In driving through the region this past week I have seen several fields where water was ponding and others where water was running over fields.  The primary damage to plants from flooding or ponding is oxygen depravation.  Oxygen content in water is much lower than air, even air within the soil.  When soils are waterlogged or flooded, there is much less oxygen available to plants.  Plant tissues need oxygen for respiration to make high energy compounds that are needed for nearly all other life reactions.  Low oxygen availability means the process of respiration slows.  If oxygen levels drop too low, plant respiration changes to a pathway similar to fermentation.  In this process, energy production can be reduced up to 95 percent.  This can greatly reduce yields and if it goes on long enough, some plant tissue dies.  The duration of the flood is an important factor on the effect of low oxygen levels on the plants.  Local conditions influence the effects, but 36 to 48 hours is often the tolerable limit.

Soil drainage properties can aggravate the flooding effect.  Soils high in clay content “Gumbo” soils prolong the flooding and remain waterlogged after flood waters recede.  In general corn can tolerate flooding better than soybean.  Although if flooding occurs in the spring, the corn growing point is near the soil surface or below ground, and may be submersed longer.  The soybean growing point is at the tip of the stem and may remain above water.

Soybean plants may turn yellow because oxygen for nodule function is reduced.  Corn may lose nitrogen from denitrification.  In corn, if more nitrogen isn’t added, permanent yield loss may occur.  If soil is deposited on the leaves of crops, this reduces photosynthesis because the leaf tissue cannot absorb the sunlight.  Finally roots may be damaged making them more susceptible to disease. 

The wet weather has also been conducive to many of the seedling soybean diseases.  Some parts of the region have been very wet and soil has just begun to work well for soybean planting.  While there haven’t been too many reports of seedling diseases in soybeans, soybeans that were just planted and then received a heavy rain will be vulnerable.  You may want to check recently planted soybeans for disease incidence.     With warm soils and plenty of moisture, soybeans should pop right out of the ground and really take off unless they have some seedling diseases, such as Phytophthora or Rhizoctonia.

Weed control has been and will be a challenge in fields because of very windy conditions and wet field conditions.  Weeds such as waterhemp and sunflowers are really growing in soybeans.  It is important to use the recommended rates to control the size of weeds you have, otherwise you may have to spray again.  This also increases the   potential for herbicide resistance to develop on weeds that are not killed.  There have also been reports of spray drift on gardens and horticultural crops, and fields of corn being damaged by spray.  Remember to watch the wind speed and always follow label directions.

With the excellent planting weather we had earlier, it is approaching the time for the first application of glyphosate to Roundup Ready soybeans.  It is important to remember that waiting too long to make the first application can cost you.  In some recent research conducted in Nebraska, row widths of 7.5, 15, and 30 inch were evaluated.  The results indicated a 5% yield loss when weed removal occurred at V3 for 7.5 inch rows, V2 for 15 inch and V1 for 30 inch rows respectively.  These weed removal timings corresponded to 19 days after emergence for 7.5 in rows, 15 days after emergence for 15 inch rows and 9 days after emergence for 30 inch rows.  The number of days after emergence will vary by location and growing conditions.  This data emphasizes two important points related to weed management in Roundup Ready soybeans.  First, the critical time for weed removal comes much earlier in wide rows than narrow-rows.  Second, waiting too long to make a glyphosate application will cause soybean yield reductions, regardless of the row spacing adopted.

When Roundup Ready soybeans were introduced, many growers delayed their first application of glyphosate in an effort to save the costs of a second in-crop application.  This was based on the premise that the cost of the second application would be greater than the yield and revenue that could be gained by spraying earlier.  With the availability of generic glyphosate, the cost of a glyphosate is now lower, to where in some instances, the cost of the custom application is more than the chemical itself.  The research in Nebraska indicated that there is a 2 percent yield loss for each plant growth stage of delay past the critical time of weed removal.  At current prices, an early and timely application of glyphosate will almost always offset the loss of revenue associated with yield reductions of this magnitude, even if a second herbicide application is needed later in the season.