Dog Days of Summer – Soybean & Corn Development

August 04, 2022

The dog days of summer have begun to take it’s toll on the developing crop.  Areas of sandy, low organic matter soils have begun to some drought stress.  However, the news is not all bad.  Digging with a tile spade to a 14” depth yesterday here in Plattsmouth, I could still make a mud ball in my fist.  I realize that is not the case everywhere.  But, conducting this same practice in your area can give you an idea of how much soil moisture you currently have in place.

Soybean development has progressed to full bloom (R2) and is approaching the R3 Stage (pod initiation).  Irrigation is on-going where it exists. In some instances where the plants are lacking plant height (less than 12” tall) the irrigation is beneficial to provide the plant structure needed for proper flowering and pod set.  Even in the driest of weather soybeans are most efficient with water use during the current stages of development.  The total water use by a soybean crop (evaporation plus transpiration) is 21 to 24 inches per year.  About 65 percent of this water is used during the reproductive stages. The peak water use rate, about 0.3 inches per day, occurs during pod development. The average rate during the reproductive stages is about 0.25 inches per day. The most important times for soybean plants to have adequate available water are during pod development (R3-R4) and seed fill (R5-R6).  Soybeans use between 10 and 11 inches of water from full flower (R2) to beginning maturity (R7). Therefore, effective irrigation plus rainfall should equal about 3 inches during full flower (R2), 3 inches during pod development (R3-R4) and 4.5 inches during seed fill (R5-R6).*(courtesy Elmore, R.W., D.E. Eisenhauer, J.E. Specht, and J.H. Williams. 1988.)  Long story short, a couple of good 1-2 inch rains in August will provide the biggest benefit for soybean yield potentials.

Corn development has progressed into the reproductive stages. Dryland soil moisture is generally poor across much of Nebraska.   Some fields of corn are approaching the permanent wilting point in the soil profile, which can turn the corn white/brown, and the plant begins to die. Much of the corn is finishing pollination and is the R2 (blister) stage of development. Once the silks turn brown, fertilization is complete and the now fertilized kernels will “pop up” out of the cob and begin the blister stage of development.  Starch within the kernels has begun to accumulate and most of the nitrogen and phosphorus accumulation in the plant is directed toward filling the ears.  The number of ears and kernels on the ears is now fixed. Moisture use within the plant is directed mainly into filling the kernels of the ears.  Adequate moisture enables the ears to fill to the tip of the ear and increase size and test weight. Environmental stress, especially moisture and heat stress, can cause poor pollination and seed set. Yield losses of 3 to 8% can occur each day from drought-like conditions during the 2 weeks before and 2 weeks following silk emergence. Reduced number of kernels can occur due to failure of silk emergence, failure of emerged silks to pollinate or kernel abortion. Although silks normally pollinate within a day of being exposed, they remain receptive for 6 to 7 days after emerging from husks. The plant continues to adjust seed production for the environment through kernel size. Under stress during grain fill, kernel numbers per ear are adjusted first, completely filling some ears rather than partially filling all kernels. Kernel size is adjusted last. Kernels at the tip are pollinated last, which is why they are smaller than those at the base.

Water use in corn is still at its highest use rate and after pollination as we approach the milk stage in some of the earlier planted hybrids, water use will slow down. Much of the corn is currently using between 0.25-0.30 inches of water per day.

When irrigating we begin to think of how much water is needed and when.  It has been hard to keep up this year, with many producers currently at or already above their normal yearly irrigation totals.  At our current corn growth stage, the root zone of the crop is about 4 feet deep and we can predict how much water the crop uses through the following table.  We still need quite a bit of water.

EXAMPLE:  If your soil type holds 1.5 inches of water per foot, there are 8 inches of water in the 4 foot crop root zone.  Research conducted through the University shows that soil moisture levels can be reduced up to 40% per foot of soil and not reduce grain yield. If the root zone was full of water, the crop could pull out 4.8 inches of water (8 inches x 60%) before the crop would show signs of stress.  If the crop was between the dough and beginning of dent stage, you would not need to irrigate.

Weed Escapes—Are you done spraying yet?   OR  Have you surrendered and have quit spraying?   As we march into the 2nd half of summer, soybean fields will get no cleaner from weed pressures than they are today.  Regardless of the trait platform of the genetics you have planted, soybean fields will continue to culture added weed growth.

What is the Answer? 

