• Many different “bugs” can be found with dead/dying seedlings.
• Most are attracted by, and feed on, decaying plant material.

Any seed in the ground has been punished with seemingly constant rains and cold soils. Corn that has emerged is yellow, sad-looking and waiting for the sunshine. There has already been much banter about replanting, or digging out the old rotary hoe from the back of the barn. Field inspections, and digging row skips, may reveal challenged seeds/seedlings. Many times, a range of soil organisms are found in association with these struggling plants and they are often implicated for poor stands. In reality, they are likely decomposers just doing their job…”clean up in aisle 3.”

Millipedes are wireworm-like arthropods (like insects, they belong to the Phylum Arthropoda-means “jointed foot”), having two pairs of legs per body segment. Centipedes, which they are often confused with, are typically predators and have only one pair of legs per body segment. Millipedes have become more prevalent as no-till production becomes more widespread. They are often found in large numbers, but are rarely a pest. This is because they typically feed as scavengers, feeding on dead or decaying materials often associated with seedling blights. Several pest managers have reported numerous millipedes in and around corn kernels/sprouts that have been in the ground for two or more weeks. These kernels were probably fell victim to pathogens of some kind (bacteria/fungi), after sitting underwater and opportunistic millipedes were merely acting as the “clean-up crew” and hollowing out kernels that were in early stages of decay.

Juvenile (“baby”) earthworms and potworms are closely-related, common animals found in soils. They are small, generally very pale in color, and often less than 1/4 inch long. These worms feed on damaged and decaying plant remains, not live tissue. Therefore they are closely associated with the decaying plant parts and surrounding soil and sometimes wrongly accused of damaging seedlings – in fact, they usually arrive after the seed is dead. Their mouthparts are incapable of causing damage to live tissue – they don’t have “teeth” and instead are specialized to suck up partially-liquefied material. The point of all this is to reiterate that pest managers should keep an open mind when diagnosing field problems. As one submitter confessed, he was so convinced that it was an insect problem and therefore looked for anything moving when he couldn’t find grubs or wireworms.

Many other critters, e.g., mites, symphylans, and springtails, have been observed on or around rotting seeds/seedlings. All are small, some fast moving, and certainly are unfamiliar to most. These animals never see the light of day and work beneath the soil. But they are not causing the poor emergence/growth, but taking advantage of weak and dying plants (including dead weeds and crop debris from previous years) in various stages of decay, as well as the hospitable environment supplied by atypically wet soils. In short, they’re “good bugs” turning decaying plant material into soil.

Happy scouting!

Wk 1 = 3/16/17 - 3/22/17; Wk 2 = 3/23/17 - 3/29/17; Wk 3 - 3/30/17 - 4/5/17; Wk 4 - 4/7/18 - 4/12/17; Wk 5 - 4/13/17 - 4/19/17; Wk 6 - 4/ 20/17 - 4/26/17; Wk 7 = 4/27/17 - 5/3/17; Wk 8 = 5/4/17 - 5/10/17

Indiana has planted 19% of the soybeans as of May 8th, which is similar to the 5-year average of 17%. The majority of these soybeans were planted the week of April 24th and have not emerged. It is not uncommon for April and early May planted soybeans to sit in cool and wet soil for two and even three weeks. Many areas of Indiana and the Midwest have received excessive rain coupled with cooler temperatures. The question on many people’s mind is how long does it take soybeans to emerge and more importantly, will they emerge in conditions like these.

Obviously, fields that are flooded and are excessively saturated with cold temperatures are the most likely to be replanted. The fields that are characterized as “cool and wet” over the past 2 to 3 weeks may still have hope. Over the past three years, we have been evaluating planting dates and planting operations for several management scenarios as well as documenting soybean phenology (development). The following information is really to help provide some guidelines to forecast soybean emergence. Heat unit accumulation is used in estimating the development of many crops (emergence to successive leaf development). However, field conditions can alter the precision/reliability of heat units needed for soybean emergence such as planting depth, residue cover (e.g., no till vs. conventional till), rainfall (and really, soil moisture), soil temperature, and soil crusting.

