Frost can occur at any time during the growing season. Frost events in July and August can occur in northern Wisconsin, although these events are rare. Most of the time farmers deal with early- or late-season frosts that impact spring plant establishment or affect yield and harvest timing in the fall.

Early-season frost

Corn plants will not be killed by frost unless temperatures get cold enough to kill the growing point that is 3/4 of an inch below the soil surface. So corn that has not emerged typically is well insulated from frost damage. So corn that has not emerged typically is well insulated from frost damage.

Frost should not be a problem with corn until the growing point moves aboveground around V5 to V6. Farmers and agronomists usually do not worry about frost at these early stages of development. Early frost can have an impact on grain yield, but the trade-off between planting date impact on yield is greater than for frost damage impact on yield. Delayed planting further impacts profitability due to greater moisture and consequential drying costs.

Symptoms of frost damage will start to show up about 1 to 2 days after a frost. Symptoms are water soaked leaves that eventually turn brown. After 3 to 4 days watch for new green leaves emerging n the whorl. If new leaves are not emerging check the growing point for discoloration. Any deviation from a white, cream or light yellow color indicates that the growing point is killed.

Experiments were conducted in 2001, 2002, 2003, 2004, and 2005 to measure the impact of early defoliation on corn grain yield. Clipping treatments were applied at V2, V4 and V6. Clipping treatments consisted of cutting the plant at ground level with a scissors. Plants in the control treatment were not clipped. In another treatment, all plants in the plot were clipped. In another set of treatments, half of the plants were clipped in 2-, 4-, and 8-plant patterns. For example in the 2-plant pattern, the first 2 plants in the row were not clipped, the next 2 plants were clipped at ground level, the next 2 plants were not clipped, and so on.

Although these treatments do not fully simulate the light frost damage that recently occurred on corn over the last couple of evenings, they do provide some guidance on what a hard frost might do that completely defoliates the plant without killing it. Figure 1 describes the impact of complete defoliation on corn grain yield at the V2 stage of development. When all plants were clipped, grain yield decreased 17 bu/A from 210 to 193 bu/A (8%). When half of the plants were clipped in various patterns, grain yield was not affected; the trend was a decrease of 8 to 9 bu/A (4%).

These data indicate that frost early in development has relatively little impact on corn grain yield. If all of the leaves are removed from every plant in the field at the V2 stage of development and plants are not killed, then the expectation is that grain yield would decrease up to 8%. If the recent frosts were hard enough to kill plants then use the publication UWEX 3353 for guidance on whether or not to keep a stand and what to look for when assessing plant health.

Figure 1. Impact of clipping corn leaves at V2. Experiments were conducted in 2001, 2002, 2003, 2004, and 2005 at Arlington, WI. Treatments consisted of clipping at ground level (or not clipping) consecutive plants in 2-, 4-, 8-, and all-plant patterns.

Late-season frost

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.

Will corn mature before a killing frost?

Typically in a normal year, corn should be "silking at the end of July and denting on Labor Day." After corn silks, it normally takes about 55 to 60 days (1000 to 1200 GDUs)for it to mature (Table 1). Normally during September, growing degree units in Wisconsin accumulate at the rate of 12 to 19 units per day for a total accumulation of 400 to 450 units (Table 2). Likelihood of a 32 ° F freeze by September 20 is 3 years out of 5 in northern, and 1 year out of 5 in southern Wisconsin.

Table 1. Required growing degree units between corn development stages and maturity (black layer).
Corn	Relative maturity zone (days)
development	85-90	95-105	110-120
stage	Growing degree units
R1 (silking)	1000	1100	1200
R2 (blister)	800	880	960
R3.5 (late milk / early dough)	600	660	720
R4.5 (late dough / early dent)	400	440	480
R5 (dent)	200	220	240
R6 Maturity (black layer)	0	0	0
Harvest (kernel moisture at 25%)	150	150	150
derived from Carter, 1991

Table 2. Corn growing degree unit accumulation in Wisconsin.
	North			South
Month	Daily	Monthly	Total	Daily	Monthly	Total
	Growing degree units
May	8-11	300	300	10-13	350	350
June	11-17	400	700	13-20	500	850
July	17-20	575	1275	20-23	650	1500
August	20-17	575	1850	23-19	650	2150
September	17-12	400	2250	19-13	450	2600
October	12-8	300	2550	13-10	350	2950
derived from Mitchell and Larsen, 1981

