Harvest time is not the time to determine if a disease affected your crop

It happens every year.

A field is about to be harvested and something is awry.  Perhaps the plants are lodged, ears are poorly filled, or pods shrunken.  What happened to my crop?

From a plant disease perspective, it is nearly impossible to provide any useful information to the producer.  Many pathogens that can cause crop diseases are also excellent saprophytes.  That means they utilize dead or dying plant tissues for nourishment.  Consequently, when plants prematurely senesce, these organisms see these plants the same way as I viewed Golden Corral in college- it’s chow time!  As a result, it is common to observe senesced plants in fields with multiple “pathogenic” organisms present in or on tissues.  For example, I recently read a report form the diagnostic clinic where four pathogenic organisms were detected in soybean stems.  Was it organism 1,2,3, or 4 that caused the disease?  All of them?  Was it something else related to the environment that killed the plant and all 4 moved in afterwards?  Therein lies the problem- when plants are dead there is no way to know what killed it.    The most important thing needed to properly manage a disease is a confidant identification of the pathogen and knowledge that it was the cause of reduced plant performance.

This is why it is so important to be checking fields throughout the season.  Assessing the health of the crop while most plants are still green allows you to understand if the issue is related to environment, disease, insects, or some other factor.  During the season, make a point to assess your fields at least 4-5 times throughout the season, from planting through maturity.  Look for plants that exhibit abnormal growth or symptoms.  Send samples to your state diagnostic clinic for assessment.  By doing this, you arm yourself with the information you will need to defend yourself from potential yield limiting diseases in subsequent years.

Don’t be that guy.  Don’t wait until it is dry.


Tips for small grain planting in an odd 2019 field season.

Soon many producers will be starting to plant small grains.  Below is my “Top six” list of important things to consider when planting wheat and other small grains in the coming weeks.

1) Ensure that you remove green bridges at least 10 days prior to planting.  This season, prevent plant acres may have favored the development of grassy weeds and other potential hosts of aphids that transmit barley yellow dwarf (BYDV) virus in small grains.  Aphids acquire BYDV upon feeding, and then can transmit it to susceptible hosts.  This group of viruses has a wide host range, and many grassy weeds, in addition to volunteer small grains, serve as a means for this virus to persist during the summer.  When aphids such as the Bird Cherry Oat aphid (and some others that feed on wheat)feed on these grassy hosts and a fresh field of wheat or barley is planted nearby, they can move into these fields and begin to transmit the virus early in the growing season.  Fall infections of BYDV in wheat and barley result in the greatest  potential yield loss in the following season.  To minimize BYDV, ensure that any weeds are killed 10 days prior to planting.  In addition, other obligate pathogens that require a living host to survive the winter, such as stripe rust, can be maintained on volunteer wheat and native grasses during the summer.  This is another reason to ensure to ensure weeds are burned down prior to planting.

2) Plant after the Hession fly free date.  Some think this insect is a myth, but it can appear on occasion.  However, planting after this date is one way to minimize the window of opportunity for diseases such as the aphid transmitted BYDV, stripe rust, and powdery mildew to impact diseases early in fall growth and establishment.  The earlier infections occur, the more likely you will see yield losses the following Spring.

3) Plant certified seed.  Several diseases can be transmitted on seed.  Minimize potential issues by purchasing high quality, disease free seed.

4) Ensure good soil to seed contact.  The faster your plants emerge from the ground, the less chance you will observe potential issues with seedling blights.

5) Plant after soybeans.  This season the potential opportunity to do this will be limited.  Fusarium graminearum, which causes Fusarium head blight (FHB), grows much less efficiently on soybean residue than corn residue.  Thus, planting behind soybeans can reduce the amount of local FHB inoculum in a field the following season.

6) Select varieties with the highest yield potential and best disease resistance packages for common yield limiting diseases.  Growers should focus on FHB resistance (click here for university ratings) then check leaf blotch ratings.  Other issues that are problematic for you on a local scale should also be considered.

