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ISSUE 12  July 19, 2001



Planting date influences corn hybrid maturity and the thermal (or heat units needed to grow) responses of the crop during the season. In a study run in 1993 at four locations through Indiana and Ohio, researchers determined that delayed planting shortens the thermal intervals for corn throughout the season.

In other words, delayed planting decreased the time between planting and silk emergence, especially at the more northern locations. In fact, the growing degree days (GDDs) to mid-silk were decreased as much as 126 GDDs for mid-June plantings as compared to early May plantings. One way corn adapted to a late planting in order to compensate for the shorter season was to slightly decrease the final leaf number on plants in the mid-June planting for four of the five hybrids planted.

Delayed planting also decreased the thermal interval between mid-silk and kernel black layer development, especially in northern locations. And, as expected, the later maturity hybrids were more affected than earlier hybrids. On the average, the GDD change from mid-silk to black layer was decreased from 4.5 to 7.4 GDD per day of delayed planting among the hybrids used in the study. Thus, season-long effects can be seen from delayed planting of corn and can affect more northern locations substantially.

Averaged over three years of a similar study, delayed planting was shown to decrease the thermal time from planting to kernel black layer by 4 to 9 GDD per day of delayed planting for the three hybrids that were evaluated during all three years. Growth and development of corn has been known to be closely related to GDDs accumulated over calendar time. And, the "short-season" hybrids typically require fewer GDDs to reach silking and black layer than do the "fuller-season" varieties.

Many seed companies provide days to maturity ratings on the hybrids sold and often use the GDD ratings for the product as a secondary descriptor. Unfortunately, a standard GDD-based maturity rating scheme is not used in the seed industry. The differences are from different GDD calculation methods, initiating GDD accumulation at different times (date of seedling emergence rather than from the planting date), differences in defining the actual date of black layer formation or with genotype by environment interactions that vary the relation between GDD accumulation and growth stage development.

To complicate matters, GDD accumulations required for corn germination and emergence vary among locations and years, often due to the variability of soil temperatures and photoperiod. Once emerged, however, the vegetative growth rate on corn is fairly consistent with GDD accumulation as long as plant stress is not present. This parallel is not as clear between silking and black layer formation and can even be varied depending on hybrid grown. Some hybrids can modify their GDDs requirements a bit, while other hybrids are less able to compensate.

In fact, the limited compensation ability in corn may be due to day length or hybrid photoperiod sensitivity. In a 1983 study, it was determined that corn sensitivity to photoperiod begins 4 to 8 days prior to tassel initiation and continues until shortly after tassel initiation. However, later studies in 1986 modeled corn growth rate under a range of day length variations prior to tassel initiation and still found that hybrids adapted differently based on early photoperiod changes. However, to confirm both studies, the effects (timing requirement) of delayed planting on hybrids from emergence to silking were relatively similar across hybrids tested but was an even greater linear decrease in thermal interval and with greater differences among hybrids for the effects of delayed planting from silking to black layer formation. In other words, more of the corn compensation ability is seen during the silking to black layer formation development period. And, more "grain fill time" is lost with delayed or late planting than is with a more tailored and early planting date. This does seem to indicate that corn "monitors" the day length and "adapts" based on this monitoring as well as on any plant stresses that the corn plant may be experiencing.

Besides hastening development (requiring less GDDs), delayed planting primarily is seen as affecting grain fill, ultimately yield. The time line between silking and black layer formation in corn is when the hybrid is kernel filling from initiation at pollination to blister to milk to dough to dent to finishing grain fill at physiological maturity (or black layer). In the three year study, the interval from silking to milk line development was the time interval when the GDD requirement was decreased the most per day of delayed planting, emphasizing again the importance of the time period during pollination and beyond for corn yield gain as well as the need to plant in a timely manner.

Other studies have repeated the above information. And, while corn can't "compensate" quite as well as soybeans in cutting back on GDDs requirements toward physiological maturity, corn has been found to frequently adapt by lessening the time required by 3 to 4 days (rarely more than a week) during poor seasons or with delayed planting.

Now is a good time to look at your hybrid maturity rating and determine where you stand in the season against the average date of the first fall frost. Commonly the GDDs required for corn in North Dakota and the northern growing regions of Minnesota are as follows:


Relative Maturity (days)

















The average first fall frost (first 32 F date) varies through the region from September 16 to September 25, going from the south end of the Valley up to the Canada border. Use the NDAWN (North Dakota Agricultural Weather Network) on the internet at: http://www.ext.nodak.edu/weather  to access the program for GDD determination for corn to more accurately pinpoint the amount of GDDs accumulated from your location using your date of planting in order to determine the approximate number of GDDs you need for corn to reach physiological maturity. GDDs from this point to September 15 range each day from 5 to 15. Figure a minimum number and a maximum number of remaining GDDs for your area based on the range or based on the normals seen in other years in your location to determine how your corn crop is progressing for the season and if your corn is on target for good yields.



Late season problems in soybeans are usually very limited; however, there are some cases where a few diagnostics can help point you in the right direction or jog your memory for other, related ideas toward determining the causes for field conditions. While by no means complete, perhaps the following conditions and probable causes will help you investigate your soybean fields more closely from bloom to harvest.

