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Improving Manure Nitrogen Use Efficiency

By Charles Wortmann, Juan Pablo Garcia Montealegre, Rich Little, and Aaron Nygren
 
Land application of organic materials for soil management in Nebraska is important.
  • Organic N applied annually to Nebraska cropland is equal to 150 lb/ac fertilizer N applied to about 1.3 to 1.6 million acres of corn.
  • Beef feedlot manure, other livestock manure, municipal biosolids, and organic industrial wastes are important for application of organic N.
  • The availability measured as fertilizer N equivalence of applied organic N, expressed as pounds of N fertilizer equivalent per pound of organic N applied, is not well-predicted.
  • The uncertainty of fertilizer N equivalence for applied organic N leads to over-application of fertilizer N, resulting in low N use efficiency.
  • Pre-plant estimates of fertilizer N equivalence might be improved in consideration of organic material properties and field conditions.
  • Canopy sensor directed in-season N application practices have been validated for corn produced on unmanured fields and may greatly improve N use efficiency for manure N.
Research Objectives
  1. Validate or adapt canopy sensor-directed in-season N application practices for fields with manure or other organic material applied.
  2. Improve the prediction of the fertilizer N equivalence for organic materials.
These objectives were addressed with research conducted at six sites for three years and two sites for two years. The research included work with four livestock manures, three municipal biosolid products and an industrial by-product. The algorithm for interpretation of sensor readings was:
 
 
Results for Crop Canopy Sensor-Directed In-Season N Application for Manured Fields
 
The use of crop canopy sensor-directed in-season N application on manured fields for N use efficiency with high corn grain yield accounted for manure N and was validated.
 
Benefits associated with sensor-directed in-season N application included:
  • The in-season 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, respectively.
  • Grain yield was 10% more with manure plus fertilizer N compared with 178 lb/ac fertilizer N applied before planting for the first crop after manure application.
  • The mean in-season N rate was 70 lb/ac for rainfed and 80 lb/ac for irrigated sites.
  • The in-season N rate decreased with increased manure rate.
  • In-season N application increased the allocation of plant biomass to grain (a higher harvest index).
  • The crop responded to in-season N within 10 days of application, but recovery from N deficiency stress was often incomplete by tasseling
  • The mean agronomic efficiency of the in-season N was 30.5 lbs of additional grain yield per lb of in-season N applied.
  • 82% of the in-season N was recovered by the crop.
Best practices for sensor-directed in-season N application on manured fields
  • The sensor directed in-season N was most accurate with 60 lb/ac fertilizer N applied pre-plant because:
  1. Recovery from N deficiency stress was sometimes delayed beyond tasseling if the pre-plant N application was less than 60 lb/ac, and
  2. Pre-plant N application of more than 60 lb/ac N often masked differences in N deficiency and reduced the sensitivity of sensor assessment.
  • The best time for sensor-directed in-season N application was between V12 and V14.
  1. If N deficiency stress is severe, in-season N application before V12 is desired to give more time for recovery from stress by tasseling.
  • At least 30 lb/ac N should be applied in-season even when the sensor information indicates a low or 0 in-season N rate.
  • The Solari algorithm slightly underestimated the in-season N rate and should be revised to N rate = 317.
  • The in-season N rates determined from sensing at V8, V10, and V12 were well-correlated, indicating that earlier sensing, such as with a drone, could help determine:
  1. no in-season N is needed;
  2. in-season N is needed, but variable rate application is not justified;
  3. variable rate application is needed, but it can be done using a prescription map developed from the early sensed image; or
  4. variable rate application should be directed by on-the-go sensing during application.
Results for Improved Prediction of the Fertilizer N Equivalence for Organic Materials
  • The average fertilizer N equivalence of the applied organic N was 44% for the first crop, 21% for the second crop, and 10% for the third crop after manure application.
  • The fertilizer N equivalence was:
  1. similar for rainfed and irrigated;
  2. similar for all organic materials;
  3. greater for lower soil organic matter sites (less than 3%) compared with higher (more than 6%) soil organic matter sites;
  4. not affected by C:N ratio or the hemicellulose, cellulous, and lignin content of the organic materials; and
  5. decreased with subsequent crop following manure application.
  • However, there was much variation in fertilizer N equivalence from site to site, indicating low predictability.
Conclusions
 
The results validated the use of sensor guided in-season N application as a means to efficiently use the N available from manure and other organic materials. The results do not provide a basis for improving the prediction of fertilizer N equivalence of manure organic N with the exception that higher fertilizer N equivalence is expected with low soil organic matter soil compared with high soil organic matter soil. Decisions regarding pre-plant fertilizer N use for manured fields need to be made by balancing the risk of over-application to protect yield potential with the risk of under-application for environmental protection.