A portion of Iowa’s corn crop is likely to experience a frost before naturally reaching maturity as a result of cool temperatures later this week. Natural maturity is often determined when a black layer is formed at the kernel tip. At this timeframe grain moisture is typically 28–35% moisture. If a frost or freeze occurs a black layer can still form, however, grain moisture will be greater than 35%. A light frost of greater than 28oF for only a couple hours may kill some leaves, especially in the upper canopy. A freeze of 28oF or less for a couple hours will result in whole plant death.
Whether maturity is natural or induced by a frost, grain dry matter accumulation has finished. Incomplete grain fill from a frost or freeze often results in corn with low test weight that is greater than 35% moisture. The severity depends on how far the milk line has progressed at the time of the cold temperatures. At ¼ milk line the kernel moisture will be higher and the kernel weight will be lower compared to ¾ milk line. Expect dry down in the field to take longer because of a higher kernel moisture and slower dry down rates because there is continued moisture exchange with the cob.
Grain yield decreases more the earlier a killing frost occurs (ranges from 12% in late dent up to 41% at beginning dent stage). Yield losses are also less when only leaves are killed (ranges from 6% in late dent up to 27% at beginning dent stage). In addition to lower yield, incomplete grain will occur resulting in a lower test weight. Expect test weight to be in the low 50s to 40s (pounds/bushel). Lighter kernels will require fine adjustments to combine settings. Recognize the effects frost and freeze to corn ahead of natural maturity results in more susceptibility to breaking, lower grain protein, and lower digestibility
Grain moisture of frost injured corn is deceptively low because the outer portion of the kernel dries quicker than the interior. Often grain moisture is 1–2% higher than many grain moisture meters, however, the newer 150 mhz units used by many elevators are more accurate. Dry frost killed corn 1–2% lower than the typical 14–15% and cool grain as quickly as possible. It will take more energy per unit of moisture removed to dry frost damaged corn. Storage life drops rapidly below 53 pounds/bushel test weight.