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Fluid Journal : Fluid Journal 2005-2007
Spring 2005 Variable-rate N Management: One Option For Better Profits Drs. James Schepers and John Shanahan Also increases N-use efficiency and minimizes potential environmental consequences. Fluid Journal 1 Deciding where and how much N to variably apply is probably the biggest challenge facing producers. If they are confined to using a soil-based N management strategy, producers either have to plan for an average year or perhaps try to compensate for moderately extreme conditions. If water availability is the limiting factor, it is probably best to Summary: Variable-rate nitrogen (N) management seemingly has many positive attributes, but is only slowly being adopted by producers. The primary reason at this time is probably the lack of a consistently adequate economic incentive unless high-value crops are involved. Several commercial products and approaches are available to help producers evaluate the technologies. These include components that might be used to assess N needs, make variable-rate N recommendations, and/or variably apply products for individuals who would like to build their own management programs. Others include commercial services that provide a comprehensive package of activities for producers who are not prepared to handle the intimidation component of new technologies or find it more convenient and less risky to hire others to provide the services. Users of the technologies have multiple options in terms of which inputs to the N recommendation process might logically include a spatial component and which do not. plan for an average year because more favorable water conditions for the crop will also result in additional mineralization. For crops like corn, sorghum, and wheat, a late-season N stress will reduce grain protein content considerably more than it will yield. There probably isn't a right or a wrong way to make variable-rate N fertilizer recommendations because so much depends on the local situation, information available, and availability of someone with Geographic Information System (GIS) skills to analyze and help integrate the information. Considerations to make Yield goal. A uniform yield goal for the field is commonly used and probably is the least appropriate considering the sophistication and capabilities of variable-rate fertilizer application equipment. Yield maps from previous years can provide useful information related to yield stability. Maps with four to six yield categories are usually appropriate depending on the size of the field. If a management zone approach is used for soil sampling and making nutrient applications, then it may be convenient to develop a yield map with the same number of categories. An image taken during grain fill can be used to characterize the spatial variability that will likely occur in yield. Using this approach, someone would need to provide the yield goal for the field so that relative yield for each segment or area can be calculated and the yield redistributed to generate a proxy yield map. Another strategy being tested for estimating yield potential of corn is based on crop vigor at about the V8 growth stage. Crop normalized difference vegetation index (NDVI) values, along with growing degree days (GDD), are used to predict yield from which a fertilizer N recommendation is derived. This latter strategy seems to work for wheat in Oklahoma where winterkill can be an issue, but with corn the strategy needs additional evaluation in that corn yields are not usually related to plant population as much as they are to water status and climatic conditions later in the growing season. The applicability of this approach in corn is questionable because climatic records clearly illustrate that the difference between average and exceptional corn yields is having favorable light and temperature conditions during the grain filling period. Residual N. Responsible N management practices do not usually result in excessive residual N levels in the root zone at the end of the growing season. As such, residual soil N may be one of those inputs that is appropriately represented by an average for the field or that is proportioned according to soil color or other physical attributes. If management zones are delineated and soil samples are available for other purposes, then it might be appropriate to analyze soil samples at that level of resolution. Grid sampling for residual N is on the decline because of the cost and marginal value of the data. As such,
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