# Manure Nitrogen Use for Increased Profit and Environmental Protection

February 17, 2020

Land application of organic materials is important in Nebraska. This article builds on an earlier CropWatch article. Since then, the data were further analyzed and the results further interpreted. This article avoids much repetition of the 2019 article but gives some new or revised information.

Land application of manure and other organic materials supplies much N to Nebraska’s crop production. In contrast to most other nutrients applied in organic materials, the availability of manure-N and its fertilizer-N substitution value is not well-predicted. Supplementing manure N with in-season fertilizer-N application according to crop need expressed by canopy light reflectance is a means to most profitable manure- plus fertilizer-N use.

Research was conducted at six sites for three years and two sites for two years in which the organic N availability was assessed and sensor-directed in-season N application was fine-tuned involving a comparison of six livestock manures, three municipal biosolid (treated sewage) products and an industrial by-product. All were solids.

Organic N Availability

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 (44-21-10) for a total of 75% availability during the three years following application. For comparison, with current UNL recommended equivalences for organic N in year 1, 2 and 3 after application are:

• 25-15-7 for feedlot manure, 63% of average availability;
• 15-15-7 for compost, 49% of average availability;
• 30-15-7 for poultry manure with litter, 69% of average availability; and
• 35-15-7 for poultry manure without litter and for uncomposted solid municipal biosolids, 76% of average availability.

The current estimates of organic N availability which are lass than 20 years old are intentionally conservative to protect against yield loss, such as due to uneven application. However, manure application equipment and awareness has improved for much more uniform application. The manure N credit can be increased for maximizing profit and reduced loss of N to the environment such as nitrate-N leaching to aquifers and emission of nitrous oxide. If such losses are of sufficient environmental concern, the manure N credits may be even greater than the 44-21-10 due to the variations in availability across locations and years.

There was much variation in the fertilizer N equivalence across site-years but, on average, it was similar for all of the organic materials tested although these differed for C:N ratio and the hemicellulose, cellulous, and lignin contents. 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.

In-Season N Application to Manured Fields

The variation in fertilizer-N equivalence from site to site indicates the low predictability and an opportunity for profit gain and environmental protection through sensor-guided in-season N application to manured fields. With 60 lb /ac of fertilizer-N applied pre-plant, the average in-season fertilizer-N rate was 43% less with manure applied compared with no manure for the year of application and 17% less for the second and third year after application. The in-season fertilizer-N rate decreased with increased manure rate. The in-season fertilizer-N was efficiently used resulting in 30.5 lbs of additional grain yield per lb of in-season fertilizer-N applied and 82% of the in-season fertilizer-N was recovered by the crop.

Sensor-directed in-season N was most accurate with

• a high N reference strip,
• 60 lb/ac fertilizer-N applied pre-plant,
• in-season N application between V12 and V14,
• a minimum of 30 lb/ac in-season N even when the sensor information indicates adequate N availability,
• variable rate for in-season N only if the N need is sufficiently variable,
• use of NDRE (Normalized Difference Red Edge Index) for canopy reflectance and,
• use of the following algorithm to calculate the in-season N rate.

$${\text{In – season N rate }(\frac{lb}{ac}) = 317\sqrt{0.97-\text{SI}}}$$ $${\text{Where SI (sufficiency index)} = \frac{\text{NDRE}}{\text{NDRE for the reference strip}}}$$