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Variable Rate Debate: Straight Rate vs Variable Rate Applications

Often when we are reviewing soil test results with farmers our recommended actions include variable rate application of fertilizer. Those recommendations often lead to the question: Is variable rate really worth it? We say if your field has variability, yes.

Let’s take a look at our corn yield map from a couple seasons ago and some notes about this field’s characteristics to use as a reference for the discussion. The four yield points selected across this field have a 179 bu range, from the low at 133 bu/ac to the high at 312 bu/ac. The total crop removal rate by corn for Phosphorus (P2O5) is 0.6 lbs/bu and Potassium (K20) is 1.3 lbs/bu. Therefore, across this field there is a significant difference in the amounts of Phosphorus and Potassium removed in each of the yield zones.

This amount of variability in yield across a field is very common and can result from many factors including those indicated on the example map: soil changes (eroded side hills vs loam), management practices (starter only strip vs in-season Nitrogen), and abundance or lack of field tile.

If the amount of nutrient removed by each crop is variable it would make sense to variable rate apply the nutrients you replace on that field instead of using a straight rate across a field with variability because a straight rate is only going to exaggerate the nutrient variability over time. With a straight rate application on a variable yielding farm, over time the nutrient levels will fall in the high yielding areas—eventually causing yields to become stagnant or even decrease, and nutrient levels will increase in the low yielding areas where there is not as much removed by the crop, thereby increasing the risk of nutrient loss through erosion or leaching.

The debate then moves to the capabilities of the application equipment. Yes, there is still a long way to go to be able to variable rate spread fertilizer in less than 40 to 60 foot boom widths and have rate transitions occur instantaneously instead of taking a few seconds and 100 feet to change the rate. But given the huge advances in machinery automation in the last 5 years, we truly believe that these equipment shortcomings will be overcome in the not too distant future. Every field has variability, VR technology helps you manage it.

The variable rate debate comes down to making a decision on each farm: straight rate or variable rate? Based on each farm’s characteristics, you must weigh the risk of applying a single rate and not applying enough in the high yielding areas and too much in the low yielding areas vs variable rate applying and the equipment limitations resulting in over or under application in transition zones.

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