGuide G1335 for calculation procedure for manure’s Crop N Credit per acre.