By: Peter Thomison OSU Extension

Last week, I received reports of several ear oddities showing up in corn fields including the following:

Shortened husk leaves with normal ears protruding beyond the husks

Most corn fields have a few ears with exposed tips. In extreme situations, a high proportion of ears outgrow husks by 1/3 to 1/2. According to Aldrich et al., 1986, Modern Corn Production, observed this where “… extreme [drought] prevailed during the time of ear set with abundant rainfall and good growing conditions thereafter.”

With corn at so many different stages of growth in Ohio this year, it is likely that some corn was subject to heat and drought earlier in the season during husk formation followed by cooler and wetter conditions. Ears protruding beyond the husks are likely to be more susceptible to bird and insect damage followed by molds that may produce mycotoxins. It is advisable to harvest earlier and dry corn down to minimize these potential problems. Avoid hybrids prone to ear tip exposure. However, ear tip exposure problems may be more the result of environmental conditions or an interaction of a particular hybrid with environment than genetics per se.

Fig. 1. Ears with exposed ear tips at maturity.

“Barbell” or “Dumbbell” Ears

Barbell ears (Fig. 2) are characterized by kernel formation at the base and tip of the ear but absent from the middle of the ear.

The barbell ears I have heard about this year were observed in sweet corn fields. Barbell ears have been associated with chilling injury during ear formation and seem to be more common in certain sweet corn genetic backgrounds. Some agronomists have suggested that low temperatures disrupt normal kernel development resulting in anomalous ear growth.

Barbell ears also appear to be a glyphosate injury symptom in non-GMO corn. It has been observed when non-GMO corn hybrids are sprayed with a low level of glyphosate in spray tank contamination.

Fig. 2.  Sweet corn ears exhibiting barbell deformity. Source: Kevin Black, 2019.

In 2016, agronomists at the University of Nebraska reported the widespread occurrence of dumbbell shaped ears and short husks in certain dent corn hybrids (https://cropwatch.unl.edu/2019/corn-ear-formation-issues). They concluded that the widespread nature of the symptoms suggests a weather-related stress event interacting with genetics and management practices. They also attributed the problem to loss of the primary ear node followed by development of a secondary malformed ear.

For more on ear development problems and others ear abnormalities, check the following: “Troubleshooting Abnormal Corn Ears” available online at http://u.osu.edu/mastercorn/

By: Kelley Tilmon, Pierce Paul, Andy Michel, OSU Extension

There have been recent reports of high corn earworm populations in certain grain corn fields. Corn earworm is a pest with many hosts including corn, tomatoes and certain legumes. In Ohio it is typically considered a pest of sweet corn rather than field corn, but this past week substantial populations have been found in certain field corn sites. Corn earworm moths are most attracted to fields in the early green silk stage as a place to lay their eggs. These eggs hatch into the caterpillars that cause ear-feeding damage, open the ear to molds, and attract birds. With a wide range of planting dates this year, different fields may be at greater risk at different times.

It is open to debate how well corn earworm can overwinter in most parts of Ohio, and the majority of our population probably immigrates each summer from more southern states. Weather fronts from the south can help carry influxes of moths our way. Compounding the problem, many of these southern moths are resistant to some of the Bt hybrids used against them in the past. Dr. Celeste Welty, OSU vegetable entomologist, maintains a trapping network for corn earworm in sweet corn which can be found here:

https://docs.google.com/spreadsheets/d/10gh3rHahdxLKkXQapGyEPxWsjHYRmgsezOoFHnwtyEo/edit#gid=0

Corn earworms are damaging as caterpillars laid by moths in the silks near the developing ear tip, and are all but impossible to find by scouting. They vary quite a bit in color – with individuals that are dark brown, brown, tan, green, or even pinkish. Typically only one caterpillar is found per ear but in heavy infestations more may be found. They enter corn ears at the tips where the majority of feeding occurs. This also opens the corn ear up to the potential development of ear rots. Unlike western bean cutworm caterpillars, corn earworm caterpillars do not spend any time out on the plant surface before migrating to the ears – they are protected in the ear structure from the beginning and so insecticide application does little good against the caterpillars. When corn earworm moths are immigrating, sweet corn growers rely on frequent sprays to kill adult moths, which is not economical in field corn.

The Bt protein Vip3A (in Viptera) is still deemed effective against corn earworm. For a current infestation in field corn, because chemical control is ineffective, the scouting emphasis should be on assessing mold and disease levels in infested corn.