In our speed and seed rate trials, we were intensely monitoring emergence and seed spacing. Again, these can be used as guidelines. The wet spring of 2015 delayed the planting of that trial until May 24th near West Lafayette in no-till conditions. In 2016, we added the 12.5 mph and included 50 and 170 thousand seeds/acre to the seed rates. We were able to plant it April 19th south of Lafayette in conventionally tilled field. In Table 1, you will find the number of heat units (modified GDD formula with 50°F base) to reach 25%, 50%, 75%, and 90% emergence across the planting speeds and seed rates. For practical purposes, planting speed and seed rate had little variation in the time to emergence in these two trials. Although these were planted in 30-in rows, the seed spacing of 50 and 90 thousand seeds/ac would analogous to those of 15-in rows seeded at 100 and 180 thousand seed/ac (4.2 and 2.3 inches between seeds, respectively).

Table 1. Heat unit accumulation recorded for 25%, 50%, 75%, and 90% emergence of soybeans in 2015 and 2016. These values were averaged across seeding rates (50, 90, 130, 170 thousand seeds/acre) and planting speeds (5, 7.5, 10, 12.5 mph).

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Based on the previous study, we would anticipate soybean emergence (greater than 50% or VE) with the accumulation of 140 to 160 GDDs. In our 2017 planting date trial (Figure 1), we planted soybeans on April 25th in a conventionally tilled field. Heat unit accumulation from April 25 to May 11 is only 132 GDDs with a few hypocotyls cracking the soil line. However, soil temperatures were near 50°F for five days and the rainfall has been high over the past 2.5 weeks. Saturated conditions will limit oxygen for plant respiration (i.e., burning energy for growth), and thus, extending the time (calendar days and thermal time) for emergence. Emergence is expected over the coming days with the accumulation of another 20 to 30 GDDs.

If your planted soybean fields are in the “cool and wet” situation and not emerged after 160 GDDs, you should determine the viability/progress of the seedlings in preparation for replanting decisions.

Growth and development of corn are strongly dependent on temperature. Corn develops faster when temperatures are warmer and more slowly when temperatures are cooler. For example, a string of warmer than normal days in late spring will encourage faster leaf development than normal. Another example is that a cooler than normal grain filling period will delay the calendar date of grain maturity.

The phrases "string of warmer than normal days" and "cooler than normal grain filling period" can be converted mathematically into measures of thermal time by calculating the daily accumulations of heat using temperature data. Commonly used terms for thermal time are Growing Degree Days (GDDs), Growing Degree Units (GDUs), or heat units (HUs).

Different methods exist for calculating heat units depending on a) the crop or biological organism of interest and b) the whim or personal preference of the researcher. The calculation method most commonly used throughout the U.S. for determining heat unit accumulation relative to corn phenology was first evaluated by Gilmore & Rogers (1958) and termed "Effective Degrees". Barger (1969) later proposed that the same method, which he termed "Modified Growing Degree Days", be adopted as the standard heat unit formula by the National Oceanic and Atmospheric Administration.

This method calculates daily accumulation of GDDs as the average daily temperature (^oF) minus 50. The "modification" refers to the limits imposed on the daily maximum and minimum temperatures allowed in the calculation. Daily maximums greater than 86 ^oF are set equal to 86 in the calculation of the daily average temperature. Similarly, daily minimums less than 50 ^oF are set equal to 50 in the calculation.

Example 1:
If the daily maximum temperature was 80^oF and the minimum was 55^oF, the GDD accumulation for the day would be ((80 + 55) / 2) - 50 or 17.5 GDDs.

Example 2 (Illustrating the limit on daily maximums):
If the daily maximum temperature was 90^oF and the minimum was 72^oF, the GDD accumulation for the day would be ((86 + 72) / 2) - 50 or 29 GDDs.

Example 3 (Illustrating the limit on daily minimums):
If the daily maximum temperature was 68^oF and the minimum was 41^oF, the GDD accumulation for the day would be ((68 + 50) / 2) - 50 or 9 GDDs.