 


Diagrams to help you rate foliar disease on corn

Many in the agricultural community, as well as researchers annually rate corn for disease as a means to assess hybrid response, hybrid effectiveness, or potential disease level on field productivity.  It can be difficult to rain the eye to accurately measure disease on foliage, and differences in the type and size of the structure or lesion associated with the pathogen varies significantly.  The four links below will direct you to disease area diagrams we developed in order to help you obtain accurate disease severity estimates in your fields. The method you use to assess disease severity may differ depending on the overall objective. The diagrams below are cor grey leaf spot, northern corn leaf blight, common rust, and southern rust.  These can be printed, laminated, and taken to the field with you to assist your ratings.

SAD_CommonRust SAD_GreayLeafSpot SAD_NorthernLeafBlight SAD_SouthernRust


New maps for tracking Southern rust in corn (repaired link)

Southern rust is caused by a fungal pathogen that does not overwinter in Illinois.  Instead, it blows in from warmer regions during the growing season.  When conditions favor spread and development of this disease significant damage can occur, especially if it arrives before tasseling (VT).  For more information on Southern rust, check out the Crop Protection Network publication by clicking here.  Yesterday our colleagues detected this disease in Southeast Missouri.  With the hurricane/tropical storm remnant pushing northward, and warm weather forecast, there is a good chance we could see some movement into the Southern / Western  part of the state within the next 7-10 days, and there may be some in Southwest Illinois already.  Scouts should be keeping their eyes out for this disease, as things can escalate quickly.  Suspect samples should be sent to the University of Illinois Plant Diagnostic Clinic for confirmation.  Images can also be sent to me via twitter @ILplantdoc, or email at  nathank@illinois.edu.

We have a new tracking system for rusts in corn that we are using this year.  To access the maps click here.  These maps are very useful for tracking a disease that does not overwinter in the area as it needs to spread, establish, and produce more spores to move, unlike diseases that overwinter on residue, alternate hosts, etc.  That means you can see the disease spread and know if you are at risk for it moving into your area.  If your fields are at a critical point in growth, and the disease is detected nearby, in season management can be considered, and unneeded treatments avoided.  We also have a tar spot map.  This disease overwinters in the region, and little is known about disease movement and spread within a season.  Data indicate it can move at least from field to field; however, after last season there is reason to believe it may move longer distances, but we simply are speculating at this point.  This map therefore can tell you when the disease is starting to be detected in certain regions, and also if the disease has been detected in new counties.  For more on tar spot click here.

Locations where tar spot was confirmed in 2018. This map shows incidence not severity at county level. Severity in general was highest in Northern Illinois and extremely low in central Illinois

 

 


Avoid the cosmic freakout

Today colleagues in Indiana reported tar spot presence in some of their research plots located in North West IN.  They found an extremely low number of stroma (less than 10) when assessing approximately 500 feet of plots.  When you see or read about the report keep a few things to keep in mind:

1) The amount detected was exceptionally low, and not close to the widespread severity we saw early last year.  For example, on  July 5th, 2018, we detected tar spot in DeKalb at 100% incidence (every plant had some) with an average of 6% severity at the ear leaf at VT.  Those were wet fields, closed canopies, and a history of moderate tar spot.

2) It will be hot and dry for the foreseeable future.  Tar spot likes moderate temperatures and persistent humid conditions.  In corn that is still in the early vegetative stages, the persistent levels of humidity the pathogen likely needs to sporulate, then transfer those spores to plants, germinate, and infect, might not be met.  Last year at this time our fields in DeKalb and Monmouth were at or approaching VT around this time.  This season we are at V6 and V7, respectfully.  There is not much canopy to retain moisture, especially when conditions 3 weeks ago were favorable for disease onset.

3) Continue to scout, but be aware that the majority of the chatter out there about tar spot being detected in the Midwest is based on misdiagnoses of insect frass.  Spraying poop with fungicide is not going to benefit your crop.  Click here for more information on that particular issue.  If you have any suspect samples, send them to the UI plant diagnostic clinic, send me images, and let us know the approximate location of the putative detection.  We are collecting samples as we did last season.

4) We have observed tar spot in Illinois every year since it was first detected.  This disease overwinters in the region, just like grey leaf spot, white mold in soybeans, and Fusarium head blight in small grains.  Last year was the first time that the disease was severe enough to cause yield loss.  Detecting it is not uncommon.  When it arrives and the amount of symptoms expressed during critical periods of grain fill is what is most important.  Last year was the perfect storm of susceptible crop, environment conducive to disease for a prolonged period of time, and infection during a period critical for yield.  We will observe it this season, the question is when, and how severe and widespread it will be.

