Field yield fell: the variety, the field, or the year?

A field’s relative yield is its yield in one season as a percentage of the farm’s average that same season. Write it down for every field and season, and you can tell whether a low harvest came from the field, the variety, or the year, before you drop a variety. In Brazil, a dry season took about 30 percent off a soybean field’s yield. At 55 bushels an acre and an illustrative $10 a bushel, that is $165 an acre lost to a dry season, not to the variety. Start the series with the field you doubt most, and switch a variety only when the series points at it.

How much can one dry season take off a field’s yield?

About a third. Amado and colleagues followed six crops (soybean, corn and wheat) on a 57-hectare (141-acre) commercial field in Palmeira das Missões, Rio Grande do Sul, Brazil, with a combine monitor logging yield point by point. In the dry 2003/04 season the field’s average yield fell about 30 percent. Taking 2002/03, a season of normal rain, as 100 percent, the soybean yield was 96 percent in 2000/01 and 67 percent in 2003/04.

The year also outweighed the field’s own best ground. In the dry seasons, the yield that only the best quarter of the field reached was lower than the yield that three quarters of the field reached in the normal season. Across 338 fields in the US Midwest with at least three years of yield maps, variation from one year to the next was larger than variation inside the field, for every crop.

Does the variety move yield as much as the year?

Much less, in the studies that measured it, though the two numbers below come from different studies and measure different things. In a soybean trial network across the Brazilian Cerrado, 559 trials at 57 sites over seven seasons, sites accounted for at least 80 percent of the variation among the treatment factors. When a site took the winning variety of its neighboring sites instead of its own winner, average yield fell by under 2 percent, almost every time.

At 55 bushels an acre and $10 a bushel, 2 percent is $11 an acre, against the $165 that the dry season took. Read the comparison as an order of magnitude, not a measurement.

Can one season on my field judge a variety?

No. One season on one field is a single observation with no direct comparison to other varieties. When two varieties grow side by side in a field, the comparison may favor field variability rather than variety differences.

Virginia’s extension service shows what follows, with numbers. Its relative yield is a variety’s yield as a percentage of the trial’s average. A variety chosen on one year of data at Suffolk had a relative yield of 114, and 100 the next year, no better than the average entry. A variety chosen on the data of several sites always scored at least 100 percent of the test average the following year, and averaged 105 across all sites and years.

Judge a variety on trials from several sites and seasons. To compare two varieties on your own ground, plant a strip with a check row beside it, and see what your on-farm trial tests.

What does one field’s series show?

The example is a soybean farm near Suffolk, Virginia, with three fields of 80 acres each, so the farm’s average is the plain average of the three. The values are illustrative, and the farm is not real. The back field grew variety A in 2022 and 2023. After the poor 2023 harvest the owner blamed A and planted variety B in 2024.

The back field fell from 50 to 36 bushels in 2023, which looks like variety A’s fault. The other two fields fell as much, and the farm’s average dropped from 55 to 40.

Read as a percentage of the farm’s average, the back field sat at 91, 90 and 90 percent. The 2023 drop belonged to the year, because every field fell. The switch to variety B changed nothing, because the field stayed at 90 percent. What stays constant is a gap that belongs to the field, worth $4,000 a year.

Three seasons is the minimum the research behind this article used. Amado and colleagues ask for at least three years of mapping to characterize yield variability, and Maestrini and Basso kept only fields with three or more years of data. A field below the farm average for three seasons is one to take to the agronomist, who may ask for a compaction reading, measured and written down.

Which field do I start with, and what goes on its lines?

Start with the field you doubt most, and fill its lines from the scale tickets and monitor files you already hold, before the next planting. Each line holds the yield in one unit, the farm’s average and the percentage, the variety, and the reason for choosing it, dated.

Write the reason before planting. People who know how an uncertain event turned out typically feel that the outcome was always likely, so a reason written after a poor harvest can drift toward blaming the variety. Choosing the variety stays with you and your agronomist (see what the agronomist decides), and this article names no variety.

Measure every season the same way. On the Brazilian field the combine had no moisture sensor, so the researchers corrected each yield file against the cooperative’s load-intake record and standardized every yield to 13 percent moisture. A monitor reading, a scale ticket, and a remembered number measure one field three different ways. Write the source beside each number, and leave out a season measured differently from its neighbors.

• For each field, open one line per season with the yield in the same unit and the source of each number
• Beside each yield, write the farm’s average that season and the field’s yield as a percentage of that average
• On the same line, write the variety, the reason for choosing it, and the date; from next season on, write them before planting
• Before switching variety over a single season, check that variety in trials at several sites and seasons in your region
• Mark any field that ran below the farm average for three seasons or more, and take it to your agronomist
• When you change the variety on a field, keep its line going, so the next seasons show whether its percentage moved

Keeping this record is part of farm management. More on reading crop records field by field is in the production library.