6.7   Variable Rate Nitrogen Using Yield Maps

Principal Investigators

Lloyd Murdock, Extension Professor, Agronomy
Paula Howe, Research Specialist, Agronomy

Cooperator

Wayne McAtee, Farmer, Trigg County, Kentucky

Introduction

            At the present, most farmers in Kentucky apply nitrogen (N) to corn at constant rates over all the fields.  Most farmers use the same rate within each field, as well as on all fields.  Many farmers indicate that they use this method, along with higher than recommended rates of N, to ensure high corn yields on all parts of the field, even though they don’t expect all parts of the field to be able to use the highest rate of nitrogen.

            The amount of nitrogen fertilizer needed by a corn plant during a season will depend to a large extent on the yield potential that exists for that year.  The yield potential is determined by a number of factors such as the amount of available moisture, insects, diseases, soil type, etc.  One of the greatest determining factors on upland soils in Kentucky is the amount of topsoil present.

            A study beginning in 1998 on two fields with upland soil types indicated the 2/3 to ¾ of the yield variability of corn was due to topsoil depth (Murdock and Howe, 1998).  The results included a two-year history of data from each site.  On severely eroded sites in the field, the yields ranged from less than 30 bu/ac to about 100 bu/acre, depending on the severity of the erosion.  In areas with none to slight erosion, the yields were in the 120 to 170 bu/ac. range.  In areas of soil accumulation, the yields ranged from 170 bu/ac. to over 200 bu/ac.  Large differences in each field existed and the differences were highly correlated to topsoil depth.

            It appears that the yield potential of the eroded areas is strongly limited by the amount of topsoil.  Therefore, it makes agronomic and economic sense to limit the amount of nitrogen applied to these areas.  The areas of soil accumulation and highest yields are in lower areas of the field and might benefit from higher rates of nitrogen due to higher yield potential and loss of N because these soils will remain saturated longer during wet periods of the year.

Therefore, the amount of nitrogen needed by the crop should change as one enters the low, medium and high producing areas of the field.  The nitrogen added should increase as the yield potential increases.  There is strong evidence that this will lower production costs and possibly raise yields, as well as overall profits.  In 1998, Barnhisel, et. al. increased profits by $32/ac on upland field sites by varying both seed planting rate and nitrogen application rates using mainly top soil depth as a criteria.  Most of their profit increase was due to variable rate nitrogen applications.

One of the most difficult tasks in the procedure is determining the areas of a field that have different amounts of topsoil that would allow one to make a judgment on the areas of the field to receive the different rates of nitrogen.  A field survey using a soil probe would be very time consuming and expensive and require expertise that many farmers may not have.

Present soil survey maps are not accurate enough to delineate most of these areas of low, moderate and high yield on a field.  A possibility would be to use a 2-3 year history of yield monitor results to delineate the low, medium and high producing areas in each field using GIS procedures.  This seems a strong possibility on upland sloping soils, especially in the karst topography area of Kentucky based on the research accomplished in 1998 by Murdock, Howe, Barnhisel, et. al.  Based on the present research, this method may be most applicable in the upland karst area of Kentucky.  It may well be expanded to other upland areas in the future, but additional research would need to be carried out to prove its worth.

Specific objectives for this project are:

1)                  To determine if variable rate N applications can be profitable on upland soils in karst topography;

2)                  To determine if a 2-3 year history of yield maps is sufficient information on which to base a variable rate nitrogen plan on upland soils in karst topography; and

3)                  To determine the proper rate of nitrogen to be used on the low and high yield potential producing areas of upland soils in karst topography.

Background

            Yield maps can be valuable tools to evaluate N response within a field.  Davis et al. (1996) examined the variability of corn yield and its relation to N response.  Franzen and Kitchen (1999) determined that management zones for variable rate N application based on soil types and yield potential as determined by use of yield maps, soil maps, and drainage maps would allow precision farming to be more practical for commodity crop producers.  Dinnes (1998) used yield maps to determine N response on corn for three N rate areas (262 lb. N/ac, 157 lb. N/ac, 50 lb. N/ac).  He found that drier conditions resulted in minimal leaching and an elevated degree of N mineralization in high organic matter soils.  This resulted in a low N fertilization requirement to optimize yield on lowland soils.  The economic benefit of variable N application was studied by Barnhisel et al. (1999) based on landscape position.  The economic benefit for N ranged from $6.50 to $34.00 for one year and six site locations.  The end-of-the season test for cornstalk nitrate provides a site-specific assessment of N sufficiency for corn growth according to Balkcom and Blackmer (1999). Observed relationships among sufficiency levels and soil characteristics were used to identify the best N recommendations.