5) Fields at highest risk for tar spot will be no till, corn after corn fields experiencing moderate temperatures and persistent humid conditions, and had tar spot last season.  Our collaborative research team has preliminary data indicating that any infested residue on the surface of fields can produce viable spores.  Tillage may potentially reduce the overall number of spores available for local infection of a particular field by reducing the amount of surface residue on the field, but there is no reason to expect the act of tillage alone to impact survival and viability of spores produced on the residue remaining on the field surface.  Planting into fields that were soybean last year may reduce initial disease onset.  This disease isn’t a rust.  Keep in mind, until we have hard data these are simply assumptions based on experience and similar pathosystems.

6) It is evident that there is a lot that is not understood about this pathosystem and in particular, pathogen biology and ecology.  Our tar spot coalition, which consists of a group of pathologists and breeders from the Midwest and Florida, is working on coordinated trials and collaborative projects to learn as much as possible about this disease in an effective, efficient manner.  We are working hard to help our producers minimize potential losses due to this disease.

In sum, keep scouting, don’t freak out, and stay hydrated- it’s going to get hot out there!

On a side note, I’d be more concerned about the recent report or Southern rust from Southeast Missouri, especially for our #corn growers in the southern portion of the state.  That disease blows around, and with hot temperatures and a predicted hurricane remnant moving in, it could move a bit, especially in some of these late plated corn fields.


RSVP for the Champaign Pest and Pathogen Field Day!

Come to Champaign, Illinois on July 22nd for the first annual field crop Pest and Pathogen Field Day from 9am-noon.  Registration, doughnuts, and coffee will start at 8:30 am. Parking for the event will be available at the Agricultural and Biological Engineering farm on the UIUC South Farm Facility, located at 3603 South Race Street, Urbana, IL, 61802.  Click HERE to register.

Join us to walk research plots and learn about insect and disease identification in field crops, current research on field crop entomology, nematode, and plant disease research, and discuss local and regional production issues with entomology and plant pathology experts from the University of Illinois Department of Crop Science.

Examples of some of topics that will be discussed:

Seed treatments for suppressing soil borne diseases of soybean and corn

Lesion nematodes in corn and soybean

Understanding HG types and resistance to soybean cyst nematode

Current research projects of tar spot on corn

Bacterial leaf streak of corn

Red crown rot in soybeans

Fungicides in crop production

Mycorrhizae in crop production

Corn root worm research

Defoliators in field crops

Thrips and Soybean Vein Necrosis Virus

Understanding residual control of insect pests

Cover crops and insects

and much more!

RSVP today- this is a free field day, bring sunscreen, a hat, and plenty of questions!


Fusarium head blight-assessing FHB in your field and late season considerations

A few weeks ago we wrote an article on how to assess severity of Fusarium head blight (FHB) in small grains as well as some practices to consider that can help improve potential profitability in cases where outbreaks are severe.  Now that symptoms of FHB are starting to develop in the earliest flowering wheat, it is a great time to assess your fields and determine if any considerations for harvest need to be made.  To access the article click here.


Another Look at Soybean Planting Date

As we wait for things to dry out so planting can begin in Illinois, I’ll provide an update on soybean planting date, including addition of some recent data and more detail on what planting date studies are telling us.

Between 2010 and 2018, we ran a total of 30 soybean planting date trials at four sites—Urbana and Perry in central Illinois and DeKalb and Monmouth in northern Illinois. We also ran trials at two southern Illinois sites in some of those years, but wet springs in many cases limited the data amount or quality—poor stands from heavy rainfall makes a mess of planting date response—and we generally found there that planting date responses are similar to those in central Illinois.

Most trials included four planting dates, with target date ranges of April 15-20, May 5-10, May 20-25, and June 5-10. When planting couldn’t be done within the target range, later plantings were adjusted; the last planting was later than June 10 in about one-third of the trials. Each trial had four replications, with plots consisted of four 30-inch rows (or seven 15-inch rows) by 25 to 50 ft. long, depending on the location. Two 30” or four 15” rows were harvested with a small plot combine.