A. Plants are dead or wilted

1. dead plants with no evidence of disease

a. plants are broken off at the ground but secondary root system was in place; however, callus tissue is seen on the plant ·alfalfa hopper ·lesser cornstalk borer ·surface applied dinitroaniline herbicide · hail damage

b. dead areas across the field show plants that are scorched looking · spider mites

c. a circular pattern of dead plants are seen in the field · lightning

2. Plants are dead but roots show disease problems

a. white fungal growth is seen with sclerotia (large, black, irregular structures inside and outside the stem)· Sclerotinia rot (white mold)

b. stem is discolored with minute, black fruiting structures · charcoal rot

c. stem is discolored with reddish fruiting structures present · black root rot

d. stem is discolored but no fruiting structures are present; however, cankers are seen near the stem nodes ·stem canker

e. roots with galls, cysts, root proliferation or stubbiness · nematode (usually not present in ND)

B. Flowers are not present due to damage or because they are eaten off

1. flowers were fed on or eaten off · corn earworm · bean leaf beetle

2. flowers fell off or were not present · drought damage · boron deficiency · zinc deficiency

C. Pods are damaged or lost

1. pods show insect damage

a. pods show that the seed cavity has been fed on · corn earworm · grasshoppers

b. pods show on superficial feeding · bean leaf beetle · a late season caterpillar (less likely with the growing degree days this year)

c. pods are completely cut off the plant and lying on the ground · a late season caterpillar (following very heavy defoliation only)

2. pods are stunted, discolored or shriveled

a. fruiting bodies are present on pods · pod and stem blight · anthracnose

b. pods have fallen off the plant but were not fed on · drought damage · boron deficiency

c. pods are shriveled with one or more aborted seeds · stink bugs · pod and stem blight · anthracnose

d. pods are hardened, knotty and show spots on the outer covering and the seed are smaller than normal (may be shriveled) ·stink bug

e. pods are abnormal, distorted and small (one or no seed per pod) · bud blight virus · phenoxy or benzoic acid or pyridine misapplication or drift

3. pods are split along the midline · pod is too small for developing beans (can occur during the season with alternating wet and dry conditions, especially on certain varieties)

D. Physical damage to the leaves or stems is seen on the soybeans

1. leaves show insect feeding

a. leaves are skeletonized or lacy looking · bean beetle · caterpillars

b. leaves show holes or leaf margins show feeding damage · beetles or caterpillars

c. leaves show a mottled, yellow appearance or may appear scorched · spider mites

d. leaf tips and upper leaf margins are yellow with possible cupping · potato leafhopper

2. leaves are hanging down, shriveled/collapsed petioles or stems

a. insect damage is suspected · alfalfa hopper

3. the stem terminal hangs from the plant

a. insect feeding injury is seen · corn earworm

4. soybean plants easily snap or lodge when touched or blown during wind or heavy rains

a. breakage along any part of the plant · alfalfa hopper (old damage) · cornstalk borer (old damage) · dinitroaniline injury (old, surface application) · paraquat injury (old post-directed injury--rarely seen/used) · phenoxy injury (old post-directed or drift injury) · old hail damage

E. discoloration or stunting of leaves or stems

1. interveinal chlorosis remains on upper leaves

a. disease present · black root rot · stem canker · brown stem rot · Phytophthora rot · Sudden death syndrome (not currently present in ND or MN)

2. near maturity, stems appear blighted with fruiting bodies present

a. soybeans show effects later in the season · pod and stem blight · anthracnose · black root rot · Sclerotinia rot

3. plants show cankers from the lower to the mid-section of the main stem

a. most cankers are near the axils of the branches · stem canker

4. leaves are discolored, spotted or blighted

a. many diverse causes may be contributing to the problem · frogeye leaf spot · bacterial blight ·aerial blight · downy mildew · brown spot · sudden death syndrome (not yet seen in ND or MN) · sunburn · purple seed stain · brown stem rot · Fusarium spp. · Air pollution

5. leaves have a white, powdery growth on the upper-side

a. disease is apparent · powdery mildew

6. areas within the field appear scorched

a. damage is not known to be due to insects ·spider mites · chlorine toxicity

7. leaf yellowing is followed by a silver-gray discoloration of the lower stem

a. disease is suspected ·charcoal rot

8. stem is tunneled making plants wilt or die

a. insect damage is noted · stem borer

9. roots are fed on or cut off

a. insect damage is suspected · white grubs · wireworms

F. the entire plant appears stunted and may exhibit abnormal growth

1. leaf crinkling, mottling or distorting is seen · viruses · phenoxy, benzoic acid or pyridine injury ·manganese toxicity

2. leaves yellow and roots have galls, cysts, proliferation or stubbiness · nematode (not currently found in ND)

3. leaves are yellow with some plants stunted

a. look for damage to roots and stems · white grubs · beetle larvae · stem borer

G. Some plants within the field remain green past the remainder of the field

1. few pods present, otherwise plant appear normal but green · stink bugs

2. some leaves are crinkled but few pods are present, of which many only have one-bean or appear in clusters · bud blight virus · phenoxy, benzoic acid or pyridine injury

3. pods mature in color but stems and petioles remain green ·green stem condition (suspect stink bugs or varieties with certain environmental stresses; some viruses and growth regulator drift damage can also cause stems to remain green)

H. Seed is discolored or damaged

1. brown or black streaks from the hilum (bleeding hilum) · viruses · genetic

2. purple color patterns on the seed · purple seed stain · stain of crushed black nightshade berry juices

3. moldy, dark or deformed seed · any one of various diseases (such as pod and stem blight, anthracnose, downy mildew)

4. seed coat is split · excessive rainfall with high humidity, often following a drought

5. small and shriveled seed · stink bugs (puncture marks may be visible) · various diseases (charcoal rot, brown stem rot) ·drought damage

6. orange to brick red or light brown seed

a. as damage increases, seeds are more shriveled · stink bugs

7. small, shriveled, green seeds · frost damage


Denise McWilliams
Extension Crop Production Specialist

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