Use Tables 1 and 2 to determine the likelihood that a field will mature. For example, if on September 1, your field is at R3.5 (late milk / early dough) and you are in a 95-105 relative maturity zone, it will take about 660 growing degree units to mature the crop before it is killed by a frost. Since corn is usually killed in 3 out of 5 years by September 20 the field in all likelihood will accumulate about 300 to 380 growing degree units and be at the early dent to dent stage of development when it is killed by a frost.

Temperatures required to kill corn plants

For fields that only had light frost damage, it is too early to harvest. Growing conditions may improve during September allowing the crop to mature and produce reasonable grain and silage yields.

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.

Silage moisture drydown

Corn silage should be harvested at the appropriate moisture content for the type of silo in which it will be stored (Table 3). If corn is frosted prior to 50% kernel milk, the moisture content of corn may be too high to be properly ensiled. However, during the drydown period, dry matter yield will decrease due to leaf loss, plant lodging and ear droppage. Thus, a trade-off exists between moisture and yield.

For corn silage frosted prior to the dent stage, the moisture content will be too high for successful ensiling. The silage crop should be allowed to dry in the field for several days and moisture content should be monitored. For corn frosted during the dent stage, harvest should begin quickly to prevent yield loss as damaged leaves are shed or break off the plant.

Since mold can occur on the ears before the desired moisture level is reached, harvest may have to begin immediately. To help control problems with excess moisture, wet silage can be mixed either with ground grain, straw, or chopped hay to reduce the overall moisture of the stored silage, The rule of thumb is about 30 pounds of dry material per ton of silage will be needed to reduce silage moisture one percentage unit.

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.

Handling silage from fields with uneven maturity

Many corn fields in Wisconsin can be uneven for maturity. There is some concern about harvesting these fields for silage and the potential for mold development. Mold problems in silage occur when corn is harvested too dry. When harvesting a corn field differing in maturity handle field sections separately where possible. In fields where the chopper must move through areas differing in maturity (i.e. low spots) chop when the majority of the field is at the proper moisture. The immature spots will be wetter than the rest of the field and might seep in the silo, but as long as the seepage does not leave the silo, nothing is lost. Fermentation should be adequate for preservation of the corn silage. However, corn that is too dry might develop a "hot spot " where mold can develop, thereby increasing the chances for mycotoxin development.

Harvesting silage at the correct moisture

If the decision is made to harvest the crop for ensiling, the main consideration will be proper moisture for storage and fermentation. Recommended whole-plant moisture contents for fermenting corn and producing silage vary for different storage structures (Table 3). In general, more moisture is required to get good packing in storage structures that allow easy diffusion of air such as bunkers.

Table 3. Recommended moisture content (%) for corn stored in various types of storage structures.
Upright oxygen limiting silos	50-60
Upright concrete stave silos	60-65
Bag silos	60-70
Horizontal bunker silos	65-70
Roth et al., 1995

For many years, corn was harvested for silage at the black layer stage of development. Lower forage fiber levels, higher digestibility and highest yields were observed slightly earlier than the black layer stage, and recently this recommendation was modified to begin corn silage harvesting at 50% kernel milk and be finished by 25% kernel milk (Wiersma et al., 1993).

Growers often find that corn is too wet and seepage occurs in the silo when corn is harvested at 50% kernel milk. On average, the recommendation of using kernel milk to predict whole plant moisture is closely correlated with previous work. For example, at 50% kernel milk whole-pant moisture equals 63%. However, the range at 50% kernel milk is 53 to 73% whole plant moisture, with the majority of the hybrid environments around 70%. Many hybrids grown in Wisconsin have a "stay-green" trait that improves standability by keeping the stalk and leaves green while husk leaves turn brown and open allowing the ear too dry.