We dropped the data from the trials at Perry (Orr Center) in both 2011 and 2012, leaving 28 trials with usable data. In 2011 at this site, yields were low and the last date was very late (June 22), while in the drought of 2012, yield increased with later planting, from only 15 bushels when planted on April 15, to 31 bushels when planted on May 25. That happened because rain that fell beginning in late July was too late for early-planted soybeans, but was of some help to later-planted ones. While we can’t rule out the possibility that this could happen again, including “outlying” data from those two trials had a very large effect on response over all trials, and that lowers the ability of the data to predict planting date response.

We converted the yield data into percent of maximum yield in each trial, then did a regression of yield against planting date and planting date squared, producing a curve that shows acceleration of yield loss as planting date gets later (Figure 1.) Yield loss from planting delays is a little less than I reported previously: according to the curve, planting on May 1, May 15, May 30, and June 10 produced yield losses of 2, 7, 12, and 18%, respectively. At the maximum yield average of 71 bushels per acre, planting on these dates meant yield losses of 2, 4, 9, and 13 bushels per acre, respectively. That’s still a loss, at least at dates later than May 1, but under normal conditions, we can generally plant into mid-May without losing a lot of yield.

Figure 1. Soybean yield response to planting date over 28 Illinois trials. Data are from four sites, 2010 to 2018.

Although fitting a line to the data like we see in Figure 1 shows a small loss in yield for planting on May 1, this line is forced down by the effect of later planting; only once in 25 plantings made before May 1 was yield (at 96%) less than 98% of maximum. This shows that there is little danger of yield loss in soybeans from planting too early, although we didn’t plant in February or March like some have in order to see how yields respond to such “punishment.” This also shows that there’s no real yield penalty from planting in late April or the first few days of May, compared to planting earlier in April.

Planting date studies are not easy to do, and the data from them are somewhat tricky to summarize. Although in this case the data line up relatively well across planting dates, Figure 1 shows that there’s a considerable amount of variability in yield response to planting date, especially at the later dates. As an example, yields from planting between June 1 and June 10 averaged about 85% of maximum, but ranged from 74 to 100% of maximum.

Another way of looking at data like this is to cluster it into groups based on planting date ranges, then to see if the variability is large enough to prevent us from having confidence that planting date within that range had any effect on yield at all. To do this, I broke the data into 10-day planting windows, and ran “descriptive statistics” (using the Excel® spreadsheet) on each group of data to see if variability meant loss of certainty (Figure 2.)

Figure 2. Planting date responses clustered into seven, 10-day planting periods corresponding to the segments separated by vertical lines in Figure. The I-shaped bars atop the data bars show the 95% confidence interval for that cluster of data.

The small I-shapes atop the data bars show the “95% confidence interval”, which is a statistical calculation based on variability among the numbers. Here, if this interval includes 100% (which it does for the first two clusters, April 12-20 and April 21-30), then we can’t say with confidence that the average yield from planting dates in that range is less than 100%. As planting gets later, there’s more variability and so larger confidence intervals, but none of them overlap the 100% line. This means that once we pass May 1, the loss in yield is real, and almost certainly not due to random chance because of high variability.

What’s the point of such an exercise in statistics? In this case, we can see that the line in Figure 1 fits the data points fairly well, but there were still some relatively high yields even with early June plantings. This analysis shows that enough of the data points were low enough that we can’t reasonably expect planting on June 1 to yield as much as planting on May 1, even though that can happen once in a while.

This also illustrates the need for more than one or two studies to give us reasonable confidence that our data predict future responses, which is the whole point of doing such work. Like other factors whose effects depend on weather, planting date studies are notorious for producing widely varying results. So if we had data from only three or four trials, the confidence intervals, which increase in size as the number of points decreases, would likely be so wide that we’d have little idea what to expect when planting late. Or data from only some trials might be selected to make responses look uniform. That creates confidence—“this is what we always see”—but selecting data we “like” is just making up the story. That story might be a nice one, but it’s not one that will help us to know what to expect the next time we head to the field.


Remember to check your hybrids for tar spot ratings, scout your fields

It is that time of year again.  Soon corn will be in the ground, and the 2019 field season will be taking off.  It is no surprise that I spent the majority of my time on the speaker circuit discussing tar spot in corn.  We have learned a fair amount since then, but there are many more things that need to be researched and learned before we have excellent tar spot IPM management programs.  However, there are a few points you should keep in mind this season that can help you determine your risk for tar spot and management practices that can help your bottom line.

