If I've encountered this scenario once, I've ran into it a 100 times throughout my career. The product or practice being tried changes, but the outcome is the same. It goes like this.
"We applied product X this year for the first time."
"Did it pay?"
"Well, we think it did, but we treated the entire field, so we don't have anything to compare to."
Answers as to why the whole field was treated without at least one round or a few rows being left for comparison vary. Sometime it's because shifting form one set of conditions to other would require adjustments or changes in machine settings that may or may not be easy to make. Sometimes it's because once someone is convinced of a practice and its value by a dealer or someone else, they don't want to miss out by not using it on all of their acres.
The only problem is without a control strip to compare to, there is no way to figure out what would have happened in that field in that year if you hadn't used the new practice or product. This is why Steve Dlugosz, and agronomist with Harvestland Co-op in east –central Indiana, recommends that if you're doing something you haven't done before, like applying sulfur in your fertilizer mix for corn, then leave at least a check strip where no sulfur is added. Otherwise there will be no practical way, not even with the modern miracle- a yield monitor- to tell what would have happened if the sulfur had not been applied.
The ultimate proof would be in a replicated test, but most farmers don't have time to become researchers. Strip trials would be the next best alternative if they're set up correctly. But if you don't have time for that, either, at least leaving one round where you don't do the new thing you do in the rest of the field will give you a point of reference. And if it turns out that the new product is a bin buster, then you've only given up the advantage on a small amount of acreage.