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. Research Question In variable rate application technology (VRT), crop production input rate is changed within a field in response to variable factors that affect the optimum rate of application. Individuals involved in many aspects of production agriculture are asking if they should adopt VRT. If it is widely implemented, then a major change must occur in how production agriculture views and manages crop production inputs—a change from the predominant procedure of applying inputs as uniformly as possible and adjusting rates only between fields. Background Why should VRT be considered? Uniform application of crop production inputs does not allow optimum efficiency or profitability because factors that affect crop production are not always uniform within fields. Properly implemented, VRT is a correct concept. It has the potential to improve, or ideally maximize efficiency of inputs and profitability of individual fields by targeting application where needed and at optimum rates. Variable rate application is not a new concept. The University of Illinois published a circular in 1929 outlining practices to intensively soil sample fields for the purpose of mapping soil pH variation and determining treatable areas for variable limestone application. This was an important aspect of crop production at that time. Forage legumes, such as clover (which is very sensitive to low pH), were important in crop rotations. Also, liming was an expensive and labor intensive practice. It was important to apply limestone for pH correction and to efficiently use the limestone that was applied. Perhaps the complete process was not quite as high‐tech as today's methods, but it was deemed important enough to develop the management it required. Is the same situation present today? Technology certainly has the potential to make many things possible and easy to accomplish. But, are the costs of products, the costs associated with product supply and application, and the costs of improper application associated with uniform application worthy of VRT and its associated costs and management? Can VRT substantially improve the correctness of applications. Can VRT improve environmental stewardship? Or is the equipment and computational ability more detailed and precise than the world in which we conduct the business of production agriculture? Has the technology outpaced our ability to understand and accurately map soil properties that affect crop yield? Applied Questions Is there improvement from VRT fertilizer application? Yes and no. Field research to date is not exhaustive. It shows VRT can improve input efficiency and field profitability, but it indicates that positive economic return to VRT implementation does not always occur. Several possible shortcomings affect potential benefits. These include: (i) the crop is not responsive to the input; (ii) the crop‐input response function used is not specific or appropriate; (iii) within‐field variation is in a range that does not affect yield; (iv) variation is small or does not exist, and therefore VRT cannot improve upon results from a uniform rate; (v) variation is not correctly (or at least adequately) identified, measured, or delineated (mapped); (vi) measurement and recommendation practices available today are neither adequately accurate or reliable for VRT; (vii) identified variation is not correctly managed or there is incorrect interpretation of expected crop response; and (viii) costs of implementing VRT (sampling, mapping, equipment, and personnel) outweigh the value of crop yield increase or input saving. The bottom line, will VRT pay? 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 economical. It must improve profitability of fields and provide environmental benefits, or it should not be used by farmers. Will VRT be adopted and implemented by production agriculture? As is the case for many practices, no definitive answer exists as to whether VRT should be used in every field or is the best crop input management for all farmers. The answer is—it depends. It depends on the expected crop response to lime, nutrients, pesticides, and the value of the crop. It depends on the characteristics of the variability that affect crop production and the capability to measure, map, and manage it. It depends on the capability of production agriculture to understand and manage the new technology. It depends on the importance of such potential benefits as environmental improvement. There is no guarantee that VRT will provide economic benefit, the same as for other management or production practices. As always, assessment of each field is needed to help ensure use of profitable practices and greatest chance of economic benefit. Variable rate technology is one of many management tools with the potential to optimize crop yield and profitability. If no other benefit occurs, at least the VRT process demands critical field evaluation and management. Recommendation To effectively implement VRT one must have the capability to accurately identify and reliably interpret within‐field variation. This is the basis of VRT and is a prerequisite for its success. Additional research is needed to fully develop these capabilities. There must be improved and reliable input‐crop response models and cost effective practices to accurately assess variation. Also, it must make economic sense to implement VRT (which can include environmental benefits). Once the models and practices for VRT are in place, then predictive economic simulation could target individual fields or geographical regions for successful VRT use.