The incidence of tar spot was fairly widespread last year.  Incidence is simply asking the question, “Do I have any tar spot in my field?”  Incidence does not incorporate the severity of infection.  One could have a field with a high incidence of tar spot, yet the severity (number of lesions on leaves of plants) could be low.  This link shows the tar spot incidence in 2018 :Tar Spot established in the United States-2018

If we were to estimate where the greatest severity of disease was last year, it likely tracked with the late season storms that pushed through the region in August and September.  In Illinois, severity was greatest in the region North of I-90, and most severe in the north central part of the state.  Increased severity likely means increased local inoculum for this season.  If you are planting corn in a region that was hit hard by tar spot last season, your risk for disease is elevated compared to areas where disease was sparse or absent.

The fungus that causes tar spot overwinters in residue, and spores are released from the stromata (raised black spots on foliage, stalks, husks) at night during periods of moderate, humid weather.  These spores spread locally and also can move at a minimum to nearby fields on rain and wind.  If you are planting into a field of corn residue from plants that were severely affected by tar spot, you may be at increased risk for disease compared to if you are following a field that was in soybean last year or is tilled.  That does not mean tar spot will not occur, as it can spread from nearby fields; however, planting after soybeans or tilled fields may reduce local inoculum levels, reducing disease onset and potentially severity.  The later the disease starts, the less impact it is likely to have on your crop.

All commercially available hybrids are susceptible to tar spot, but some hybrids are more tolerant than others.  No particular brand is better than another.  Ask your seed dealer or check out Dr. Smith’s website    for information pertaining to specific hybrids and tar spot response.

Tar spot severity by brand. Numbers indicate individual hybrids within a brand. Data from DeKalb OVT, 2018. Rated at R5/6

 

Scouting is critical for this disease.  CCA’s and producers should ensure that fields are being scouted frequently and often, especially in the days/weeks approaching tasseling.  If you notice tar spot showing up prior to VT, a fungicide may help.  There are several products with a label or 2ee for tar spot suppression.  Like rusts, this is an obligate fungus, and you want to ensure that the ear leaf and leaves above are protected during the critical periods of grain fill.  You do not want to chase this disease-revenge sprays will not work.

Lastly, although tar spot is the hot topic, our most severe and widespread disease last year was, without a doubt, grey leaf spot.  Do not lose sight of this disease and other diseases that are observed and encountered more frequently and consistently in Illinois.  Tar spot is likely to be episodic, much like Fusarium head blight in wheat and white mold in soybeans.  It may be a while before we see significant disease as we did in 2018 (I hope this is the case).


Early-Season Soybean Management for 2019

Average Illinois soybean yield first exceeded 50 bushels per acre in 2004, when it was 50.5 bushels. It was 51.5 bushels in 2010, and 50 bushels in 2013. Over the five years beginning in 2014, it was 56, 56, 59, 58, and, in 2018, an astonishing 65 bushels per acre. Yield in each of the past five years was above trendline, which is a first—the longest stretch of above-trendline yields in the previous 30 years was for three years. In each of the three years 2004, 2014, and 2018, soybean yield exceeded the previous record by about 10 percent. Illinois corn yields were record-high in each of these three years as well, showing that both crops tend to respond similarly to unusually good growing conditions.

Variety selection and maturity

Most people have already selected varieties for 2019. There are a lot of good varieties and a lot of information available on their performance. The University of Illinois soybean variety trials are only a small part of this information, but there are comparisons there that might not be available from other sources. Still, seed companies remain the primary source for information about the varieties they sell, and finding topnotch genetics shouldn’t be too difficult.

One issue that continues to attract interest is how varietal maturity affects yield. In the UI variety trials, entries at each location are separated in roughly equal numbers into two sets: one with longer- and one with shorter-maturity varieties. These sets are planted in separate trials, mostly so the early-maturing ones can be harvested first if a long delay will compromise their performance. Averaged across 5 regions and 13 sites in 2018, the “early” varieties averaged 74.8 bushels per acre, and the “late” ones averaged 73.5 bushels per acre. So across a range of maturities within and among regions running from north (average MG 2.7) to south (average MG 4.2) in Illinois, maturity was not consistently related to yield. We might want to choose a mixture of earlier and later varieties to spread harvest some, but should concentrate more on yield potential than on maturity.

