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Cutting Costs Without Hurting Yields? Check Fertilizer N Rates In Corn After Alfalfa

Feb 23, 2017
 
University of Nebraska-Lincoln
Institute of Agriculture and Natural Resources
CROPWATCH
 
With tight crop margins for the 2017 growing season, many farmers are looking for ways to cut input costs without hurting yields. One way to do this is by giving the appropriate N credit when calculating how much N to apply to corn grown after a prior alfalfa crop.
 
When alfalfa is terminated, decomposing alfalfa shoots, crowns, and roots provide N to subsequent corn crops. The contribution of N from alfalfa and subsequent reduction in the fertilizer N requirement for corn is known as the alfalfa N credit. The alfalfa N credit can vary from 40 to 150 lb N/ac depending on soil type and alfalfa plant stand. (See Table 1, adapted from Nebraska Extension Fertilizer Suggestions for Corn.)
 
Table 1. Estimated nitrogen credit from alfalfa.
 
 
In this study we investigated the variation in yield across fertilizer N rates in irrigated farmer fields planted with corn after a prior alfalfa crop, assessing the potential for reducing N fertilizer inputs without hurting yields. For this analysis, we used farmer field data on yield and applied N fertilizer collected by the Central Platte NRD during six years (2004-2010). We selected irrigated fields planted with corn after a prior alfalfa crop in three counties (Buffalo, Hall, and Merrick). We excluded fields affected by hail or flooding and also those that received manure application.
 
 
Figure 1. Irrigated yields in fields planted with corn after alfalfa plotted against fertilizer N in the Central Platte Valley (Buffalo, Hall, and Merrick counties). Values inside boxes indicate the average corn yield (bu/ac) for each N fertilizer rate class. Parenthetic values indicate the number of fields for each fertilizer N rate class.
 
Selected fields were grouped into six classes depending upon the applied N fertilizer: <90, 90-110, 110-130, 130-150, 150-170, and >170 lb N/ac. In order to graphically show the range of corn yields across farmer fields, for each N fertilizer class, we used a statistical technique called “box plots.” For each fertilizer rate range:
  • the box represents 50% of the fields,
  • the box plus the vertical bars account for 80% of the fields, and
  • the range between the top and bottom dots includes 90% of the fields.
The median value (50% of the fields lie above this line and 50% below) is indicated with a horizontal line inside each box. We also included the average yield (values inside boxes) and number of observations (numbers in parenthesis at the bottom of the chart) for each fertilizer N class. Different letters, on top of the boxes, indicate statistically significant differences in corn yield among N rate classes (Tukey’s test, P<0.05).
 
The most important finding of the analysis is that there was no difference in corn yield across fertilizer N rates ranging from 90 to 170+ lb N/ha, with yield consistently averaging about 200 bu/ac. Despite this, it was remarkable that 80% of the famer fields received fertilizer N rates larger than 110 lb N/ac, with most of them receiving 150-170 lb N/ac. This finding indicates there is a large opportunity to reduce costs (and the environmental footprint) associated with N fertilizer in corn after alfalfa with little risk of hurting corn yield.
 
Analysis of the data also revealed that average corn yield in fields that received fertilizer N applications smaller than 90 lb N/ac was substantially lower (by 27 bushels) than average yield of fields that received larger N rates (>90 lb N/ac). However, there was large variation in yield across fields that received little N fertilizer, ranging from 110 to 225 bu/ac, and some of these fields achieved yields similar to those in fields that received larger fertilizer N rates. We speculate that the wide yield range with low fertilizer N rates can be attributed to variation in residual soil N and groundwater N from irrigation across fields and variation in soil type and alfalfa plant stand at termination, and, hence, different N credit from the prior alfalfa crop (Table 1). Unfortunately, we did not have data to test these hypotheses.
 
Table 2 below shows the potential cost saving that can be realized by reducing the current N rate in corn after alfalfa. Cost savings were calculated for different scenarios of N fertilizer reduction (from 20 to 100 lb N /ac) and fertilizer N prices (from 0.20 to 0.60 $/lb N). For example, a farmer who reduces current fertilizer N rate by 60 lb N/ac in corn after alfalfa, could save $24/ac (fertilizer priced at $0.40/lb N). This is equivalent to $3,120 for a 130-acre pivot-irrigated field… and without any yield penalty!
 
Table 2. Dollars saved per acre associated by reducing current N fertilizer rate in corn after alfalfa for different fertilizer N prices.
 
 
Conclusions
A major take-home message comes out from this analysis: for most NE farmers growing corn after alfalfa, there is room to reduce current fertilizer N rates, without hurting yield, by assigning the proper N credit when calculating the N fertilizer requirement. Information on how to calculate fertilizer N rates for corn and assign the proper alfalfa N credit can be found in the Nebraska Extension resource, Fertilizer Suggestions for Corn.