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When is your corn crop safe from frost?

Agriculture.com Staff 10/09/2006 @ 2:19pm

The Illinois State Water Survey has compiled some "ballpark" estimates for when the first frost would typically occur for different areas of Illinois.

For northern Illinois, the first frost usually occurs during the first week of October; for central Illinois, during the second week of October; and for southern Illinois, during the third week of October.

How does this frost data potentially impact the maturing of our 2006 corn crop? According to Bob Frazee, University of Illinois natural resources educator, as of September 10, corn development throughout Illinois is slightly behind the five-year average due to the cool, wet weather experienced the past two weeks across the state.

However, with 95% of the crop in the dent stage and 39% reported as already mature, very few farmers are concerned about the impact of an early frost on the maturing corn crop.

Frazee says the same criteria that are used to determine when the corn crop is safe from frost damage can be also used to help determine when the corn crop is mature and harvest can be scheduled. Physiological maturity is the term used to describe the point of grain development at which the kernels have maximum dry weight and the plant is safe from yield loss by frost. The formation of the black layer is the signal of full kernel maturity.

To determine the maturity of the corn crop, Frazee suggests breaking a corn ear in half, and remove several of the kernels from the cob. The presence of the black layer can be determined by using your fingernail to cut into the tip of the kernel. The black layer is formed and becomes visible after the maturity line reaches the inward tip of the kernel, where the kernel attaches to the cob. A dark brown line first appears on the developing kernel and later turns black. Kernels at the butt end of the ear will develop the black layer first.

Frazee reports that once the black layer appears, the corn can no longer increase in weight and will begin a gradual reduction in moisture content during the drying period before harvest. Kernel moisture is usually around 30% to 35% at physiological maturity, depending upon the hybrid. Corn harvest can begin at physiological maturity, but many producers usually choose to leave the corn dry naturally in the field until it reaches 22% to 28% moisture.

According to UIUC research, if a corn crop is prematurely killed 10 days to two weeks before black-layer maturity, a four- to five-percent grain yield reduction can be expected. However, Frazee cautions that if the corn crop is killed three weeks before physiologically mature, the yield loss may approach 10% to 20% with considerable reduction in grain quality. A corn crop killed by freezing temperatures one month before normal maturity (or 30 to 35 days after silking) is reduced in potential grain yield by 35% to 50% and the grain is probably not marketable. Consequently, Frazee expects that an early frost this year would have a negligible impact on most corn fields throughout the state.

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