Early corn and soybean growth and seed treatments.

Soybean planting is nearly wrapped up now across the Midwest.  As you know, soybean planting in southern Minnesota and elsewhere occurred at two distinctly different time periods this year.  Some growers started planting soybean immediately following a record early completion of corn planting in April this year.  In fact I heard of isolated instances in which a few Minnesota soybean growers had also completed soybean planting in April.  Then at the end of April and the first half of May, cold, wet conditions substantially slowed down soybean planting.  The second half of May was again warmer and drier, allowing completion of soybean planting.

Widespread frost May 8 – 9, followed by several days of cool temperatures, caused damage to both emerged corn and emerged soybean.  Corn appears to be growing out of frost damage at this point since the growing point for corn was still under ground when frost occurred.  However I have seen isolated fields where 50% or more of the corn seedlings had severely wrapped whorls and “leaf-laddering” caused by the corn seedlings trying to grow out of the frost damaged tissue.  As a result, some plants were under so much stress that they were dying.  However these were isolated cases.  For the most part, with the warm temperatures and generally favorable soil moisture presently available, early-planted corn is growing quite well at this time.

Early planted soybeans may not have withstood the frost so well however.  The growing point of soybean is above ground as soon as the cotyledons emerge.  Emerged soybean plants exposed to frost in early May were killed in several cases, and needed to be replanted.  One “old-timer” with a penchant for sardonic wit told me that “if farmers want to plant their soybeans twice, they HAVE to start planting early!”

Aside from the potential frost damage, the question regarding the effect of cool soil temperatures on emergence and growth of both corn and soybean is valid and relevant.  Cold conditions put additional stress on the germinating seeds and emerged seedlings.  As a result, seeds and seedlings are at greater risk of attack from soil-borne fungal diseases.  The most common of these are phytophthera (Phytophthera sojae), pythium (Pythium species), and rhizoctonia (Rhizoctonia solanae).  Soybean fungicide seed treatments are starting to be widely used across the Midwest to combat these diseases especially in cool, wet conditions encountered with earlier planting.  These treatments may also include insecticides and/or nematicides for seedling control of various insects and soybean cyst nematode.  Control from these seed treatments is generally only effective during early season stand establishment and vegetative growth of the soybean since they protect the germinating seed and seedling roots against attack from the soil-borne fungi. 

Soybean seed treatment, just like any other seed treatments, is simply an insurance policy against adverse early-season pests.  The cost effectiveness of soybean seed treatments depends on many factors, including your soil history and weather conditions each year.  Some years you likely won’t need it, while other years you wish you had it.

Herbicide Application and Effectiveness in Stressed Crops

            Temperatures have warmed considerably compared to those we experienced in early May in southern Minnesota.  However, I would like to remind growers that when crops are under serious stress – due to cold temperature, hot temperature, drought, or just about any other kind of stress – herbicide application will impose additional stress on the crop regardless of whether it is soybean or corn.  Some herbicides are more stressful than other herbicides, but all herbicides (even glyphosate) will impose some stress on the crop simply because the plants cannot as quickly metabolize the active herbicide ingredients into non-toxic compounds when under stress.

            If conditions are, or have been, putting significant stress on the crop, consideration should be made to delay post-emergence herbicide application, especially if weeds are still small.  Additional herbicide stress during vegetative growth of corn or soybean can lead to delayed flowering and/or maturity, or reduced vegetative growth, both of which may reduce yield potential.  Corn which suffered from frost injury in early May should be allowed some additional time to grow out of the injury before additional herbicide stress is imposed.  Ideally, herbicides should be applied when crop growth conditions are optimum.  Fortunately, this is usually also the same time when weed control is optimum.  Unfortunately none of us live in an absolutely ideal world.  Trade offs must sometimes be made between optimizing herbicide application timing and managing work load.  Herbicide stress should be considered when making your management decisions.

            An additional note about herbicide application on frost-damaged corn should be made here.  Most herbicide labels have some maximum crop size restrictions for herbicide application.  Most of those restrictions are based upon the number of visible leaf collars on the corn plant.  If corn was injured by frost, two or three of the lowest leaves may have been burned off by the frost, and may not be visible.  The consequence is that you may see five leaf collars on the plant, but if three leaves were destroyed by frost (and are not visible), the corn plant may be at stage V8 (8 visible leaf collars) rather than stage V5.  If the herbicide is labeled for application up to stage V6, the corn development may already be beyond labeled usage of the herbicide.  Always use herbicides according to labeled directions.

I’m REALLY NOT trying to make life any more complicated than it already is for you.  However, the take-home message here is that corn with prior frost damage can sometimes be further along in development than it first would appear to be. 

Insect and disease update.           

            So far this spring soybean rust appears to be non-existent in the southern USA.  That is good news for soybean growers across the country.  Monitors across the gulf coast states have not found any soybean rust thus far in 2010, even in places where it has been reported in previous years.  Soybean rust has never been found in the Midwest, but sentinel monitors have been in place across most states for at least a couple years now, in order to make sure we know if it ever does occur.  Soybean rust spores cannot overwinter in our cold midwest winter conditions.  Therefore all soybean rust in the Midwest must develop from spores blown in from the far south.  If no rust is present in the far south so far this year, it seems a good bet that it will not be a problem for northern growers in 2010.  Quite possibly, a colder-than-normal winter in 2009-2010 in the gulf coast states has greatly reduced the amount of rust that survived the winter in that region. 

            In areas where cold wet weather has been followed by above normal precipitation, it is likely that phytophthera, rhizoctonia, and pythium diseases have the potential for development in soybean.  This may be exacerbated by compacted, water-logged, or low-fertility soils or other conditions that hinder good root development, and put the plants under excessive stress.  Symptoms of these diseases can begin to occur any time after emergence.  Infected seedlings can damp off and die during early vegetative development.  Older plants will develop brown discolored lesions on the main root and lower stem.  Even if these older plants do not die immediately, they will become less productive and eventually die during seed-filling.  As of this writing, it is too early to have seen any symptoms of these diseases.  However they can become visible at V2 growth stage or later.

            In corn, black cutworm feeding at low levels, well below economic threshold levels, has been reported in eastern Iowa.  I am not aware of any other parts of Iowa or Minnesota that have reported any cutworm feeding activity yet this spring.  With the current rate of heat unit accumulation, maximum cutting and wilting caused by black cutworm larval activity should have been occurring about May 23 - 27 in northern Iowa and southern Minnesota.  The time for scouting for black cutworm feeding is right now, if you haven’t already started.

            If black cutworm larvae are less than ¾ inch long and 2 – 3 % of plants are cut or wilted, an immediate insecticide treatment should be considered.  If larvae are longer than ¾ inch, then about 5% of plants cut or wilting should prompt an immediate insecticide treatment.  You can find the larvae at or just below the soil surface wrapped around wilting corn seedlings.  When the larva has cut the plant off at the ground surface, it simple moves on to the next plant and continues feeding until it pupates.  The reason for a higher percentage of cut and wilting plants needed to reach economic threshold levels when larvae are larger than ¾ inch is because larger larvae are nearing the pupation stage of their life cycle, and thus will not be feeding much longer anyway.

Memorial Day

            I hope all of you have had, or are having, a pleasant Memorial Day weekend.  While spending time together with family and friends during this holiday, please remember those who gave the ultimate sacrifice in defending the freedoms that we all enjoy every day.  In addition, say thank you to everyone you know who is currently serving or has served in our armed forces.  They deserve our gratitude.