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 for it to mature. In "cool " growing seasons we find many fields that are between the silking and milk stages of development on Labor Day. These fields will require 700-1200 growing degree units in order to mature and another 150 units to be at a harvestable moisture (Table 1).

Table 1. Required growing degree unit accumulation between corn development stages and maturity .
Corn development stage	Relative maturity zone (days)
Corn development stage	85-90	95-105	110-120
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 (25% kernel moisture )	150	150	150
derived from Carter, 1991

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). The 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. 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.

Table 2. Corn growing degree unit accumulation in Wisconsin.
	North			South
Month	Daily	Monthly	Total	Daily	Monthly	Total
	growing degree days
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

What Is a Killing Frost in Corn?

Corn is killed when temperatures are near 32 F for a few hours, and when temperatures are near 28 F for a few minutes. Less damaging frost occurs when temperatures are around 32 F and conditions are optimum for rapid heat loss from the leaves to the atmosphere, i.e. clear skies, low humidity, no wind. The stem on a corn plant is a temporary storage organ for material that eventually moves into the kernels. 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 3).

Table 3. Potential grain yield losses after frost.
Corn Development stage	Killing frost (Leaves and stalk)	Light frost (Leaves only)
	percent yield loss
R4 (Soft dough)	55	35
R5 (Dent)	40	25
R5.5 (50% kernel milk)	12	5
R6 (Black layer)	0	0
derived from Afuakwa and Crookston, 1984

Handling Silage from Fields with Uneven Maturity

Many corn fields in Wisconsin are 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

A very critical aspect in producing and preserving quality corn silage is harvesting at the proper whole-plant moisture for the storage structure. Recommended whole-plant moisture contents for fermenting corn and producing silage vary for different storage structures (Table 4). In general, more moisture is required to get good packing in storage structures that allow easy diffusion of air such as bunkers.

Table 4. 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. Figure 1 describes the relationship between whole-plant moisture and kernel milk for over 400 hybrid environments between 1991 and 1995. 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.

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).

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 MILK91 or MILK95 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 3). 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.

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.

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.

Late planted immature corn resulted in lower yield and quality than early planted corn harvested around R5.5 (Table 7). Little grain was produced on corn planted after June 22 even with shorter-season hybrids. Milk per acre and milk per ton of immature corn was about 1/3 that of corn harvested at R5.5 to R6.

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.

Key References

Afuakwa, J.J., and R.K. Crookston. 1984. Using the kernel milk line to visually monitor grain maturity in maize. Crop Sci. 24:687-691.

Burger, B.A., and K.D. Hudelson. 1993. Effect of maturity on silage yield and quality. In E.S. Oplinger (ed.) Wisconsin Research Report of studies on cultural practices and management systems for agronomic crops. pp. 143-144.

Carter, P.R. 1991. Corn development and growing degree days. Agronomy Advice Mimeo Series 28.10.

Lauer, J.G., and K.D. Hudelson. 1994. Effect of maturity on silage yield and quality. In E.S. Oplinger and J.G. Lauer (ed.) Wisconsin Research Report of studies on cultural practices and management systems for agronomic crops. pp. 359-360.

Mitchell, V.L., and R.W. Larsen. 1981. Growing degree days for corn in Wisconsin. UWEX. 22 pp.

Roth, G., D. Undersander, M. Allen, S. Ford, J. Harrison, C. Hunt, J. Lauer, R. Muck, and S. Soderlund. 1995. Corn silage production, management, and feeding. American Society of Agronomy, Madison, WI. 42 pp.

Undersander, D.J., W.T. Howard, and R.D. Shaver. 1993. Milk per acre spreadsheet for combining yield and quality into a single term. J. Prod. Agric. 6:231-235.

Wiersma, D.W., P.R. Carter, K.A. Albrecht, and J.G. Coors. 1993. Kernel milkline stage and corn forage yield, quality, and dry matter content. J. Prod. Agric. 6: 94-99.

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)

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).

Table 7. Corn silage yield and quality response to planting date during 1994 at Arlington, WI. Corn was harvested near R5.5 or after a killing frost. Derived from Lauer and Hudelson (1994) and Undersander et al., (1993).
Hybrid	Planting	Kernel development	Whole plant moisture	Dry matter yield	Crude protein	ADF	NDF	In vitro digest.	Milk production
	date	stage	%	T/A	%	%	%	%	lb/T	lb/A
Pioneer 3417 (108 d RM)	May 11	R5.6	62	8.2	7	24	45	79	2100	16900
	May 31	R5.5	55	7.4	7	28	52	75	1600	11700
	June 22	R5.0	71	4.8	9	32	59	73	1100	5300
	July 11	R2.0	79	2.7	10	33	62	68	900	2500
Pioneer 3751 (98 d RM)	May 11	R5.8	58	7.8	7	24	46	79	2000	15800
	May 31	R5.5	58	8.3	7	26	49	77	1800	15000
	June 22	R5.0	65	2.6	9	29	55	76	1400	3700
	July 11	R2.0	76	2.2	10	31	60	73	1100	2400
Pioneer 3921 (85 d RM)	May 11	R6.0	52	6.6	7	23	44	76	2200	14200
	May 31	R5.6	64	6.9	7	26	47	77	1900	13000
	June 22	R5.0	66	1.3	8	33	59	69	1100	1400
	July 11	R4.0	74	2.4	10	32	61	69	1000	2400
Corn development stage: R2 = Blister; R3 = Milk; R4 = Dough; R5 = Dent; R5.5 = 50% kernel milkline; R5.8 = 80% kernel milkline; R6 = Black layer (physiological maturity).

Fig. 1. The relationship between silage moisture and kernel milk (Lauer unpublished).