Feeding sites or exit holes when the caterpillar matures and leaves the ear leave holes in the corn husk, which provide a potential entry wound for pathogens like Fusarium and Gibberella. Some of these organisms can then be a further source for mycotoxins, including Fumonisins and deoxynivalenol, also known as vomitoxin. In some cases, damaged kernels will likely be colonized by opportunistic molds, meaning that the mold-causing fungi are just there because they gain easy access to the grain. However, in other cases, damaged ears may be colonized by fungi such as Fusarium, Gibberella and Aspergillus that produce harmful mycotoxins. Some molds that are associated with mycotoxins are easy to detect based on the color of the damaged areas. For instance reddish or pinkish molds are often cause by Gibberella zeae, a fungus know to be associated with several toxins, including vomitoxin. On the other hand, greenish molds may be caused by Aspergillus, which is known to be associated with aflatoxins, but not all green molds are caused by Aspergillus. The same can be said for whitish mold growth, some, but not all are caused by mycotoxin-producing fungi.

So, since it is not always easy to tell which mold is associated with which fungus or which fungus produces mycotoxins, the safe thing to do is to avoid feeding moldy grain to livestock. Mycotoxins are harmful to animals – some animals are more sensitive to vomitoxin while others are more sensitive to Fumonisins, but it is quite possible for multiple toxins to be present in those damaged ears. If you have damaged ears and moldy grain, get it tested for mycotoxins before feeding to livestock, and if you absolutely have to use moldy grain, make sure it does not make up more than the recommended limit for the toxin detected and the animal being fed. These links provides more information on ear molds and mycotoxin contamination and identification:

https://agcrops.osu.edu/newsletter/corn-newsletter/2018-28/ear-rots-corn-telling-them-apart

https://ohioline.osu.edu/factsheet/plpath-cer-04

By: Peter Thomison OSU Extension

Lately I have received questions as to whether corn at various stages of development, especially the blister (R2) and dough stage (R3) stages, will mature before the 50% average frost date. According to the National Agricultural Statistics Service, as of August 18, 37 percent of Ohio’s corn acreage was in the dough stage (R4) compared to 70 percent for the five year average, and three percent of the corn acreage was in the dent stage (R5) compared to 21 percent for the five-year average. Many areas of the state corn are considerably behind the five-year average because of late planting. Late maturation of the corn crop had led to questions about the likelihood for frost damage and whether more fuel will be needed to dry corn.

Physiological maturity (R6), when kernels have obtained maximum dry weight and black layer has formed, typically occurs about 65 days after silking. At physiological maturity (kernel moisture approximately 30-35%), frosts have little or no effect on the yield potential of the corn crop.

Dr. Bob Nielsen has summarized research findings from Purdue University and Ohio State University that provide insight into both the calendar days and thermal time (growing degree days, GDDs)  typically required for grain at various stages of development to achieve physiological maturity (kernel black layer, R6). This research was conducted at two locations in Indiana (west central and southeast) and two locations in Ohio (northwest and southwest) with three hybrids representing 97, 105, and 111-day relative maturities planted in early May, late May, and mid-June. The calendar days and thermal time from silking to black layer for the 111-day hybrid maturity are shown in Table 1 from http://www.agry.purdue.edu/ext/corn/news/timeless/RStagePrediction.html. The calendar days and thermal time from silking to black layer for the 97-day hybrid and 105 maturity are also available from this Purdue webpage.

The study indicated that corn planted in mid-June compared to early May requires 200 to 300 fewer GDDs to achieve physiological maturity.  According to Dr. Nielsen, while slightly different responses among the four locations of the trial existed, there did not seem to be a consistent north/south relationship. Therefore, growers can use the results summarized in the following table to “guesstimate” the number of calendar days or heat units necessary for a late-planted field at a given grain fill stage to mature safely prior to that killing fall freeze.

How many GDDs can be expected from now until an average date of a killing

frost for a 111-day hybrid planted in mid-June?  To answer this question, estimate the expected GDD accumulation from Aug. 19 until the average frost date (50% probability) for different regions of the state (Table 2).  These GDD expectations are based on 30-year historical normals reported by the Ohio Agricultural Statistics Service. The GDD accumulation was calculated using the 86/50 cutoff, base 50 method.

If you want to determine the “youngest stage of corn development” that can

safely reach black layer before the average frost date at a given weather

station, use the information in Table 2 on remaining GDDs in conjunction with

Table 1 which indicates GDDs needed to reach black layer at various

stages of grain fill. Compare “GDDs remaining” for the site with the GDDs

required to achieve black layer depending on the corn’s developmental stage.