In late April to early May, normal daily GDD accumulations for central Indiana are about 10 GDDs. By late July, the normal daily accumulation rises to about 23 GDDs. For a typical corn growing season in central Indiana, say from late April to late September, the total seasonal accumulation of GDDs is about 2800 GDDs.

The USDA-funded Useful to Usable (U2U) multi-state research and Extension project developed a GDD decision support tool that is now hosted by the Midwestern Regional Climate Center at http://mrcc.isws.illinois.edu/U2U/gdd/. The GDD Tool estimates county-level GDD accumulations and corn development dates based on current and historical GDD data plus user selected start dates, relative hybrid maturity ratings, and freeze temperature threshold values. The GDD and corn development predictions are displayed graphically and in tabular form, plus the GDD accumulation estimates can be downloaded in a Comma Separated Value (.csv) format for you to work with in your own spreadsheet program. The GDD Tool is currently available for the states of North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, Illinois, Michigan, Indiana, Ohio, Kentucky, and Tennessee.

Figure 1 shows a screen capture from that calculator in which I selected "Tippecanoe Co., IN", a start date (aka planting date) of Apr 20, a relative hybrid maturity rating of 111 "days", and a freeze temperature threshold of 28^oF. The tool automatically adds estimated GDD values from planting to silking and black layer based on the "corn maturity days" you enter, but each is customizable if you know the GDD values specific to your hybrid. The tool displays estimates of actual cumulative GDD from planting to today's date, then estimates of cumulative GDD for the remainder of the season. Estimates of silking and black layer dates are displayed, as well as the early and late ranges of those estimates. When you are viewing the actual graph on the Web site, estimates of GDD accumulations at specific dates "pop up" when you hover your computer mouse over parts of the line graph.

Related References

Barger, G.L. 1969. Total Growing Degree Days. Weekly Weather & Crop Bulletin 56:18. U.S. Dept. of Commerce and USDA, Washington, D.C.

Gilmore, E.C. and J.S. Rogers. 1958. Heat units as a method of measuring maturity in corn. Agron. J. 50:611-615.

Nielsen, RL (Bob). 2014. Determining Corn Leaf Stages. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/VStageMethods.html. [URL accessed May 2017].

Nielsen, RL (Bob). 2014. Use Thermal Time to Predict Leaf Stage Development in Corn. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/VStagePrediction.html. [URL accessed May 2017].

Nielsen, RL (Bob). 2017. Hybrid Maturities for Delayed Planting. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/HybridMaturityDelayedPlant.html. [URL accessed May 2017].

U2U@MRCC. 2017. Corn GDD. Useful to Usable Project, Midwestern Regional Climate Center. http://mrcc.isws.illinois.edu/U2U/gdd/ [URL accessed May 2017].

The risk of damaging spring frost events is one of the downsides to planting corn earlier than normal, but is one growers often accept when early spring field conditions are otherwise suitable for planting. However, the threat of low temperatures in late May or early June also raises the specter of frost or low temperature damage to young corn plants, regardless of planting date. Early morning temperatures in the 30s^o(F) coupled with clear calm conditions overnight certainly are favorable for frost formation on exposed surfaces, including leaves of young corn plants. In other words, temperatures do not need to drop to 32^oF or cooler in order for frost to form.

When significant frost develops on young corn plants, it is tempting to jump to the logical conclusion that significant plant mortality will soon follow. However, frost by itself is not a guaranteed "kiss of death" for young corn plants. What is more important is whether the temperature that accompanied the frost event was lethal or not. Most agronomists agree that "lethally cold" temperatures for young corn are those that dip to 28^oF or lower for 1 to 2 hours.

The effect of frost on young corn when it is accompanied by temperatures no lower than about 30^oF is primarily damage and death of the exposed above ground leaf tissue. As long as the growing point of the young plant (aka the apical meristem) is still protected below the soil surface, the injured plant usually recovers from the effects of the superficial leaf damage.

Within 3 to 5 days of the frost event (more quickly with warm temperatures, more slowly if cool), elongation of the undamaged leaf tissue in the whorl will become evident. As long as the recovery is vigorous, subsequent stand establishment should be not be affected.

