Where the money in variable rate actually comes from

Variable rate does not add yield everywhere. It moves inputs off ground that cannot use them and onto ground that can, and it only pays where those two kinds of ground sit in the same field. On a farm that is uniform end to end there is nothing to move, and the honest answer is close to zero.

So the arithmetic has three steps, and skipping any one of them is how a calculator ends up quoting a number nobody ever collects. Start with what a full variable-rate program returns on ground that responds. Published trials across corn, cereals and oilseeds put that between roughly 10 and 45 euro per hectare, with corn on the Cornbelt at the upper end and dryland cereals at the lower. Then cut it to the operations you are actually going to vary, remembering that nitrogen and seeding rate are chasing the same productivity difference, so the second one is worth less than the first. Then, and this is the step that gets left out, apply it only to the share of the farm that responds.

That share is the variability setting. A field with under 10 percent yield swing has perhaps a quarter of its area worth a different rate. At 10 to 25 percent, where most fields sit, call it half. Above 25 percent, three quarters. The rest of the farm is already close to its right rate and a prescription changes nothing there except your diesel.

What to do with the figure

Treat the total on the right as the ceiling for what you can spend finding and working your responsive acres. Soil sampling is usually the largest item and it lasts several seasons, so year one is the tight one and years two and three are where the program looks obvious. If your drill or spreader cannot vary rate yet, that controller comes out of the same budget.

Then go and find the acres. Several seasons of satellite growth, elevation with slope and curvature, harvest maps and soil tests will tell you where the field is genuinely different rather than where it looked bad in one wet June. That map is what turns a per hectare figure on a web page into a prescription your terminal can read, and next harvest is what tells you whether the split was right.