Plant indicators for determining corn harvest date

September 8, 1994 1(24):161-162

Joe Lauer, Corn Agronomist

With the generally good growing season experienced so far and the increased acreage planted to shorter season hybrids, corn development is "on-pace" for the year. Growers should anticipate a "normal" harvest date for corn silage and high-moisture corn. According to the Agricultural Statistics Service, nearly half the corn in Wisconsin had begun to silk by July 24 (only 21 percent was silked by this time over the last five years). The relatively cool August has slowed accumulation of heat units to "average" amounts, but there are some fields around the state near maturity. As of August 28, about 65 percent of the corn was in the dough stage and 20 percent was dented, compared to the five year average of 52 percent in dough and 17 percent in dent.

Physiological maturity of corn is normally reached about 60 days after silking. At physiological maturity, maximum grain dry weight has accumulated, corn is safe from frost, kernel moisture is 30 to 35 percent, and whole plant moisture is 60 to 65 percent.

The plant indicator for corn physiological maturity is kernel black layer formation at the base of the kernel. No further increases in yield will occur in the kernel after formation of the black layer. An easy method for monitoring kernel development and approaching physiological maturity is the kernel milk line. Loss of all milk from the kernel or "no milk line", occurs at the same time as kernel black layer formation under normal conditions. Kernel milk line can also be used to assess approximate whole plant moisture.

To find the kernel milk line, break a corn ear in half and look at the top or tip half. After dent stage begins, a line can be seen on the smooth side of the kernels. It marks the boundary between the solid and liquid parts of the immature endosperm. The progression of this kernel line from kernel crown to the kernel base can be monitored and used to predict physiological maturity. It generally takes about 11 days to move from the early dent stage of development to half milk line, about 6 days from half milk line to 25% milk line, and about 7 days from milk line to no milk (physiological maturity or black layer formation).

The optimum time to harvest corn for grain or high moisture corn is after black layer formation at the proper moisture for the storage system (Table 1). The optimum stage to harvest corn for silage depends upon whole plant moisture content, yield and forage quality (Table 2). Whole plant moisture content is within the acceptable ensiling range of 60 to 70 percent beginning at 25% milk line. Crude protein declines with increasing maturity, indicating that harvest delays to no milk line will result in lower forage quality. Whole plant acid detergent fiber, neutral detergent fiber, and in vitro dry matter disappearance are at an optimum between early dent and 50% milk line. Based on these criteria, corn silage should begin at 50% milk line and conclude by 25% milk line.

Table 1. Optimum moisture for corn harvesting to minimize field losses and storage problems in different storage systems. Harvest should be finished at the mid-point of the preferred range.
Storage system Preferred moisture range
Dry grain Kernel moisture (%)
Picked/cribbed 20 to 25
Combine/Dry 21 to 25
High moisture Ground shelled Kernel moisture (%)
Conventional silo ---------
Oxygen-limiting silo 25 to 30
High moisture Ground ear corn Kernel moisture (%)
Conventional silo 32 to 36
Oxygen-limiting silo 25 to 32
Whole-plant silage Whole plant moisture (%)
Horizontal (Trench/Bunker) silo 65 to 70
Upright stave (Tower) silo 60 to 65
Bag silo 60 to 70
Oxygen-limiting silo 50 to 60


Table 2. Whole plant dry matter, crude protein, ADF, NDF, and digestibility for corn silage at five stages of kernel maturity.
Maturity stage Dry matter Yield Crude protein ADF NDF Digestibility (in vitro)
  % ton/A % % % %
Soft dough 24 5.4 10.3 27.2 52.7 77.1
Early dent 27 5.6 9.9 24.3 48.0 79.0
1/2 milkline 34 6.3 9.2 22.8 45.1 80.0
3/4 milkline 37 6.4 8.9 23.8 47.3 79.6
No milkline 40 6.3 8.4 24.0 47.3 78.6

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