There is not ONE thing that can make your weed control better or worse.  There are many things to consider. This is part of the reason way that industry continues to provide options that are “add-on” modes of action to the wide based platform of Roundup Ready Crops. Combinations with Dicamba, 2,4-D, Glufosinate  (Liberty) are all current herbicide traited  options available for use for post-emergence weed control within Roundup Ready Crops.   As the industry continues to develop additional herbicide tolerant combinations for soybeans, it is inevitable that scrutiny of these herbicide tools will also increase.   As an example:  Stories about glyphosate (the active ingredient in Roundup, Traxion, Buccaneer, Touchdown, and a host of other compounds) resistant weeds and concern about the use of Roundup Ready crops have circulated for years since the introduction occurred into row-crop use back in the mid-late 1990’s.  Most of these stories actually create fear of using beneficial technology and provide use recommendations not based not on scientific data but largely on emotions and assumptions.  This can create consumer concern about using technology that has proven to provide many benefits including facilitation of conservation tillage systems, excellent weed control and excellent crop safety.  BUT, as the years of broad spread application have continued to develop, the incidence of more weeds (waterhemp/marestail mainly) escaping a treatment have also increased.

Does this mean that all marestail, palmer amaranth and waterhemp populations are resistant??

DOUBTFUL.  Monsanto introduced Roundup herbicide in 1974, providing growers with a new and effective tool for broad-spectrum weed control.  Roundup Ready crops were introduced beginning in 1996, providing growers with additional tools to obtain excellent weed control and crop safety along with greatly improved ease, flexibility, and potential cost savings.  Both of these novel technology introductions have revolutionized weed control practices in crops since then.  It is hard to argue that Roundup agricultural herbicides and Roundup Ready crops are excellent, economical solutions to weed control while offering a relatively low risk of resistance development.  Remember the pre-1996 alternatives?—cultivating, “walking beans in the summer heat”, etc.  More of a nightmare for many growers, as we continually seek better options for soybean weed control.

Over the past 10 years or so, weed species such as pigweeds (waterhemp, palmer amaranth, redroot pigweed), and marestail, have become routine candidates for weed control escapes from a Roundup treatment—primarily in soybeans.  This year is no different as we have experienced a number of fields treated with a glyphosate based treatment that was applied 2 to 3 weeks ago, now have marestail and pigweed escapes from the original treatment.


  •  Why does the treatment kill one waterhemp plant fine and 3 feet away NOT
  •   kill another waterhemp plant of the same size?
  • Why is there seemingly more escapes this year than past years?
  • Why do certain fields that tillage done earlier have more weeds/more escapes now

Glyphosate has no soil residual, and is tightly bound to soil organic matter and dust.  These two facts play important roles in why there continues to be weed escapes after application—even with high use rates. These facts also aid in the potential development of resistance.

I have seen most escapes in the areas that are directly behind where the sprayer traveled in the field.  I have also seen many escapes along roads where excessive dust from the adjacent road has covered the leaves.  Dust/dirt on the weeds leaves will limit/tie up the glyphosate from being absorbed/translocated in the plant.  Areas that incurred tillage this year are seeing increased weed pressure.  This is likely due to the weed seed dormancy being broken via the tillage practice in the soil.  I won’t argue the justification for the tillage, I just notice that weed pressure is generally greater in areas that have had a tillage pass since that last cropping season.

I do not believe that every waterhemp, marestail or any other weed escape makes the weed automatically resistant to glyphosate.  The inconsistency of the weed control is LIKELY a matter of really all of the following:

  • no pre-emergence herbicide use with a dual mode of action
  • application timing with no increased rate change for larger weeds
  • the non-use of an additional post-emergence herbicide/mode of action and residual for control of these weeds in the treatment
  • hot, dry, dusty dirty conditions at application time—actively growing non “dirty” weeds are easier to kill..

The long and short of this discussion really comes down to this.  A balanced weed management strategy combined with the “right” genetics is always key to your success.   Not many growers I visit with fault their herbicide programs in October if the soybeans don’t yield.  Yes, you might have/will have weed escapes, but I doubt whether they will have any yield reduction capabilities if you are just seeing them now.  You must combine proven genetics with the herbicide trait options, regardless of the trait options you select.    I also encourage you to consider the continued use of pre-emergence applications along with multi-mode post-emergence applications if you are having problems controlling palmer amaranth or waterhemp late into the growing season (August).

Dr. John MacNamara, Wilbur-Ellis Agronomist