Table 2. Estimated GDDs remaining from Aug. 9 to the first fall frost for Ohio.

If your corn is in the milk stage (R3) as of Aug. 19, will it be safe from frost? Table 1 indicates that corn planted in mid – June required about 681 GDDs to reach black layer from R3 and Table 2 indicates that all regions of the state can accumulate that number of GDDs before the 50% frost date.

However, if your corn is in the blister stage (R2) as of Aug. 19, it might be a different story. The kernel development – GDD accumulation relationships in Table 1 indicate that corn planted in mid-June that is at R2 needs about 781 GDDs to reach black layer. Table 2 indicates that three regions of the state, South Central, Central, and Southwest, accumulate that number of GDDs before the 50% frost date. Several other regions, West Central, and Southeast, come close to accumulating this number whereas, the Northeast, Northwest, and North Central regions are least likely to accumulate the GDDs required to achieve physiological maturity.

The research results in Table 1 demonstrate that late-planted corn has the ability to adjust its maturity requirements, and most of this adjustment occurs during the late kernel development stages. In previous growing seasons when GDD accumulation was markedly less than normal, corn planted by mid-June has usually achieved physiological maturity before the first frost occurred.

References

Nielsen, R.L. 2011. Predicting Corn Grain Maturity Dates for Delayed Plantings

By: Bob Nielsen, Purdue University

The grain fill period begins with successful pollination and initiation of kernel development, and ends approximately 60 days later when the kernels are physiologically mature. During grain fill, the developing kernels are the primary sink for concurrent photosynthate produced by the corn plant. What this means is that the photosynthate demands of the developing kernels will take precedence over that of much of the rest of the plant. In essence, the plant will do all it can to “pump” dry matter into the kernels, sometimes at the expense of the health and maintenance of other plant parts including the roots and lower stalk.

A stress-free grain fill period can maximize the yield potential of a crop, while severe stress during grain fill can cause kernel abortion or lightweight grain and encourage the development of stalk rot. The health of the upper leaf canopy is particularly important for achieving maximum grain filling capacity. Some research indicates that the upper leaf canopy, from the ear leaf to the uppermost leaf, is responsible for no less than 60% of the photosynthate necessary for filling the grain.

Kernel development proceeds through several distinct stages that were originally described by Hanway (1971) and most recently by Abendroth et al. (2011). As with leaf staging protocols, the kernel growth stage for an entire field is defined when at least 50% of the plants in a field have reached that stage.

Silking Stage (Growth Stage R1)

Silk emergence is technically the first recognized stage of the reproductive period. Every ovule (potential kernel) on the ear develops its own silk (the functional stigma of the female flower). Silks begin to elongate soon after the V12 leaf stage (12 leaves with visible leaf collars), beginning with the ovules near the base of the cob and then sequentially up the cob, with the tip ovules silking last. Consequently, the silks from the base half of the ear are typically the first to emerge from the husk leaves. Turgor pressure “fuels” the elongation of the silks and so severe drought stress often delays silk elongation and emergence from the husk leaves. Silks elongate about 1.5 inches per day during the first few days after they emerge from the husk leaves. Silks continue to elongate until pollen grains are captured and germinate or until they simply deteriorate with age.

Silks remain receptive to pollen grain germination for up to 10 days after silk emergence (Nielsen, 2016b), but deteriorate quickly after about the first 5 days of emergence. Natural senescence of silk tissue over time results in collapsed tissue that restricts continued growth of the pollen tube. Silk emergence usually occurs in close synchrony with pollen shed (Nielsen, 2016c), so that duration of silk receptivity is normally not a concern. Failure of silks to emerge in the first place (for example, in response to silkballing or severe drought stress) does not bode well for successful pollination.

Pollen grains “captured” by silks quickly germinate and develop pollen tubes that penetrate the silk tissue and elongate to the ovule within about 24 hours. The pollen tubes contain the male gametes that eventually fertilize the ovules. Within about 24 hours or so after successfully fertilizing an ovule, the attached silk deteriorates at the base, collapses, and drops away. This fact can be used to determine fertilization success before visible kernel development occurs (Nielsen, 2016a).