Procedures

            A progressive farmer in Trigg County will be the cooperator.  He has several years of yield history on his farms using a yield monitor.  From this history (2 to 3 years of corn yields on the field), 2-3 upland fields with significant yield variability will be chosen.  The farmer’s anhydrous ammonia nitrogen (NH₃-N) applicator will be modified to make it capable of applying variable rate NH₃-N according to applicator maps taken directly from yield maps.  The equipment will be calibrated to assure proper NH₃-N applications prior to field applications.  Either before or after N application of treatments, the low, medium and high producing areas in the field will be surveyed and sampled for topsoil depth, available nutrients, texture, organic matter and penetrometer resistance.  This will aid in understanding the yield results at the end of the year.  It will allow for determining any yield limiting factors and allow a correlation of these factors with yield to determine the most yield limiting factors.

            The areas of the field that will be identified from yield history as low potential yields (less than 120 bu/ac); medium potential (120 to 170 bu/ac); and high potential (over 170 bu/ac).  These would be the criteria if yields over 200 bu/ac existed.  Otherwise, the criteria would be: low (less the 60% of highest yield); medium (60 to 85% of highest yield); and high (above 85% of highest yield).  The low, medium and high yield producing areas of the field will be transferred to a GIS map to be used by the N applicator computer for determining areas of low, medium and high N application rate.  The nitrogen application rates will be based on AGR-1 Nitrogen Recommendations.  The medium yielding areas (none to slight erosion) will receive the AGR-1 application rate.  The low yielding areas will receive 40 lbs/a less and the high yielding areas will receive 40 lbs/a more.  The variable rate will be compared to a constant rate of an AGR-1 recommendation.

            The first year, emphasis will be on using the 3 above rates in a replicated trial.  The three rates will be applied at constant rates across the 3 different yield potential areas multiple times over the length of the field to determine the adequacy of the N rates for the different yield potential areas.  The nitrogen will be side-dressed using anhydrous ammonia and each rate will be replicated four times through the field.  Most of the background research used in the AGR-1 recommendations was from areas with none to slight erosion.  This trial would help determine the proper rates for the low and high yielding areas.  Most of the nitrogen used in these experiments will be side-dressed.

            In all cases, it will be important to determine if the amount of applied nitrogen is deficient, adequate, or excessive in the three different yield potential areas.  This will be determined by the following methods: 1) grain yield measurements at harvest using the grain monitor and weigh wagon or on-farm scales; 2) soil NO₃-N from soil samples taken when the corn is 6-12 inches of height; 3) measure the chlorophyll content of corn leaves prior to grain fill; and 4) measure NO₃-N in the lower part of the corn stalks after harvest.

Information from each one of these measurements will help determine the proper rate of N fertilizer for the three different yield potential areas.  The 2^nd and 3^rd years of the trial will concentrate on using variable rates of N on these upland soil types using yield maps as the determining factor for the different N rates.   Rate trials will also continue on a limited scale.  The variable rate part of the trial will be in a part of the field that has consistently shown yield variability and will run the entire length of the field.  This part of the field will be mapped into different yield areas based on previous yield maps.  High, medium and low yielding areas will receive high, medium and low rates of nitrogen, respectively.  Constant rates of nitrogen will be included as treatments to help in the evaluation.  A control strip will also be included.  Different fields will be used each year of the trial.

The success of the experiment will be determined by any increased profit that might be gained by the farmer.  The increased profit could come from reduced nitrogen application quantities and/or increased in corn yields.  Yields within the high, medium and low yielding areas will be measured as well as the yields from the entire length of each treatment strip in field.  Actual amounts of nitrogen used will also be measured.  These factors will be compared between the constant and variable rate nitrogen treatments.  This will allow for economic comparisons to be made based on differences in yields, amount of nitrogen applied, and technology investments.

Expected Benefits

            This experiment would help determine if variable rate nitrogen could be successfully and profitably applied to the upland corn producing areas of Ky.  It would also determine if a quick and inexpensive method could be used to determine the areas in the field where the nitrogen rates should be varied.  A history of past yield maps would be used for this purpose.

            If this project is successful and profitable, I would expect the corn producers in Kentucky would change rapidly to adopt this new procedure.  Based on recent research in Ky, (Barnhisel, et. al 1999, and Murdock, 1990-92) one might expect a return of $10 to $30/ac.  If the profit were $10/ac, the increased income to a farmer with 1,000 acres of upland corn would be $10,000/year or $100,000 over 10 years.  If one-half of the corn acres in Kentucky are considered upland regions, then the potential increased profit each year to Kentucky farmers would be on the order of 3-4 million dollars/year.  If the profit were $20/ac (which it might well be), the above figures would double.

Deliverables

Below is a list of ways that the results of this project will be reported and made available to the public: 1) publications:            UK Agronomy Notes, UK Soil Science News & Views, UK Fact Sheets, Publish in Southern Soil Fertility Proceeding, North Central Soil Fertility Proceedings, American Society of Agronomy Abstracts; 2) instructional methods: UK Fact Sheets, In-state proceedings for how to make this program successful; 3) conference and field days: This information will be presented at scores of county and multiple county meetings and field days.  State Conferences and field days for producers and agribusinesses, as well as out-of-state conferences and field days; and 4) web page: deliverables will be placed on the UK Precision Ag Web Page and any other appropriate web pages.