After a frost the crop will look drier than it really is, so moisture testing will be critical. Be sure to test whole-plant moisture of chopped corn to assure yourself that acceptable fermentation will occur. Use a forced air dryer (i.e. Koster), oven, microwave, electronic forage tester, NIR, or the rapid "Grab-Test" method for your determination. With the "Grab-Test" method (as described by Hicks, Minnesota ), a handful of finely cut plant material is squeezed as tightly as possible for 90 seconds. Release the grip and note the condition of the ball of plant material in the hand.

If juice runs freely or shows between the fingers, the crop contains 75 to 85% moisture.
If the ball holds its shape and the hand is moist, the material contains 70 to 75% moisture.
If the ball expands slowly and no dampness appears on the hand, the material contains 60 to 70% moisture.
If the ball springs out in the opening hand, the crop contains less than 60% moisture.

The proper harvest moisture content depends upon the storage structure, but is the same for drought stressed and normal corn. Harvesting should be done at the moisture content that ensures good preservation and storage: 65-70% in horizontal silos (trenches, bunkers, bags), 60-65% in upright stave silos, and 55-65% in upright oxygen limiting silos.

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.

Some growers have expressed concern about nitrate poisoning. If frosted corn is ensiled at the proper moisture content and other steps are followed to provide good quality silage, nitrate testing should not be necessary. However, it is prudent to follow precautions regarding dangers of nitrate toxicity to livestock (especially with grazing and green-chopping) and silo-gasses to humans when dealing with drought-stressed corn. Nitrates absorbed from the soil by plant roots are normally incorporated into plant tissue as amino acids, proteins and other nitrogenous compounds. Thus, the concentration of nitrate in the plant is usually low. The primary site for converting nitrates to these products is in growing green leaves. The highest concentration of nitrates is in the lower part of the stalk or stem, so raising the cutter bar on a corn silage chopper will leave most nitrates in the field. Nitrate concentration usually decreases during silage fermentation by one-third to one-half, therefore sampling one or two weeks after filling will be more accurate than sampling during filling. If the plants contain nitrates, a brown cloud may develop around your silo. This cloud contains highly toxic gases and people and livestock should stay out of the area. The only way to know the actual composition of frosted corn silage is to have it tested by a good analysis lab.

Calculating the value of normal corn silage

Due to late planting dates and a cooler than normal growing season this year, many corn fields will probably be harvested for silage. There is even great potential for corn in these fields to be too immature for proper corn silage harvest. How should the value of corn silage be adjusted for frosted immature corn? Typical calculation methods for pricing normal corn silage include:

Relative feed value of a known forage market.
- Silage ($/T) = 1/4 to 1/2 value of hay
- Silage ($/T) = 6 to 8 times the price of a bushel of corn. If already harvested, then 10 times.
Feed replacement or substitution costs
Use market prices for energy, protein, and digestibility (NEL of corn, soybean meal, hay)
Contracted price above the cost of production (280-320 $/A).

A handy spreadsheet for calculating the value of corn silage can be found at http://corn.agronomy.wisc.edu/Season/DSS.aspx.

Immature corn silage

For most crops, forage quality and value decreases with maturity, that is fiber levels increase and digestible energy decreases. Corn is somewhat unique in that quality increases with maturity. In corn silage most of the digestible energy is in the grain portion. Immature corn will have a lower proportion of grain in the silage. Two approaches to consider for calculating the value of immature corn silage are:

Reduce the value of immature corn silage by the cost of buying back grain to bring the grain:stover ratio to a more normal proportion.
Use MILK2006 to calculate the milk per acre and milk per ton that could potentially be produced from immature corn silage.

Afuakwa and Crookston (1984) described the grain yield impact of frost at different stages of development (Table 4). A killing frost at the soft dough stage of development would result in a grain yield loss of 55% and at least that much grain would be required to produce normal silage.

Table 4. Potential grain yield losses after frost.
Corn development	Killing frost (Leaves and stalk)	Light frost (Leaves only)
Stage	percent yield loss
R4 (Soft dugh)	55	35
R5 (Dent)	40	25
R5.5 (50% kernel milk)	12	5
R6 (Black layer)	0	0
derived from Afuakwa and Crookston (1984)

The relationship between kernel maturity and silage yield and quality is shown in Table 5. Milk production per acre is 35% less when corn is harvested at the immature soft dough stage compared to the optimum stage at 50% kernel milk. Milk production per ton of immature corn silage (soft dough) was 25% lower than the optimum stage of 50% kernel milk. Thus, the milk production potential would be reduced between 25 and 35% with immature corn harvested for silage. The value of the corn silage should be adjusted accordingly.