Kernel Blister Stage (Growth Stage R2)

About 10 to 12 days after silking, the developing kernels are whitish “blisters” on the cob and contain abundant clear fluid. The ear silks are mostly brown and drying rapidly. Some starch is beginning to accumulate in the endosperm. The radicle root, coleoptile, and first embryonic leaf have formed in the embryo by the blister stage. Severe stress can easily abort kernels at pre-blister and blister stages. Kernel moisture content at the beginning of R2 is approximately 85 percent. For late April to early May plantings in Indiana, the thermal time from blister stage to physiological maturity is approximately 960 GDDs (Brown, 1999).

Kernel Milk Stage (R3)

About 18 to 20 days after silking, the kernels are mostly yellow and contain “milky” white fluid. The milk stage of development is the infamous “roasting ear” stage, when you will find die-hard corn aficionados standing out in their field nibbling on these delectable morsels. Starch continues to accumulate in the endosperm. Endosperm cell division is nearly complete and continued growth is mostly due to cell expansion and starch accumulation. Severe stress can still abort kernels, although not as easily as at the blister stage. Kernel moisture content at the beginning of R3 is approximately 80 percent. For late April to early May plantings in Indiana, the thermal time from milk stage to physiological maturity is approximately 880 GDDs (Brown, 1999).

Kernel Dough Stage (R4)

About 24 to 26 days after silking, the kernel’s milky inner fluid begins changing to a “doughy” consistency as starch accumulation continues in the endosperm. The shelled cob is now light red or pink. By dough stage, four embryonic leaves have formed and the kernels have reached about 50 percent of their mature dry weight. Kernel moisture content is approximately 70 percent at the beginning of R4. Near the end of R4, some kernels will typically be starting to dent. Kernel abortion is much less likely to occur once kernels have reached early dough stage, but severe stress can continue to affect eventual yield by reducing kernel weight. For late April to early May plantings in Indiana, the thermal time from dough stage to physiological maturity is approximately 670 GDDs (Brown, 1999).

Kernel Dent Stage (R5)

About 31 to 33 days after silking, all or nearly all of the kernels are denting near their crowns. The fifth (and last) embryonic leaf and lateral seminal roots form just prior to the dent stage. Kernel moisture content at the beginning of R5 is approximately 60 percent.

More importantly, kernel dry matter content at the beginning of R5 is only about 45% of the eventual final accumulation and there remains approximately more 30 days before physiological maturity occurs. This is sobering considering that farmers and agronomists alike often breathe a sigh of relief when the crop reaches R5 because of a mistaken and, frankly, emotional belief that the “crop is made” by this grain fill stage.

Description of the corn ear.

Interesting Exercise:
You can get a sense of the importance of the final 30 days of grain filling by calculating a number of “what-if” grain filling scenarios using the traditional pre-harvest yield estimation formula for corn with a range of kernel weight “fudge factors” from about 65 to 105 (representing kernel weights equivalent to 65,000 to 105,000 kernels per 56-lb bushel.)

Within about a week after the beginning of R5, a distinct horizontal line appears near the dent end of a split kernel and slowly progresses to the tip end of the kernel over the next 3 weeks or so. This line is called the “milk line” and marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernels.

For late April to early May plantings in Indiana, the thermal time from full dent (kernel milk line barely visible) to physiological maturity is approximately 350 GDDs (Brown, 1999). Thermal time from the half-milkline stage to physiological maturity for similar planting dates is approximately 280 GDDs. One of the consequences of delayed planting is that thermal time from the dent stage to physiological maturity is shortened, though this may simply reflect a premature maturation of the grain caused by the cumulative effects of shorter daylengths and cooler days in early fall or by outright death of the plants by a killing fall freeze.

Severe stress can continue to limit kernel dry weight accumulation between the dent stage and physiological maturity. Estimated yield loss due to total plant death at full dent is about 40%, while total plant death at half-milkline would decrease yield by about 12% (Carter & Hesterman, 1990).

Physiological Maturity (R6)

About 55 to 65 days after silking, kernel dry weight usually reaches its maximum and kernels are said to be physiologically mature and safe from frost. Physiological maturity occurs shortly after the kernel milk line disappears and just before the kernel black layer forms at the tip of the kernels. Severe stress after physiological maturity has little effect on grain yield, unless the integrity of the stalk or ear is compromised (e.g., damage from European corn borer or stalk rots). Kernel moisture content at physiological maturity averages 30 percent, but can vary from 25 to 40 percent grain moisture depending on hybrid and growing conditions.

Harvest Maturity

While not strictly a stage of grain development, harvest maturity is often defined as that grain moisture content where harvest can occur with minimal kernel damage and mechanical harvest loss. Harvest maturity is usually considered to be near 25 percent grain moisture.