Richard Taylor, Extension Agronomist; email@example.com
There have been numerous questions concerning whether soybean fields need to be replanted or not after the frequent and often excessive rainfall during June. The soybean plant has an amazing ability to compensate for inadequate stand density but in a large measure the ability to compensate depends on the uniformity of stand loss. With the recent flooding, we have seen several types of stand loss in Delaware fields. The first type results from either seedling emergence failure from seed death due to flooding; or, for already emerged plants, death of an established stand occurs from prolonged submergence or saturated soil. These events can be either localized in low areas in a field or in some cases for large portions of our relatively flat fields in the lower counties of Delaware. The proportion of the field affected can generally be calculated and a yield loss estimate determined since in this case the stand is essentially zero.
Another type of stand loss, we’ve seen is the loss of portions of rows, individual plants in a row, or scattered small areas that create a non-uniform loss in a field. For this type of loss, it becomes very difficult to determine the ultimate effect on yield. The larger the skipped area, the greater the probability that weed encroachment will occur and the lower the chance that the soybean plant’s compensation ability will be adequate to make up for the loss of stand. Uniform losses, unless very severe, will easily be overcome by the additional space (nutrient, sunlight, and water) each soybean plant has in which to grow.
I’ll go into what happens to a soybean plant when flooding occurs later in the article but first let’s go over some of the suggestions for what to do if there is a stand problem. Many of these and the explanation of what occurs during flooding come from Dr. David Holshouser, the Extension Agronomist at the Virginia Cooperative Extension Tidewater Agricultural Research & Extension Center. Dr. Holshouser discussed the impact of flooding in the last issue of Virginia Soybean Update, Vol. 14, No. 1. That issue and archived issues can be found on-line at http://blogs.ext.vt.edu/soybean-update/.
I think the most important decision involves the use of the rotary hoe on soils in many areas in Delaware where heavy rainfall after planting leads to crusting problems. This problem is even worse when planting on conventionally tilled soil that is low in organic matter or has minimal surface residue present. Almost everyone at one time or another has experienced crusting problems with soybeans so the tendency is to try to get back on the field as soon as possible to break up the crust to help seedlings emerge since they have already been severely stressed due to the heavy rainfall and possible flooding or saturated soil conditions. Stressed seedlings will have an even more difficult time emerging from crusted soil. What we need to remember is that if we get back on a field that is too wet, we will easily compact the soil either from the downward pressure of the tractor or equipment tires and from the action of the tillage equipment. Compaction will increase the stress on the crop, slow its recovery, and limit root development needed to help the remaining plants compensate for stand losses. So, the first rule to follow is to stay off the field until it has drained and dried enough to support equipment.
Next, evaluate the remaining stand to determine if it’s worthwhile to replant. Dr. Holshouser’s research indicates that after mid-June, soybean yield potential declines at the rate of about ½ bushel per acre per day of delay in planting. Research Bob Uniatowski and I did a number of years ago indicated that this decline accelerates as you move into July reaching about 1 to 1.5 bushel per acre per day in mid-July. This does not mean that on occasion a grower will find double-cropped soybean yields higher than full-season soybean yields. Late-season rainfall pattern and temperatures can dramatically improve or destroy double-crop soybean yield potential but on average we do see yields decline as planting date is delayed. What this means is that even if the current stand is below the ideal soybean population, those plants have a much higher potential yield than the seed that has yet to be planted. Try not to be over optimistic about the yield potential of replanted beans or pessimistic about the yield potential of your less than ideal soybean stand.
Stress such as weed competition, foliar burn from post-emergence herbicides, root restriction from compaction will add to the slow recovery of your soybean crop. For weed competition, we know we will need to apply some form of control to limit their impact on soybean yield potential. In particular if you are adding a tank-mix partner to glyphosate for Roundup Ready™ soybeans, choose that partner that will cause the least amount of foliar burn on the soybean crop. Be selective in choosing other partner material such as COC, NIS, and fertilizers to avoid the chance for foliar burn. Of course if you planted wide-row soybeans, you might consider cultivation for weed control but again do not get back on the field until it has dried out enough to support the equipment and the tillage practice without causing additional injury through compaction.