Table 5. Relationship between kernel maturity and corn silage yield and quality.
Corn Development	Silage moisture	Silage yield	Crude protein	ADF	NDF	IVD	Milk production
	%	T/A	%	%	%	%	lb/T	lb/A
Soft dough	76	5.4	10	27	53	77	1600	8600
Early dent	73	5.6	10	24	48	79	1900	10800
50% milk	66	6.3	9	23	45	80	2100	13300
25% milk	63	6.4	9	24	47	80	2000	12600
Black layer	60	6.3	8	24	47	79	1950	12400
derived from Wiersma et al. (1993) and Undersander et al. (1993)

Corn silage yield and quality changes substantially during the growing season (Table 6). At V11 crude protein was 18% and one ton of silage could produce 1700 lb. of milk. Like all crops, corn silage quality decreased as the crop approached flowering. Milk per ton decreased from 1700 lb./T on V11-14, to 1300 lb./T on R1.0 (silking), and was lowest at R3.0 (Milk) at 700 lb./T. Milk per ton and milk per acre then increased throughout the remainder of the growing season. During the silking and milk stages, milk per acre and milk per ton was about 1/3 of the optimum harvest dates between R5.5 and R5.8.

Table 6. Corn silage yield and quality response to harvest date for Pioneer 3578 during 1993 at Arlington, WI. Corn was planted on May 11. Derived from Burger and Hudelson (1993) and Undersander et al. (1993).
Harvest	Corn development	Whole plant moisture	Dry matter yield	Crude protein	ADF	NDF	Milk production
date	stage	%	T/A	%	%	%	lb/T	lb/A
July 11	V11	92	1.1	18	28	49	1700	1900
July 21	V14	90	2.2	15	27	50	1700	3800
July 31	R1.0	85	3.8	12	31	55	1300	5000
August 10	R2.0	83	5.0	11	33	58	1100	5500
August 20	R3.0	84	5.7	10	36	65	700	3700
August 30	R4.0	82	6.4	10	33	60	1000	6500
September 10	R5.0	76	8.0	9	27	51	1700	13400
September 21	R5.5	75	8.6	9	25	48	1900	16300
October 5	R5.8	66	8.2	8	21	43	2300	18800
Corn development stage: Vn = nth leaf collar; R1 = Silking; R2 = Blister; R3 = Milk; R4 = Dough; R5 = Dent; R5.5 = 50% kernel milkline; R5.8 = 80% kernel milkline; R6 = Black layer (physiological maturity).

The following guidelines should be considered when deciding to harvest corn silage:

Use kernel milk as a guideline for predicting when to begin silage harvest.
To insure proper fermentation for the storage structure, accurate whole-plant moisture must be determined. Immature corn is too wet to ensile and will seep out of the storage structure lowering silage quality.
In general, whole-plant moisture decreases at the rate of 0.5% per day during September.
The relationship between kernel milk and whole-plant moisture differs among hybrids. Within a hybrid the relationship between kernel milk and whole-plant moisture is correlated regardless of environment.
If there is more than one type of on-farm storage structure and since most hybrids tend to be wetter than average around 50% kernel milk due to the stay-green trait, producers may want to start by filling bunker silos and as the season progresses move to other structures.
To produce good quality silage with adequate yields, corn must be past the R4.5 to R5 stage of development. Corn which is immature should be fed to heifers or other less productive animals.

Yield impact on frost-damaged corn grain

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.

Table 7. Estimated percent corn yield loss due to defoliation occurring at various stages of growth.
Stage of growth	Percent leaf area destroyed
	20	40	60	80	100
	Yield loss (%)
Tassel	7	21	42	68	100
Silked	7	20	39	65	97
Blister	5	16	30	50	73
Milk	3	12	24	41	59
Dough	2	8	17	29	41
Dent	0	4	10	17	23
Black layer	0	0	0	0	0
derived from Vorst (1990)

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. 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 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 (from Hurburgh et al., 2007).

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
Mositure 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.

Frost