Plant appearance following damage by lethal cold temperatures (28^oF or lower for a couple hours) may initially be similar to that due to "simple" frost damage. The difference is that there will be no subsequent elongation or "recovery" of leaf tissue from the whorl like you would see in the days following "simple" frost damage to leaves. Inspection of the growing point area (by slicing down middle of stem, through the crown of the young plant) will eventually reveal discolored, soft or mushy tissue as a consequence of the lethal temperatures.

The bottom line for diagnosing the severity of frost or low temperature injury to corn is that you generally need to wait three to five days after the weather event before you can accurately assess the extent of damage or recovery. Injury to the crop can look very serious the day after the event or even two days after the event, but recovery is likely if there is no injury to the growing points of the affected plants.

 

Related References

Elmore, Roger. 2012. Imbibitional Chilling and Variable Emergence. Integrated Crop Management News, Iowa State Univ. online at http://crops.extension.iastate.edu/cropnews/2012/05/imbibitional-chilling-and-variable-emergence [URL accessed May 2017].

Nielsen, RL (Bob). 2008. Growing Points of Interest. Corny News Network, Purdue Univ. [online] http://www.kingcorn.org/news/timeless/GrowingPoints.html. [URL accessed May 2017].

Nielsen, RL (Bob). 2015. Silver Leaf Symptom in Corn. Corny News Network, Purdue Extension. http://www.kingcorn.org/news/timeless/SilverLeaf.html. [URL accessed May 2017].

Nielsen, RL (Bob). 2015. Corkscrewed Mesocotyls & Failed Corn Emergence. Corny News Network, Purdue Extension. http://www.kingcorn.org/news/timeless/CorkScrews.html. [URL accessed May 2017].

 

Delayed planting seasons create a lot of frustrations for everyone involved with planting crops. One of the agronomic questions that comes up when planting is seriously delayed is whether farmers should consider switching from their normal full-season maturity hybrids to shorter-maturity hybrids. The question is based, of course, on the perceived risk of the crop not reaching physiological maturity before a killing fall freeze and the yield losses that could result. A related, and economic, concern with delayed planting of normal full-maturity hybrids is the risk of high grain moisture contents at harvest and the resulting costs incurred by artificial drying of the grain or price discounts by buyers.

Corn development (think growth stage progress) is very dependent on temperature (warm = fast, cool = slow). The accumulation of heat on a daily basis can be quantified on the basis of calculated Growing Degree Days or GDDs. Hybrids can be characterized by how many GDDs they require from planting to physiological maturity (kernel black layer). Conceptually, therefore, one should be able to estimate the GDDs remaining from a delayed planting date to the end of the season using long-term climate data and then choose hybrids with GDD ratings that should mature no later than the date you chose to define "the end of the season".

FYI: The GDD concept and calculation are described in a related article (Nielsen, 2017a). Interpretation of corn hybrid maturity ratings is also discussed in a related article (Nielsen, 2012).

One "wrinkle" in this concept is that it appears that hybrids mature in fewer GDDs than expected when planted "late". Relative to a May 1 planting date, hybrids planted later mature approximately 6.8 fewer GDDs for every day of delay beyond May 1 (Nielsen et al., 2002). For example, a hybrid rated at 2700 GDDs from planting to physiological maturity and planted on May 31 will reach physiological maturity in less than 2500 GDDs after planting (e.g., 2700 - (30 days x 6.8)). That response of hybrid development relative to delayed planting means that normal full-maturity hybrids can be safely planted later than one would think and, consequently, means that growers can avoid switching to earlier maturity hybrids until planting dates later than one would think.

Estimating GDDs Between Two Dates at a Specific Location

The challenge in taking advantage of this relationship between hybrid GDD ratings and delayed planting lies with the estimation of available GDDs with delayed plantings for specific locations. Historical data for daily GDD accumulations exist for a limited number of weather reporting stations around the state, but accessing such data can be difficult. Currently, the Indiana State Climate Office (iClimate.org) does not offer an easy calculator for estimating the number of historical GDDs between two dates at a specific location.