Planting date

We continue to hear a great deal of talk about the need to plant soybeans early in order to get high yields. This is hardly a new discovery: ever since we saw major improvements in seed quality and seed treatments several decades ago, we have known that early-planted soybeans were capable of emerging without the need to wait until soils had warmed up to 55 or 60 degrees before planting.

In recent years some have taken “early” planting to an extreme, however, with claims that soybean planting should some before corn planting, as early in March if possible. Figure 1 summarizes the results of 26 planting-date trials conducted in central and northern Illinois between 2010 and 2016. Our target planting dates were in mid-April and then about every two weeks to early June. Planting dates were converted to days after April 1, and yields within each trial to percent of maximum yield for that trial.

Figure 1. Combined results over 26 soybean planting date trials in central and northern Illinois from 2010 through 2016.

We saw little yield decrease when planting was done by May 1 (day 31), about 7% lower yield if planting was on May 15 (day 46), and 14% lower yield if planted on June 1 (day 62). While we did not plant before April 10 in any of these trials, the fact that yields were no higher from planting on April 15 than on April 30 shows that the “early planting” advantage is generally maximized if planting can be done by the end of April.

It’s not clear what advantage there might be in planting soybeans in March, or even, as some did in 2017, in February. Emerged soybean plants are can tolerate low temperatures, with the exception of the few days when the “hypocotyl hook” appears above-ground but before it straightens (in response to light hitting its upper surface) to pull the cotyledons above the soil surface. If frost hits at this point, the exposed hook (stem tissue) can be killed, which kills the seedling. Seedlings are usually in slightly different stages down the row, so frost at this stage will seldom kill all of the seedlings, but it can certainly thin them out.

Soybeans planted in March 2018 encountered cold, wet conditions, including several snow events, during the month after planting. While any emergence under such conditions testifies to the toughness of soybeans, it’s likely that many of these were replanted. Besides stand loss, soybean plants exposed to low temperatures early in the season typically stay short, and often do not yield as much as later-planted soybeans. This shortening might have been partially reversed by increased internode elongation during very warm May weather in 2018. Still, soybeans planted in late April in 2018 also made rapid growth in May and had better stands, so probably yielded more than those that survived March planting. The goal of planting early is not to have the crop survive, but to have it yield more. Low stands and short plants aren’t generally conducive to highest yields, and issues with crop insurance coverage may be another disincentive. There certainly seems to be little reward for taking the risk of planting very early.

Should later-maturing varieties be planted first in order to take maximum advantage of the longer time in the field? There’s no problem with doing that, although early planting moves up harvest date some, so works counter to the goal of spreading harvest time by using different maturities. In 2018 we ran a trial at Urbana, supported by a seed company, to see how varietal maturity affected response to planting date. The first planting date was April 26, the last was June 6, and varieties ranged in maturity from MG 2.3 (very early for this location) to MG 3.6, which is a little later than average for this location.

Figure 2. Response to planting date by five soybean varieties differing in maturity. Data are from a trial conducted at Urbana, Illinois in 2018.

For all but the earliest-maturing variety in this trial, the planting date response was almost perfectly linear, with the loss of nearly 7/10ths of a bushel per day of planting delay—a total of more than 27 bushels—over the 41 days from the first to the last date (Figure 2). This loss rate accelerated a little for the latest-maturing variety between May 24 and June 6. The earliest-maturing variety lost only 17 bushels from first to last planting, but only because its yield at the earliest date was so much lower than yields of the later-maturing varieties.

The month of May 2018 was much warmer than normal, and this got the soybean plants off to a very fast start. Warm nights are conducive to early flowering, and this was especially notable in 2018. In the early-planted crop, first flowers appeared in early June, well before the longest day of the year, and unlike the interruption of flowering that often takes place under normal night temperatures for about a week before and after the longest day, flowering was early and continuous in 2018. As a result, nearly half of the Illinois soybean crop was flowering by July 1. The warm May probably affected the yield response to planting date as well; with warm temperatures, early-planted soybeans as fast as late-planted ones, and this widened the developmental gap between the different plantings.

