Good — I have 11 seasons of corn (2015–2025) with planting dates and yields, ~190 field-years. The key thing to get right: absolute yield swings a lot year-to-year (weather, genetics), and each year is planted in a fairly narrow window, so a raw yield-vs-date scatter would just be comparing good years to bad years. I'll normalize each field against its own year's average so the planting-date signal isn't buried by year quality.
Clear signal. Now let me pull the individual field points (each normalized against its year) to build the scatter and fit the trend.
Now I have everything. Let me compute the trend and build the plot.
Here's what your planting history says, and I'll walk through it below the chart.
Your optimal window is late March through the first week of April. Earlier is better, and the effect is steady.
What the plot shows: each dot is one field in one year, plotted by how far its yield landed above (blue) or below (red) that year's whole-farm average. I normalized against each year's own average on purpose — 2021 corn ran in the 230s and 2022 in the 140s, so raw yield vs. date would just be comparing good weather years to bad ones. Normalizing isolates the planting-date signal.
The weekly averages (gold line):
- Before Apr 1 → +16 bu/ac over the year's average (15 fields)
- Apr 1–7 → +9 bu/ac (18 fields)
- Apr 8–14 → +1 bu/ac (52 fields)
- Apr 15–21 → −5 bu/ac (58 fields)
- Apr 29+ → −8 bu/ac (24 fields)
The break-even point — where a field starts yielding below its year's average — sits right around Apr 15. The trend line works out to roughly 4 bu/ac lost per week of delay past early April.
Two honest caveats. First, the field-to-field scatter is wide (a single early-planted field can still disappoint, and a few late ones did fine), so this is a tendency across ~190 field-years, not a guarantee for any one field. Second, most of your March-planted corn came in 2024 — a genuinely strong year — so that first bin gets a little help from good conditions on top of the early-planting benefit. The Apr 1–14 signal rests on far more years and is the more reliable read.
Practical takeaway: push to get corn in the ground by the first week of April when the soil lets you, and treat anything past ~Apr 20 as giving up yield you can measure. The usual early-planting risk is a cold snap right after planting hurting emergence — so "early when conditions allow" beats "early no matter what."
Want me to layer in planting error or emergence to see whether your very-early fields ever paid a stand-establishment penalty?