Average RVRT* declines as the proportion of land in management zone 1 increases.
Management zone 1 is the least profitable management zone and increasing its proportion
relative to the other two management zones decreases expected profit and impacts RVRT*. If
management zone 2 is either non-existent or makes up 90% of the field, RVRT* is less than
$10/acre. The highest RVRT* for management zone 2 is reached when it constitutes between 20
and 30% of the field. The highest RVRT* for management zone 3 is reached when the field has
about 60% of its area in this management zone.
The additional charge for VRT versus URT application of inputs can be separated into
two ComponentsV1 = V1N + V1W, where V1N is the difference between the cost of VRT versus
URT application of nitrogen and V1W is the difference between the cost of VRT versus URT
application of irrigation water. The additional custom charge for variable-rate nitrogen
application compared to uniform-rate application was assumed to be V1N = $3.00/acre. This
additional charge was close to the mean of $3.08/ac (range $1.50 to $5.50/acre) obtained from
personal telephone interviews with firms providing precision farming services to Tennessee
farmers (Roberts, English, and Sleigh). Responding firms indicated that the additional charge
would include the difference in application costs for VRT versus URT and a charge to create a
nitrogen application map based on soil survey maps in conjunction with the consultant’s
knowledge about corn response on various soils, a visit to the field to observe conditions, and an
interview with the farmer about historical yields. Based on information developed in Georgia
(Fairchild), a center pivot system can be retrofitted for somewhere in the $5,000-to-$10,000
range depending on the number of sprinklers controlled. Assuming a 5-year life, no salvage
value and a 150-acre irrigation system, the additional cost is $9 to $18/acre. Therefore, a farmer