The standard method used to follow and predict corn growth stages is using growing degree days (GDD). This is a calculation that uses average daily temperatures to measure accumulated heat over the growing season. Most of the GDD values we use are from the Mid-West, so we have followed a few research fields the past two years to compare how DE lines up. We have also included days from planting and light accumulation (pulled from DEOS) to compare other measurement methods (Table 1 – flip phone sideways for best presentation).

Table 1: Following Corn Vegetative Growth Stages (2019-2020)
Emerge V3 V6 V9 V12 VT
Days from Planting 5-13 12-31 22-48 35-63 44-76 53-82
Growing Degree Days 84-150 219-310  492-603 787-950 1001-1337 1231-1322
Total Light 125-302 271-609 530-996 828-1343 1055-1664 1265-1508

As you would expect, our fields follow similar GDD for vegetative growth stages, with emergence, V6 and tasseling growth stages falling within expected ranges (Table 1). The same can be said for reproductive stages, with R1 (1400), R5(2190), and R6 (2700) minimum GDD all close to those observed in DE (Table 2). There is a range in values in each table, due to differences in hybrids as well as just human error.

Table 2: Following Corn Reproductive Growth Stages (2019-2020)
R1 R2 R3 R4 R5 R6
Days from Planting 59-82 60-90 67-96 83-106 78-118 106-138
Growing Degree Days 1320-1594 1559-1684 1733-2003 1957-2187 2028-2509 2686-2926
Total Light  1389-1804 1408-2003 1616-2147 1887-2367 1845-2567 2351-2952

“Days since planting” is an easy measurement, however early planted corn will suffer from slow growth due to cooler temperatures. This can be seen in Figure 1, where mid April planted corn (green circles) always takes more days to reach each growth stage. From Table 2, the 138 days to reach blacklayer is related more to the lack of heat units in early 2020 rather than the maturity rating of the crop.

Growing degree days (Figure 2) has a much tighter grouping in the vegetative stage, since it relies on actual temperature measurements. Compared to just using days from planting above, the early planted April corn lines up better with early and late May plantings. The spread in points seen in reproductive stages (e.g. R1) is due to the subjective rating of kernel development, since each ear may have kernels at two reproductive stages as it transitions. The vegetative stages use a leaf counting method, which limits human variability in staging the crop.

Using light accumulation  (MJ/m²*d) provides a similar graph to “days since planting” (Figure 3). Early germination will rely on soil temperature, and not light, so that early April planting appears to need more light to mature. Light is an energy measurement, which will correlate to heat, but probably related better to later vegetative and reproductive stages, compared to early season.

The take home point is that GDD remains a good milestone system for measuring or predicting corn stages for timing nutrient, fungicide, and irrigation applications.

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