The following is information on the effects of late-season frost injury to corn from an article by Dr. Joe Lauer, Corn Extension Specialist at the University of Wisconsin (http://corn.agronomy.wisc.edu/Management/L041.aspx).
Freezing temperatures before physiological maturity will damage corn. Maturity in corn occurs when kernels form a black layer at the kernel tip, grain will be at approximately 30 to 35 percent moisture. After maturity, no additional dry matter will be accumulated in the seed. In addition to creating quality problems, premature frost will reduce the yield of dry grain.
Temperatures required to kill corn plants
Corn is killed when temperatures are near 32 F for a few hours, and when temperatures are near 28 F for a few minutes (Carter and Hesterman, 1990). A damaging frost can occur when temperatures are slightly above 32 F and conditions are optimum for rapid heat loss from the leaves to the atmosphere, i.e. clear skies, low humidity, no wind. At temperatures between 32 to 40 F, damage may be quite variable and strongly influenced by small variations in slope or terrain that affect air drainage and thermal radiation, creating small frost pockets. Field edges, low lying areas, and the top leaves on the plant are at greatest risk. Greener corn has more frost resistance than yellowing corn.
Symptoms of frost damage will start to show up about 1 to 2 days after a frost. Frost symptoms are water soaked leaves that eventually turn brown. Because it is difficult to distinguish living from dead tissue immediately after a frost event, the assessment should be delayed 5 to 7 days.
Grain quality impact
Late season frost damage can affect grain quality and is directly proportional to the stage of maturity and leaf tissue killed. Severe impacts on grain quality can occur at mid-dough, while moderate impacts are seen at the dent stage. By the time, the kernel has reached half milk line only minor impacts will occur to grain quality. Differences among hybrids, overall plant vigor at the time of frost and subsequent temperatures will all affect final grain quality.
Other considerations
Growers should monitor stalk rot of severely defoliated plants, which have a good-sized ear. Photosynthate will be mobilized towards the ear rather than the stalk. This could weaken the stalk and encourage stalk rot development. These fields may need to be harvested early to avoid standability problems.
Yield losses are negligible if frost occurs when grain moisture is below 35 percent. Yield loss is directly proportional to the stage of maturity and the amount of leaf tissue killed. Those who will be advising growers about the likelihood of frost damage and its impact on yield should get ready by consulting the National Corn Handbook NCH-1 “Assessing Hail Damage to Corn” (Vorst, 1990). This publication has charts used by the National Crop Insurance Association for assessing yield loss due to defoliation. Knowing how to recognize frost damage and assess probable loss is important for decision making. An abbreviated version of the loss chart is shown in Table 7. For example, corn that was defoliated 20% at the milk stage would have 3% yield loss.
The stem on a corn plant is a temporary storage organ for material that eventually moves into the kernels (Afuakwa and Crookston, 1984). Grain yield will continue to increase about 7 to 20% after a light frost that only kills the leaves as long as the stem is not killed (Table 4).
Frost damaged grain drying rates
Freezing air temperatures sometimes occur in early autumn before grain is physiologically mature (“black layer). Grain drying rates can range from 0.83 to 1.16% moisture less/day (Hicks et al., 1976). Drying rates of grain following leaf blade defoliation or moderate to severe cold treatments are not different from the drying rate of normally maturing maize grain. Husk condition does not affect grain drying rates. Defoliation and freezing before physiological maturity (R6) causes grain moisture levels to be 2 to 6 percentage points greater than that of grain from control plants when grain from control plants was in the 22 to 30% harvest range. Grain frozen before R6 required 4 to 9 additional days of field drying to reach the 22 to 30% moisture range. Defoliation and cold treatments have little effect on the drying rates of cobs and ears, but moisture levels are greater than those of the control. Loose husks cause faster cob and ear drying compared to normal husks.
Characteristics of frost-damaged corn grain (http://www.extension.iastate.edu/publications/PM1635.pdf).
- Small, misshapen, soft kernels
- Undeveloped starch structure; pithy kernels
- Test weights progressively below 52 lb./bu., depending on maturity (in 1993, some corn was less than 40 lb./bu.)
- Average protein (7.5 to 8.0 percent) in corn heavier than 45 lb./bu., lower protein in corn lighter than 45 lb./bu.
- High breakage susceptibility; many fines generated in handling
- Lower digestibility compared with normal corn, especially for test weights below 45 lb./bu.
- Little or no increase in test weight after drying
- Variable amino acid levels
- Moisture meters generally read low in immature corn. Surface drying of kernels, giving deceptively low (by 1 to 2 percent) moisture readings on dried corn
Recognize that these effects are progressive, with least impact on corn closer to maturity.
Uses for frost-damaged corn
Animal feed is the best use for frost-damaged corn. Low test weight corn used for large animal feed is only slightly less valuable (2 to 5 percent) than normal corn on a per-pound basis. Poultry, however, with limited volumetric capacity, may be more sensitive to frost-damaged corn than larger livestock.
Before feeding, test light corn for protein level, amino acid level, and mycotoxins (especially fumonisin and vomitoxin). Composition will vary. Be aware that fungi invade stressed corn more readily than they do normal corn.
Wet, dry milling, and dry grind ethanol operations will not want frost-damaged corn. Using frost-damaged corn in wet milling causes low starch yields, and the separation of starch and protein cannot be clean. In dry milling, frost damaged corn sharply reduces yields of dry mill grits. Processors will discount light corn more heavily than its reduction in feed value. Fermentation will be more variable in ethanol production, with lower yields and less predictable distillers grain quality.
Handling and storage of frost damaged grain
Immature and frost-damaged corn will have marginal quality, so it’s important to manage equipment carefully to minimize further quality degradation. Set combines carefully, to balance the need to get small kernels with kernel damage. Manage the fines and chaff, which can increase mold problems in storage. Dry grain to uniform moisture levels, a tricky business because harvest moisture is likely to be somewhat uneven after a cold, short growing season. Dry frost-damaged corn at reduced air temperatures (below 160 °F) and store at 14 percent (or lower) moisture. Dry corn as gently as possible, even if it is tempting to crank it up for higher dryer capacity. Also, use slow cooling methods after gas-fired drying to minimize quality problems. If possible, aerate stored grain to cool it to 20 to 30F for winter storage (in the upper Midwest).
Frost-damaged corn breaks easily and goes out of condition quickly, even at low moisture levels. Expect storage life to be about half as long as that of normal corn. Do not harvest through low-lying frost damaged areas. The mixture will be a high storage risk. Harvest and handle them separately.
Because immature corn kernels dry on the surface, expect the moisture level of stored corn to be higher than test results. Expect to aerate the stored corn frequently. Move immature corn to market before summer. Store only clean corn and pull out the fines-laden center core of grain in bins.