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The corn crop - frost and maturity

D. R. Hicks, Dept. of Agronomy and Plant Genetics, University of Minnesota

Many corn fields over Minnesota lost some leaves to the frost of Saturday morning August 21. But, except for low areas in some fields, there remains green leaves that can continue to add grain yield. This note gives an update of corn development as the crop moves, albeit slowly, toward maturity. And since some of the crop is not likely to reach normal maturity, this newsletter also gives information regarding the effect of frost before maturity on corn grain yield, dry down, and grain quality.

Development progress

As the season progresses, the corn crop slips further behind. Mid tasseling (half of MN corn acres tasseling) occurred July 25 compared with the past 5-year average of July 20, or 5 days behind. By mid milk, the crop was 9 days behind (August 14 this year compared with the past 5 year average of August 5). The crop hasn't reached mid dough (the 5 year average date is August 15), but I've projected mid dough to occur about September 1, or 17 days later than average. The continued cool weather is stretching out the grain filling period and if this continues mid denting would occur about September 21 (24 days later than average) and mid maturity about October 10. Since it's not likely that the next killing frost will wait that long, some of the crop will not reach normal maturity and the growing season will end with the next frost.

The shorter days give us less solar radiation per day and the night low temperatures will continue to get lower in September. The end result is the plant has less photosynthetic activity (less dry matter is accumulating). The lowered photosynthetic activity and the low night temperatures are causing the plant to shut down and the dry matter growth curve is beginning to flatten. This basically is "prematurely" killing the plant. The black layer in the tip of the kernel will form which will end any further increase in grain yield. This probably will happen near the end of September. For corn that has now reached the dough stage, the test weights should be above 50 if corn continues to grow until late September.

Grain yield

Grain yield is reduced when low air temperature (frost) occurs killing leaves (some or all) before grain has reached maturity. Since the first frost has occurred, the next one will probably end the growing season, that is kill all leaves and the effect on yield and quality will depend upon the stage of corn development when the frost occurs. The yield effect of killing all leaves is given in Table 1 for corn at various growth stages. For example, if all leaves are killed at the late dent stage when 85 percent of the yield has been produced, the yield reduction due to the complete killing frost is 15 percent. Grain dry weight increases as corn continues to mature normally. Therefore, a frost before maturity has less effect on yield when it occurs at later kernel growth stages.

Table 1. Percent of total yield produced by each kernel growth stage and yield reduction resulting from a complete killing frost at each kernel stage.
Growth stage Total yield Grain yield loss Test weight
percent lb/bu
Early dent 68 32 47
Dent 77 27 50
Late dent 85 15 53
Half milk line 92 8 55
Mature 100 0 58

Drying rate

Grain on immature corn killed by frost will dry in the field if drying conditions exist. The term "soft corn" has been used to describe grain of frosted corn plants and implies that very little drying occurs after the killing frost, but this is not the case if drying conditions exist (warm temperatures, good wind movement, and sunshine).

We conducted an experiment to determine field-drying rates of corn grain that was frozen. Ears were removed, frozen, and hung on the plant in the field, another group of ears were cut and hung on the plant and the third group of ears was the normal condition (shanks not cut). Ears were removed from the field weekly to determine grain moisture. Ears with shanks cut (both frozen and non-frozen) had slightly higher moisture levels, but the drying rate (moisture loss over time) for frozen kernels was similar to that of unfrozen kernels.

Normal field drying rates

When corn has reached maturity in past years (about September 14), kernel moisture loss is ¾ to 1% per day between September 15 and 25, is ½ to ¾% per day during September 26 to October 5, is ¼ to ½% per day between October 6 and October 15, and is 0 to 1/3 % per day for the last half of October. Normally, very little field drying occurs after November 1 in Minnesota.


Animal Scientists at the U of M determined nutrient composition of immature corn and reported that crude protein, fiber, ash, and cell wall constituents were higher in concentration in the immature grain compared with mature grain. They reported that the concentration of fat and starch were lower in low-test weight corn and that gross energy did not change with test weight changes.

They used animal digestion trials with corn of varying test weights to establish the relationship of test weight and relative total digestible nutrients (TDN) as given in Table 2. Since the relative TDN is lower for low-test weights, more corn is necessary to produce the same animal performance. The amount of corn required to give equal animal performance is given in Table 3. For example, corn with a 51 lb/bu test weight has a 99% relative TDN and 56.56 pounds are necessary to equal the value of 1 bushel of normal corn. Or, the value of 1 bushel of 51-test weight corn should be 99% of current corn price. Low-test weight corn is good livestock feed and the value of low-test weight corn can be close to that of corn with normal test weight.

Table 2. Relative total digestible nutrients (TDN) and amount of corn required to equal the TDN of low-test weight corn.
Test weight Relative TDN Corn required
lb/bu percent pounds
54 100.0 56.00
53 99.7 56.17
52 99.4 56.34
51 99.0 56.56
50 98.7 56.74
49 98.4 56.91
48 98.1 57.08
47 97.8 57.26
46 97.4 57.50

Test weight outlook

For the 35% of the MN corn crop that has now reached the dough stage, there is ample time to expect that this portion of the crop will get to the late dent or beyond growth stages and the dry test weights should be above 54. If there is growing weather for the next two weeks, most of the other 65% of the crop should progress to the point that dry test weights will be 50 or higher, especially after careful drying.
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