Combine yield monitor vs weighbridge: which to trust

Short answer: trust the weighbridge for totals and anything with money attached; trust the yield monitor for the pattern within the field. They're not competing instruments — one measures tonnes, the other measures variation — and the farms getting full value from either are using the weighbridge to calibrate the monitor, then letting each do its own job.

The longer answer is worth having, because "the monitor said 9.2" gets quoted with a confidence the instrument rarely deserves.

What a yield monitor actually measures

A yield monitor doesn't weigh anything. It infers weight, typically from an impact or optical flow sensor where the clean grain elevator throws grain at the top, combined with a moisture sensor, forward speed and the header width you told it. Flow rate times time, corrected for moisture, divided by area covered: tonnes per hectare, drawn on a map.

Every link in that chain is an estimate. Which is fine — it's a genuinely clever piece of inference — but it means the output is only as good as its calibration, and calibration is exactly the step that gets skipped in a busy July.

Why the numbers drift

Several reasons, and they stack:

• Calibration is per crop, and really per condition. A flow sensor calibrated on last year's barley doesn't read this year's wheat correctly, and a hot afternoon's brittle straw feeds differently from a dewy evening's. Uncalibrated or stale-calibrated monitors are commonly 3-5% out or more; calibrated and checked through the season, 1-3% is achievable.
• Slopes and uneven feeding. Grain flow across the sensor changes on side slopes and when the header isn't full — cutting a narrow finish strip reads differently from a full bout.
• The moisture sensor has its own error, and it multiplies into the yield figure rather than adding to it.
• Header losses aren't counted. Whatever goes out the back or gets missed at the intake never crosses the sensor. The monitor reports what came through the machine, not what the field grew — usually a small gap, but not zero.

Put numbers on the drift and it stops sounding academic. A monitor running 4% hot across 100 hectares of 8.5 t/ha wheat invents 34 tonnes that don't exist — at recent feed wheat values of roughly £150-£200/t, that's £5,000-£7,000 of imaginary grain. You'll find it eventually, as a shortfall against a contract or a shed that empties early. Better not to book it in the first place.

What the weighbridge can and can't do

A weighbridge weighs what's actually in the trailer, to a fraction of a percent, every load, with no per-crop calibration ritual. For field totals, shed stock, contract commitments and settling arguments with merchants, it's the ground truth — the standard everything else gets checked against. (No bridge at home? There are workable alternatives, from public bridges at £10-£15 plus VAT a weighing to calibrated trailer counts — less convenient, same principle.)

What the weighbridge can't tell you is anything about where in the field the tonnes came from. A field averaging 8.8 t/ha might be 10 t/ha across most of it and 5.5 down one compacted headland — the weighbridge reports 8.8 and shrugs. The monitor's map is the only practical way to see that pattern at harvest, and the pattern is where the agronomy lives: the wet hole, the pH-poor corner, the tramline effect. That's what makes monitor maps worth keeping in your field history even when the absolute numbers are shaky.

Use both: calibrate early, then divide the labour

The working arrangement is straightforward:

1. Calibrate the monitor against weighed loads early in each crop. First day in the wheat, run two or three full trailer loads over the bridge and feed the true weights back into the monitor, per the manufacturer's procedure. Re-check when conditions change markedly — a new variety, a wet spell, a different field type.
2. Weights for money, maps for agronomy. Contract tonnages, shed stock, field yields you'd quote to anyone — weighed figures. Variable-rate plans, drainage arguments, headland diagnosis — monitor maps.
3. Cross-check as blocks finish. When a field is done, compare the monitor's field total against the weighed total (a yield calculator makes short work of the t/ha). If they've drifted apart by more than a couple of percent, recalibrate before the next block rather than at the end of the season.

That last habit is the whole game. A monitor checked against the bridge every few fields stays honest all harvest; one calibrated in 2023 is a random number generator with a colour scheme.

Frequently Asked Questions

How accurate is a combine yield monitor?

Uncalibrated, or calibrated once years ago, commonly 3-5% out and sometimes worse. Calibrated per crop against weighed loads and re-checked through the season, 1-3% is realistic. The error isn't constant either — it shifts with crop, moisture and conditions, which is why one calibration doesn't hold forever.

Should I trust the yield monitor or the weighbridge?

The weighbridge, for any total that matters — it weighs the actual grain to a fraction of a percent. The monitor earns its keep on within-field variation, which no weighbridge can see. Use weighed loads to calibrate the monitor, then use each for what it's good at.

How do you calibrate a yield monitor?

Cut two or three representative full loads early in each crop, weigh them over a bridge, and enter the true weights following the monitor's calibration procedure. Repeat when crop or conditions change noticeably, and cross-check field totals against weighed totals as blocks finish.

Why do the monitor total and the weighbridge total disagree?

Some gap is normal: the monitor is inferring flow from sensors with their own errors, and it never sees header losses. Within 2-3% after calibration is respectable. A bigger or growing gap means the calibration is stale — recalibrate rather than splitting the difference.

Are yield maps worth keeping if the monitor isn't calibrated?

Yes, cautiously. Even a poorly calibrated monitor usually gets the relative pattern right — where the good and bad areas are — so the map still has agronomic value. Just don't lift absolute tonnages from it, and don't compare maps across years as if the calibration held steady.