WEATHER EFFECT ON CROPS IN 2009

    Years ago, a wise man told me that the average weather year was the average of all the years of unusual or extreme weather conditions.  He may have been right.  No matter what the crop-growing conditions happen to be, we always seem to think it is too cold, too hot, too dry, too wet, too cloudy, too sunny, too humid, not humid enough.  This year is no exception.  After experiencing a great April and May planting season in southern Minnesota (with apologies to you growers in southern Iowa, Illinois, and elsewhere who had a very tough planting season), conditions turned very near record cold and cloudy in early June.  This cold weather certainly slowed down early growth and development of both corn and soybeans.  Some folks are asking whether that cold temperature will have a lasting impact on crop production in the fall, given that temperatures and growing conditions have improved as of late.
    The short answer to that question is YES.  The more relevant question is “How much of an effect will that cold temperature have?”  The answer is probably not all that much.  I often compare the crop growing season to the duration of a basketball game.  Just like in a basketball game, there are times during the growing season when your team (your crop) faces some adversities against the opposing team (the environment).  At other times, your team fares well against the opposing team.  Right now the game is early in the second quarter of play.  There is still a lot of game left to be played.  Already both corn and soybean are snapping out of the early June funk they were in during the cold snap.  The cold temperatures set back growing degree day (GDD) accumulation.  However warmer-than-normal temperatures now are allowing GDD’s to start catching up to normal.  Future weather conditions, as well as other environmental factors such as disease and insect pressure, between now and harvest will no doubt have more net impact on crop production than the cold snap during the first two weeks of June, at least in southern Minnesota and northern Iowa.

HERBICIDE IMPACT

    A couple weeks ago, when corn was still looking pale and yellow, and was growing poorly, many folks were attributing the poor color and slow growth totally to the cold weather we were experiencing.  That thought was only half correct.  Clearly corn growth was affected by herbicide application during the cold period.  Herbicide stress was even more pronounced and visible wherever herbicide application was inadvertently overlapped or where the sprayer turned around at the end of a field.  All labeled herbicides, even Roundup, will put some degree of stress on corn plants (and soybean plants, too).  Under average or above-average growing conditions, there usually are no other significant stresses bearing down on the crop in early June.  Consequently, the stress imposed by herbicide application is less visible, although it is still there. 
However, this year most corn was already under cold stress when post-emergence applications were made.  The herbicide stress in addition to the cold stress became quite visible.  Will the herbicide stress affect yield?  Again, the answer is yes.  How much depends upon how much stress there was and whether and how many plants may have died as a result of that stress.  I personally saw a few fields where corn plants actually died as a result of stress due to herbicide, cold temperature, and perhaps some soil compaction as well.  However such extreme stress was rare.  In most cases, even where herbicide stress was visible earlier, the corn has grown well out of it by now.

SOYBEAN FLOWER AND POD DEVELOPMENT

    Opening of the first flower on a soybean plant initiates the beginning of reproductive growth.  This is called stage R1, or the beginning of flowering.  Soybean plants of proper maturity for your growing area usually begin to flower in late June and will continue flowering through much of July.  Soybean is a self-pollinated plant.  That is, the pollen produced in a soybean flower pollinates the ovaries of that same flower. Therefore by the time a soybean flower is visibly blooming, it has likely already been pollinated.  Each flower has the potential to develop into one pod.  Soybean is unique in that, even under good growing conditions, more than 50% of all flowers produced on the plant will abort and not develop into pods.  Furthermore, many small, developing pods will abort and never produce seed.  This is normal for soybean.  The excess of flowers produced on the soybean plant over an extended period of time may be an evolutionary mechanism the soybean developed to allow it to overcome short periods of adverse weather during reproductive development.
    Heat, drought, insect or disease pressure, herbicide stress, low fertility, and other management practices will affect the number of soybean flowers and pods that abort, and can ultimately affect crop yield.  In breeding new soybean varieties, we continuously select lines for their ability to withstand these environmental stresses during flowering and pod development so that the varieties you grow are both high yielding and environmentally stable.  Nevertheless, practices that can minimize the potential for any kind of stress to occur during flowering and seed filling will help insure top yields.  Some examples are:
1.    Avoid or minimize soil compaction,
2.    Fertilize to soil test recommendations,
3.    Avoid applying herbicide during excessively hot weather,
4.    Minimize or eliminate soybean cyst nematode populations in your soil by use of crop rotation and resistant varieties.

    The most important period of time in the reproductive cycle of the soybean is the beginning of rapid seed dry matter accumulation.  This is called stage R5.  Physiologically, there are many energy demands being placed on the soybean plant at this growth stage.  I’ll talk more about this next month.

IRON DEFICIENCY CHLOROSIS

    It is that time of year again when iron deficiency chlorosis (IDC) appears in soybean fields.  This problem is especially prevalent in the glacial-till soils of northern Iowa and southern Minnesota.  IDC often appears in or along the edges of field depressions as yellowish, chlorotic areas in an otherwise green soybean field.  The soils in these areas are sometimes referred to as calcareous soils, and can range in size from a few feet wide up to several acres.  These soils developed [ions] eons ago as a result of large numbers of hard-shelled organisms like snails and clams flourishing, dying, and accumulating in or around shallow ponds following the retreat of glaciers.  Often the shell deposits would drift to one side of a pond where they would accumulate.  That is why you can often see iron deficient areas in the shape of a circle or crescent moon along the side of a field depression. 
IDC is caused by unusually high levels of calcium and other minerals in the soil that bind the iron and raise the pH of the soil, making the iron less available to the soybean plant.  In other words, the soil is really not deficient of iron.  Rather the iron is tied up in a form that is difficult for the soybean plant to absorb.  Some soybean varieties will tolerate calcareous soils more than other varieties.  Except under the most extreme conditions, the plants will eventually grow out of the chlorosis.  Other than for use of IDC tolerant varieties, there is little that can be done to manage soybean production in calcareous soils.  Some soil treatments are available, but economically not practical.  Galena Genetics currently screens all its germplasm for tolerance to IDC.

