CONCEPTS OF VARIABLE-RATE TECHNOLOGY WITH CONSIDERATIONS FOR FERTILIZER APPLICATION

In variable rate application technology (VRT), crop production input rate is changed within fields in response to spatially variable factors that affect the optimum application rate. It is being considered by farmers and the crop input industry because factors that affect crop yield are not always uniform within fields and, therefore, do not allow optimum efficiency or profitability from uniform application. If VRT is widely adopted, then a change must occur in the predominant management practice of applying inputs as uniformly as possible and changing rates only between fields-what has historically occurred on the majority of production fields. Properly implemented, VRT is a correct concept and can work. It has potential to improve input efficiency, field profitability, and environmental stewardship. Field research indicates, however, that positive economic return to VRT application does not always occur. There are complicating factors that limit its effectiveness. Of most importance are: (i) the cost of implementation (sampling, mapping, equipment, personnel); (ii) lack of expected increase in crop yield (little actual variation, incorrect assessment of variation or low accuracy mapping, and low precision crop-input response function or incorrect interpretation of expected crop response); and (iii) lack of input savings. These factors focus on VRT fertilization, but they apply to other inputs as well. Despite many possible and intuitive benefits, lack of consistent economic improvement causes concern about adoption of VRT practices. At today's crop value and cost of technology, VRT may not always be economically feasible. Profitability of fields or environmental stewardship must be improved by VRT, or it should not be used by farmers. VRT must be better than uniform application and ideally it should be the best application possible within a field. As is true for many management practices, no definitive answer exists as to whether VRT should be used in every field or is the best crop input management practice for all farmers. It probably depends upon the field, the crop, the geographic area, the pest, the nutrient, the soil, the variability, and the managerial ability. To effectively implement VRT, within-field variation must be accurately identified and reliably interpreted. This is the basis of VRT and is a prerequisite for it's success. Additional research should fully develop these capabilities. Improved and reliable input-crop models and cost effective mapping procedures are needed. Once models and practices for VRT are fully developed, then predictive economic simulation will help target fields for successful VRT use.