The USDA-funded Useful to Usable (U2U) multi-state research and Extension project developed a GDD decision support tool that is now hosted by the Midwestern Regional Climate Center at http://mrcc.isws.illinois.edu/U2U/gdd/. The GDD Tool estimates county-level GDD accumulations and corn development dates based on current and historical GDD data plus user selected start dates, relative hybrid maturity ratings, and freeze temperature threshold values. The GDD and corn development predictions are displayed graphically and in tabular form, plus the GDD accumulation estimates can be downloaded in a Comma Separated Value (.csv) format for you to work with in your own spreadsheet program. The GDD Tool is currently available for the states of North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, Illinois, Michigan, Indiana, Ohio, Kentucky, and Tennessee.

Figure 1 shows a screen capture from that calculator in which I selected "Tippecanoe Co., IN", a start date (aka planting date) of May 31, a relative hybrid maturity rating of 112 "days", and a freeze temperature threshold of 28F. The graph illustrates estimates of silking and black layer dates for the 112-day hybrid planted on May 31, as well as the range of the estimates. When you are viewing the actual graph on the Web site, estimates of GDD accumulations at specific dates "pop up" when you hover your computer mouse over parts of the line graph.

WORD OF CAUTION: The U2U GDD Tool does not currently account for the "wrinkle" discussed earlier in this article wherein corn hybrids typically mature in fewer GDDs than expected when planted "late" (Nielsen et al., 2002). In other words, the GDD Tool assumes the same GDDs to silking and black layer for a given hybrid maturity whether planted April 20 or May 31. Consequently, you can be led astray by the Tool if you do not modify the "Black Layer GDDs" value in the Tool's input area. For example, the screen capture displayed in Fig. 1 for a 112-day hybrid with a GDD rating of 2691 planted in Tippecanoe Co. on May 31 indicates the hybrid would mature on about October 23 when the estimated GDD accumulation exceeded 2691. If, however, you manually change the expected "Black Layer GDD" value from 2691 to 2481 GDDs (30 days after May 1 x 6.8 fewer GDDs per day delay), the GDD Tool estimates the hybrid would safely mature on about September 30, well ahead of the usual fall freeze date (Fig. 2).

Choice of "End of Season" Date

The choice of a date to represent the "end of the season" (abbreviated EOS) can be straight-forward or one of those "eyes of the beholder" decisions. If the main concern is to identify a "safe" hybrid maturity that will reach physiological maturity before a typical fall freeze date, then the spatial maps illustrated in the accompanying figures can be used to choose that date. Figure 3 depicts the historical average dates of the first 32^oF temperature in the fall throughout Indiana, while Figure 4 depicts the historical average dates of the first 28^oF temperature in the fall throughout Indiana.

TIP: Temperatures of 32^oF or slightly higher typically result in leaf injury or death due to frost damage, but the corn plant technically will survive and be able to at least continue remobilizing stored carbohydrates from the stalk tissues to immature grain. A temperature of 28^oF for several hours is considered to be lethal for corn plants.

Some growers may opt to select an "end of season" date earlier than the historical first fall freeze date to ensure that physiological maturity will occur earlier during a time period that may yet be conducive for grain drydown in the field and thus minimize their expenses of drying the grain artificially.

Hybrid Maturity Ratings for Delayed Planting

With an estimate of available growing season GDDs in hand, one can then identify approximate relative hybrid maturities that would be suitable for delayed planting (Tables 1 and 2).

Table 1 can be used to identify "safe" hybrid maturities in terms of of their GDD ratings, though it is important to recognize that the hybrid GDD ratings in this table are for accumulated GDDs from planting to physiological maturity. Recognize that some seed companies assign GDD ratings beginning at emergence, not planting. If your seed company is one of these, then add 115 GDDs to the hybrid GDD ratings and you will be in the proverbial "ball park" using this table.

