Crop Yield per Unit Area

Total harvest divided by the area it came off. It is the simplest equation on this site, and it is quoted wrongly more often than anything else here, for two reasons that have nothing to do with the division.

The first is moisture. A yield is a mass, and a mass of wet grain is not comparable to a mass of dry grain: 340 t at 20% moisture is only 322 t at 15%, so the same field yields 8.5 t/ha or 8.0 t/ha depending on nothing but which number was reported. Corn is normally quoted at 15.5%, wheat and soybeans at 13.5–14.5%, and a yield without a stated moisture is a number without units. Correct the weight first, then divide.

The second is the denominator. Gross field area includes headlands, laneways, the drowned-out corner, the shelterbelt and the hydro pole — all of which contribute area without contributing harvest. Net cropped area excludes them. The difference is commonly 3–8% and is most of the reason farm-recorded yields sit below plot yields, alongside the fact that a small plot is harvested carefully by people being watched. Both figures are legitimate; mixing them between two fields, or between this year and last, is not.

Yield monitors deserve their own caution, because they measure neither of these directly. A monitor infers mass from an impact plate or an optical sensor and infers area from GPS and a header width it assumes is full, so it needs calibration against a weighed load and it overstates yield whenever the header runs part-full — on point rows, on the last pass, and anywhere the operator narrows up. An uncalibrated monitor can be out by 10% and will still draw a beautifully convincing map.