APHID SCOUTING

    It will soon be time to start scouting fields for soybean aphids.  Aphids can infest soybean fields as early as the V1 – V3 vegetative stage, i.e. – one to three trifoliate leaves unrolled from the growing point of the plant.  We certainly already have soybeans at that stage of development in late June this year.  Scouting reports from the University of Minnesota as of June 19 have shown little or no aphid presence throughout Minnesota thus far.  However, aphid populations have the ability to develop very rapidly from virtually no infestation to economic threshold levels in just a matter of 5 – 7 days time.  Thus, scouting your fields frequently is important.
    The economic treatment threshold level for spraying soybean aphids has been studied extensively by the University of Minnesota, Iowa State University, Michigan State University, Purdue, and others.  The general agreement is that the economic treatment threshold level for spraying is 250 aphids or more per plant.  In actuality, economic damage may not be observed until more than 800 aphids per plant are present.  However by the time you scout 800 aphids per plant, and account for a time interval in which to get the spraying done, aphid damage may already have occurred.  That is why 250 aphids per plant (assuming aphid density per plant is rising) is the recommended tripping point for taking treatment action.
    Aphids are very small insects, about 1/64 inch long, and thus are difficult to count.  No one likes to take the time to count 250 aphids per plant on 10 plants in several different parts of each of your soybean fields.  The time and cost needed to take these counts can be very expensive.  The University of Minnesota developed a soybean aphid speed scouting method several years ago which can substantially reduce the amount of time required for aphid scouting.  The procedure is simpler to see on a data sheet than it is to explain.  However it basically works like this: 

1.    In a given area of the field, check 11 plants.
2.    Each plant that has 40 or more aphids should be considered a positive (infested) plant.
3.    Each plant that has 39 or less aphids should be considered a negative (non-infested) plant.
4.    From the 11 plants checked, add up the number of positive plants.
5.    If the number of positive plants is 6 or less, you do not need to treat, but should recheck the field in 7 – 10 days.  If the number of positive plants is 11, then a treatment within 7 days is recommended.
6.    If the number of positive plants is 7 – 10, then 5 additional plants from that area should be checked.
7.    If the total number of positive plants from the 11 + 5 plants checked is 10 or less, you do not need to treat, but should recheck in 7 – 10 days.  If the number of positive plants is 15 or more, then a treatment within 7 days is recommended.
8.    If the total number of positive plants is 11 – 14, you should continue checking 5 additional plants.
9.    Continue checking sets of 5 additional plants and add to the total counts of positive and negative plants until a decision can be made regarding whether to spray the field or recheck the field in 7 – 10 days.

This sampling process is sometimes called a binomial sampling procedure because sampling continues until one of two possible recommended outcomes is reached.  The advantage of speed scouting is that you do not need to count every aphid on every plant.  If you count at least 40 aphids on a plant, you can stop counting.  With practice, one can easily determine at a glance if a plant has substantially more than 40 aphids on it, or substantially less than 40 aphids on it.  It is important to keep accurate worksheet notes for each plant that you check.  You can find a field worksheet available from the University of Minnesota designed specifically for soybean aphid speed scouting at www.nwroc.umn.edu/cropping_issues/2007/Issue9/07_17_07_no4.htm.
This website and the printable worksheet explain more fully how to carry out the binomial sampling procedure.
    The binomial sampling procedure does NOT mean that the University of Minnesota has changed the economic threshold level to 40 aphids per plant.  The threshold is still 250 aphids per plant.  Rather the binomial procedure is simply a method of making statistical sampling inferences in order to estimate the number of plants that would likely have 250 or more aphids on them.
    Soybean aphid infestations in the Midwest were generally high in 2003, 2005, and 2007, and were generally low in 2002, 2004, 2006, and 2008.  This is likely not just coincidence.  As aphid infestations go up, populations of natural predators such as the Japanese beetle also go up.  Conversely as aphid populations go down, so do populations of natural predators.  Not surprisingly, populations of Japanese beetle were higher in 2002, 2004, 2006, and 2008, reducing the aphid populations.  If these trends continue and Japanese beetle populations are low in 2009, this may be a year for high soybean aphid infestations.
    Currently there are NO commercial soybean varieties in the Midwest with resistance to aphid.  Galena Genetics is currently developing lines with aphid resistance for future commercial release.

INSECT AND DISEASE UPDATE

    There are currently no significant insect or disease problems being reported in either soybean or corn in Minnesota, according to the University of Minnesota Pest Report last published June 19.  In Iowa, there are isolated reports of soybean seedling diseases, primarily Phytophthera and Rhizoctonia, due to the cool and wet soil conditions experienced in early June. 
Also Iowa has reported European corn borer moth flights in various places which could impact non-Bt refuge acres of corn.  Don’t forget, you are allowed to spray your non-Bt corn refuge acres for European corn borer, but only if infestation levels have exceeded economic threshold levels.
As the season progresses, insect and disease problems in one form or another are likely to develop, as they always do.  Please check Notes from the Pod Doctor each month for updates on developing insect and disease problems of soybean and corn.

Grant Metz, PhD
Research Director
Galena Genetics, LLC
(grant_metz@rabbeusa.com)