EXAMPLE: Using the U2U GDD Tool, you determine that for Blackford County, Indiana, approximately 2462 GDDs will accumulate between a delayed planting date of May 20 and a selected EOS of Sep 21. Using Table 1, the approximate "safe" hybrid GDD rating that most closely matches the combination of 2462 (the 2450 value in column 1) and a May 20 planting date (column 4) is 2579. What this means is that, for the planting date and EOS date you selected, you could safely plant a hybrid with a GDD rating of 2579 from planting to physiological maturity.

Some folks are more comfortable with the relative "days to maturity" ratings for corn hybrids (Nielsen, 2012). Table 2 expresses the GDD values of Table 1 in terms of CRM ratings as defined by DuPont Pioneer. Recognize that I am not by any stretch of your imagination promoting Pioneer hybrids. I simply know that Pioneer assigns GDD ratings to their hybrids based on GDD accumulations between planting and physiological maturity. The mathematical relationship between their GDD ratings and their CRM ratings is pretty good and, thus, can be used to calculate approximate CRM ratings from known GDD ratings. If you are not comfortable using Pioneer's CRM ratings, then use the GDD ratings in Table 1.

EXAMPLE: Using the U2U GDD Tool, you determine that for Blackford County, Indiana, approximately 2462 GDDs will accumulate between a delayed planting date of May 20 and a selected EOS of Sep 21. Using Table 2 instead, the approximate "safe" hybrid Pioneer CRM rating that most closely matches the combination of 2462 (the 2450 value in column 1) and a May 20 planting date (column 4) is 107. What this means is that, for the planting date and EOS date you selected, you could safely plant a hybrid with a Pioneer CRM rating of about 107.

PLEASE NOTE: Please understand that ratings for relative hybrid maturity (i.e., CRM, RM, "days to maturity", etc.) are notoriously inconsistent one seed company to another. Consequently, relationships between hybrid GDD ratings and their relative maturity ratings will vary one seed company to another. I believe the relationships listed in Table 2 are valid for Pioneer's lineup of hybrids, but cannot make the same claim for any other seed company's lineup of hybrids. Consult your seed dealer!

Caveats (e.g., disclaimers)

Recognize that actual GDDs deviate year to year from the historical "norm" because of natural variability in growing season temperatures. Also recognize that hybrids undoubtedly vary in their GDD response to delayed planting. Also recognize that hybrid GDD response to delayed planting in other parts of the country may differ from what we have documented in the eastern Corn Belt.

Table 1. Approximate equivalent hybrid GDD requirements for delayed planting relative to estimated actual GDDs available from delayed planting date to the end of the season (EOS).

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Table 2. Approximate maximum "safe" hybrid CRM for delayed planting relative to estimated actual GDDs available from delayed planting date to the end of the season (EOS).

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Related reading

National Climatic Data Center. 2015. 1981-2010 US Climate Normals. https://www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets/climate-normals/1981-2010-normals-data [URL accessed May 2017]

Nielsen, R.L. (Bob). 2012. Interpreting Corn Hybrid Maturity Ratings. Corny News Network, Purdue Univ. [online] http://www.kingcorn.org/news/timeless/HybridMaturity.html [URL accessed May 2017].

Nielsen, R.L. (Bob). 2017a. Heat Unit Concepts Related to Corn Development. Corny News Network, Purdue Univ. [online] http://www.kingcorn.org/news/timeless/HeatUnits.html [URL accessed May 2017].

Nielsen, R.L. (Bob). 2017b. The Planting Date Conundrum for Corn. Corny News Network, Purdue Univ. [online] http://www.kingcorn.org/news/timeless/PltDateCornYld.html [URL accessed May 2017].

Nielsen, Robert L., Peter R. Thomison, Gregory A. Brown, Anthony L. Halter, Jason Wells, and Kirby L. Wuethrich. 2002. Delayed Planting Effects on Flowering and Grain Maturation of Dent Corn. Agron. J. 94:549-558.

U2U@MRCC. 2017. Corn GDD. Useful to Usable Project, Midwestern Regional Climate Center. http://mrcc.isws.illinois.edu/U2U/gdd/ [URL accessed May 2017].