Will the corn crop reach maturity before a hard frost?

Growing degree days (GDD). 

Corn plants require heat in order to grow, develop and reach vegetative and reproductive milestones on their way to maturity.  Growing degree days (GDDs) are a way to determine how much heat a corn crop has accumulated after planting.  For each day, 50 is subtracted from the average of the daily high temperature (up to 86 degrees) and the daily low temperature (down to 50 degrees) to yield a daily GDD value ranging from 0 to 36.  Agronomists have determined the relationship between GDDs and developmental milestones like emergence, silking and physiological maturity.

In seed corn catalogues companies provide a hybrid’s relative maturity, typically expressed in days.  Some companies also provide additional information about the GDDs required to reach physiological maturity.  Physiological maturity is also referred to as black layer, the name for a physical barrier that forms between a kernel and cob, rendering kernels unable to further accumulate sugars and other nutrients from leaf and stalk tissue.  

Crop progress in 2019.  

To reach physiological maturity, hybrids with lower relative maturity values require the accumulation of fewer GDD units than hybrids with higher relative maturity values.  Several years ago university-based climatologists and agronomists worked together to develop an online geo-referenced corn growing degree day calculator.  To use this calculator one selects the location of  their Midwestern corn field and enters their planting date and relative maturity (or GDDs needed for black layer). The calculator then graphs the GDD that have accumulated this year. The calculator also uses the 30-year average to predict how GDD will accumulate into the future (Figure 1).   

Figure 1. Graph generated by the corn growing degree day calculator for an 80 day corn hybrid planted on May 21 in Crookston, MN (Source: U2U GDD Calculator).

Wet spring soils delayed planting throughout the southern counties in the northwest Minnesota crop reporting district while favorable planting conditions occurred throughout much of Polk County and north.  Of concern, particularly when compared to 30-year averages, is that cooler than normal air temperatures have delayed GDD accumulation in some northwest Minnesota locations (Table 1). 

Table 1. Growing degree day accumulation in 2019 and the 30-year average after estimated planting dates for five locations in northwest Minnesota (source: U2U GDD calculator)

	Planting Date
	May 21				June 7
GDD accumulation	Crookston	Thief River Falls	Humboldt		Moorhead	Ada
30-year average	1870	1765	1743		1745	1686
2019	1794	1716	1738		1605	1535

Table 2 lists when physiological maturity is projected to occur in 2019 based on 30-year average daily temperature values and the GDDs that have accumulated since planting.  Given a May 21 planting date in Crookston, projected dates of physiological maturity range from September 6 to October 24 for 73 day to 86 day hybrids, respectively (Table 2).

Table 2. For hybrids of different relative maturity, growing degree days (GDDs) and projected 2019 average date to reach physiological maturity given a May 21 planting date in Crookston, Thief River Falls (TRF) and Humboldt and a June 7 planting date in Moorhead and Ada (source: U2U GDD calculator)

		Planting Date
		May 21				June 7
Corn relative maturity	GDDs	Crookston	TRF	Humboldt		Moorhead	Ada
		Projected average date to reach physiological maturity
73	1748	Sep 6	Sep 13	Sep 12		Sep 26	Oct 7
77	1845	Sept 16	Sept 25	Sep 24		Oct 10	Nov 14
80	1917	Sept 25	Oct 7	Oct 6		Nov 2	*
82	1966	Oct 1	Oct 23	Oct 22		*	*
86	2062	Oct 24	*	*		*	*
92	2207	*	*	*		*	*

* Average black layer date occurring long after the latest spring frost date.

The growing degree day calculator is a tool that is free to use and may be a good way to pass the time while waiting for both soils to dry out to take out wheat and mature soybeans and the corn crop to mature.

References.

Corn growing degree day calculator. 2017. Useful to Useable. Online. Midwest Regional Climate Center.