The Variable-Rate Decision for Multiple Inputs with Multiple Management Zones

would have to receive an RVRT* of between $12 and $21/acre to break even with URT
application of these two inputs. This increase in net returns would have to come from either
increased yields and/or decreased input usage compared to URT application of nitrogen and
water.

If the field has no area in management zone 1 (Figure 2), management zone 2 must be
greater than 4% or less than 90% of the field for VRT application of nitrogen and water to
provide equal or higher net returns than URT application and management zone 3 has to be
between 96% (100% - 4%) and 10% (100% - 90%) of the field because management zones 2
and 3 comprise 100% of the field. If the field is 30% management zone 2 and 0% management
zone 1, the expected net return to VRT is $77/acre (RVRT*) minus $21/acre (V₁) or $56/acre.
As the percentage of a field in management zone 1 increases, the SBVP’s become narrower. If
the proportion of management zone 1 is 60%, the SBVPs for management zone 2 (management
zone 3) are 7.5% (32.5% = 100% - 60% - 7.5%) and 38% (2% = 100% - 60% - 38%). Within
these ranges of λ₂ and λ3 (given λ₁ = 0.6), RVRT* - V₁ is greater than or equal to zero and the
farmer at least breaks even by using VRT.

Conclusions

The extent that multiple-input VRT is adopted will depend on the expected net economic
benefits received by potential adopters. Fields generally exhibit yield variability; however, as
demonstrated in this paper, not all fields warrant VRT from an economic standpoint. Farmers
are interested in knowing whether VRT is economically viable on their fields. The answer to
this question varies from field to field depending on spatial variability as well as yield response
variability among management zones. The answer also varies with the crop, the inputs, their
prices, and the cost of using VRT relative to URT. In the end, no general formula exists for

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