Planted on April 26, the earliest variety reached first flower on June 9 and matured on August 28, compared to June 15 and September 17 for the latest-maturing variety. When planted on May 24, the earlier and later varieties flowered on June 15 and July 2, and matured on September 12 and September 25, respectively. So when planted late, both varieties flowered very early in their life cycles, both spent less time in reproductive stages than when they were planted early, and they ended up yielding about the same. While in this case it’s accurate to say that the later-maturing variety benefitted more from early planting, that’s only because the early-maturing one was physiologically less able to use the longer growing period allowed by early planting to produce high yield.

Seed and seeding rate

Seed quality as measured by the standard warm germination results is probably good in most cases this year, but there were enough issues related mostly to harvest time weather last fall that vigor—the ability to produce good stands under stressful soil conditions—may be lower than in some previous years. Vigor is commonly measured by the “cold test”, which consists of keeping the seed in soil at 50 degrees for several days followed by a standard warm germination test. Cold test results can be somewhat hard to interpret, but most of the time, actual emergence is higher than the cold test score. Many companies take cold scores and might be willing to share them for seed lots this year.

Although promoting low soybean seeding rate for high yields seems to have cooled a little, some still contend that planting 100,000 seeds per acre is enough. We have conducted small-plot seeding rate trials each of the past four years at six or seven sites per year around Illinois. We plant rates of 50, 100, 150, and 200 thousand seeds in replicated trials, and report yields by actual plant stands. Figure 3 below shows the results from the 2018 trials. Plant stands as percentage of dropped seeds were high in 2018, and yields were high to very high. Optimum stands ranged from 86,000 to 201,000 plants per acre.

Soybean seeding rate responses at seven Illinois sites in 2018. The yellow triangle on each curve marks the optimum stand, where added seed just paid for itself in higher yield.

The wide ranges of optimum plant stands and yields in 2018 were not unusual; across all 27 trials over the past four years, optimum stands ranged from 50,000 to 201,000, and yields at optimum stands from 53 to 90 bushels per acre, with no discernible correlation between yields and stands (Figure 4). How do we deal with such variability across sites? We certainly can’t plant enough seeds to make sure we never have too few plants—that would take planting more than 200,000 seeds per acre. With no correlation between yield and seeding rate, we also can’t reasonably adjust seeding rate by expected yield, and we can’t know actual yield. A reasonable approach is to cover most cases by planting to produce stands of about 120,000 plants per acre, which is close to the stand needed to maximize return to seed (gross return minus seed cost) in the majority of responses.

Figure 4. Optimum plant stands and yields at those stands across 27 sites in Illinois, 2015-2018.

How many seeds do we plant if we want to end up with 120,000 plants? Over the trials reported here, plant stand as a percentage of dropped seeds averaged in the low 80s for the first three years, and was close to 100% in 2018. We normally divide the desired stand by the warm germination (percentage expressed as a fraction, as in 0.95), and then by a percentage that, based on soil conditions and seed placement, we reasonable expect to produce a stand. Planting seed with high germination percentage into good soil conditions with a planter that places seed well, it may be reasonable to expect 80% establishment. If we want 120,000 plants, that would mean planting 120,000 ÷ 0.80 = 150,000.

Should we raise seeding rates to avoid having to replant if we’re planting into cool soils the last week of April? We may want to raise them a little if the cold test percentage is marginal. But the most common cause of having to replant is heavy rains after planting that leaves seeds without oxygen in the soil, causing seed death and very low stands. In few cases like this does raising the seeding rate increase the stand enough to avoid replanting.

Fertilizer

On soils heavier than loam in texture, it is unlikely that adding nitrogen fertilizer at planting, or during the season, will increase yields. On lighter soils, adding urea at planting might increase early growth and yields, but the chances of this happening are not high. There has been some data from Iowa and Indiana suggesting that adding sulfur might help soybean yields, especially (but not only) in lighter soils. As with N, we don’t have good guidelines on when S might be needed, but we do not think that the need is routine. Ammonium sulfate broadcast at planting at a rate of about 20 lb. of S (83 lb AS) can supply both N and S, but is not inexpensive. Keep in mind that most of the outstanding soybean yields in 2018 were produced without adding N, S, or other products not known to be needed for high soybean yields.