The 2004 growing season continues to be cooler than average. Many fields have tasseled the past week with silking and pollination occurring now. Although this is a typical time for pollination, it was the atypical early planting season that has been our salvation. Early planted fields are right on target for development.

We are noticing that some hybrids are silking prior to the tassels emerging. Normally silks will emerge 2-3 days after tassels have begun to shed pollen, but during cool growing seasons the reverse can happen. The plants are telling us that it is cool out there.

Last week did not help GDU accumulation. At Arlington GDU accumulation is the lowest on record for the period May 1 to July 27 with only 1162 GDUs accumulated thus far, 266 GDUs behind average (average = 1428 GDUs). Normally Arlington accumulates about 20 GDUs per day in late July. The second coolest year was 1969 when 1258 GDUs accumulated during this period.

At Marshfield GDU accumulation is the second lowest on record for the period May 1 to July 27 with 1089 GDUs accumulated so far (average = 1302 GDUs). The coolest year on record at Marshfield for this period is 1967.

The date that silking (R1) occurs on the corn plant is the reason why GDU accumulation is important. Temperature drives the enzymatic reactions that occur within corn cells which influence the rate of growth and development. Temperature is most important during the vegetative phases of development up to the silking stage. Once silking occurs, temperature plays less of a role in development since the leaves and stalk are present and fully grown. Kernel development is more influenced by the number of days that have occurred since silking, although temperature will influence the rate of kernel growth.

Thus, if we know the silking date of a field we can project when fields should achieve various developmental stages. Typically it takes 35-42 days after sillking to reach the dent (R5) stage, and 55-65 days to reach the black layer (R6) stage (Ritchie et al., 1993). The average freeze date (<32 F) at Arlington and Marshfield is October 7 and September 27, respectively. For a full-season (105-110 day RM) hybrid to achieve silking (R1) and maturity (R6) at Arlington, it requires 1250 and 2400 GDUs. For a full-season hybrid (90-95 day RM) to achieve silking and maturity at Marshfield, it requires 1150 and 2250 GDUs.

If average temperatures occur form this day (July 28) forward, we will not achieve the required GDUs for maturity of full-season hybrids at these locations. Remember though that temperature has less of an effect on development and the number days after silking is what is important for projecting harvest dates. Table 1 lists projected dates for silage (R5.5) harvest and grain maturity (R6) of full-season corn hybrids at Arlington and Marshfield. These predictions assume that temperatures will be average from July 28 on, which is highly unlikely, but it does give us a feel for the stage of development that corn will be at when frost occurs for various planting dates and the timing for harvest of silage, high moisture and dry grain during 2004.

Literature Cited

Ritchie, S. W., J. J. Hanway, and G. O. Benson. 1993. How a corn plant develops. Iowa State University CES Special Report No. 48. 21 pp.

Table 1. Projected dates when corn growth stages are achieved for various 2004 planting dates. Predictions assume that temperatures are average from July 28 on.
		Days after	Planting date
Growth stage	GDUs	silking	April 15	May 1	May 15	June 1	June 15	July 1
Arlington (Full-season hybrid = 105 to 110 d RM,)
R1 (silking)	1250	---	July 25	July 31	Aug. 6	Aug. 14	Aug. 26	Sep. 7
R5 (dent)	2150	38	Sep. 1	Sep. 7	Sep. 13	Sep. 21	Oct. 3	Frost
R5.5 (½ milk)	2275	49	Sep. 12	Sep. 18	Sep. 24	Oct. 2	Frost	Frost
R6 (black layer)	2400	60	Sep. 23	Sep. 29	Oct. 5	Frost	Frost	Frost
Marshfield (Full-season hybrid = 90 to 95 d RM,)
R1 (silking)	1150	---	July 26	July 30	Aug. 4	Aug. 10	Aug. 21	Sep. 2
R5 (dent)	2000	38	Sep. 1	Sep. 6	Sep. 11	Sep. 17	Frost	Frost
R5.5 (½ milk)	2150	49	Sep. 13	Sep. 17	Sep. 22	Sep. 28	Frost	Frost
R6 (black layer)	2250	60	Sep. 24	Sep. 28	Frost	Frost	Frost	Frost
GDUs = modified Growing degree units (base= 50 F, maximum = 86 F) Average frost date (<32) at Arlington = Oct. 7 and Marshfield = Sep. 27.