While walking your fields to evaluate the uniformity of the stand and the population count, dig up a few roots and inspect them for Bradyrhizobia nodules. If nodules are present, split some open and look for a pink color that will form as oxygen interacts with the nodule surface. If the inside is pink or red in color then the nodule is active. Black, green, or white nodules are inactive. If nodules are not present or are not active, you’ll want to recheck the field in a week or two to make sure the plants have either renodulated or the nodules have reactivated since the soybean plant depends on functioning nodules for a large proportion of its nitrogen. Another thing to do while walking the field and digging up a few plants here and there is to cut a few stems and roots open and inspect for any disease discoloration. If you suspect a disease problem, you may want to send some plants up to our plant diagnostician, Nancy Gregory in the UD Plant Diagnostic Clinic. Contact your county agricultural agent for information on forms and procedures.
There will likely be a lot of foliar fertilizer and possibly fungicides sold to help ‘kick-start’ the soybean crop back into rapid growth. I agree with Dr. Holshouser in his assessment that there is little advantage to using any of these products. The problem affecting the soybean crop from the excessive rainfall is the lack of oxygen (O2) to the roots, possible buildup of carbon dioxide (CO2) in the root zone, and the impact of ethanol toxicity from prolonged flooding. Once the soil drains enough that O2 reaches the roots again and the plants are able to begin photosynthesizing carbohydrate to support root growth, the crop will be able to obtain adequate nutrients from the soil.
Let’s next take a look at what happens to the plant during flooding or saturated soil conditions. There is limited research available on the long-term effect of flooding in soybean fields but it is thought that the impact will largely depend on the following factors: 1) the stage of development when the flood or saturated conditions occurred, 2) the duration of the flood or saturated conditions, 3) the temperature during and immediately after the flood, and 4) the rate of field drying after the flood.
What occurs during a flood is the depletion of soil O2 which causes photosynthesis to slow. Several days without O2 causes the plants to turn yellow, slow their growth rate, and possibly die. Some indirect effects can occur as well including reduced nitrogen fixation by the Bradyrhizobia bacteria located in nodules on the soybean root system, nutrient imbalances from disturbed root growth and function, and increased disease pressure.
Flooding Before Emergence
The greatest impact from flooding occurs on un-emerged soybean when the duration of the flood is greater than 24 hours, when soil temperatures are low, and when flooding occurs 2 or 3 or more days after seeds have imbibed water (imbibition is when seeds swell as they take up soil water and begin the germination process). Flooding that occurred one day after imbibition resulted in a 20 percentage point reduction in germination even after 48 hours of saturated conditions according to controlled research conducted during the late 1990s. This increased to a 33 or 43 percentage point reduction if the flooding occurred 2 or 3 days, respectively, after imbibition. The researchers found that the farther along the seed was in the germination process, the more susceptible they were to flood damage. Lower soil temperatures increased the damage. The researchers suggested that damage to the seed under brief flooding was primarily physical from seed membrane disruption/damage but with longer flood duration the injury was physiological from ethanol toxicity, O2 depletion, and CO2 build-up. In addition to a reduction in germination percentage, crusting can add to stand loss especially on worked ground low in organic matter or surface crop residue. Finally, for those seedlings that manage to emerge, there is an increased chance of seedling diseases resulting in an even greater stand loss and this is aggravated by the higher soil temperature we see at this time of year.
Flooding After Emergence
Soybeans can usually tolerate up to 2 days of flooding or saturated soil conditions but beyond this duration significant yield reductions can occur. Factors that affect the amount of yield reduction include the development stage of the crop at the time of flooding/soil saturation, the duration of the flood, the type of flood (stream overflow versus low land depressional flooding), temperature during the flood, the drainage or drying rate following the flood, and overall environmental conditions following the flood.
Soybeans are most susceptible to flood during the very early vegetative stage (V2—two trifoliate leaves fully emerged) and younger and the reproductive stages of R1 to R5 (flowering and pod set). Research suggests that varieties can differ in their tolerance to flooding. Another compounding factor is the amount of soil sediment left on the soybean plants and how long the sediment remained on the plants. The coating of sediment reduces the plants ability to photosynthesize and this reduces the ability of the plant to recover from the stress. Dr. Holshouser in his article reviews much of the research on yield loss so please refer to that article for more details. In general, it’s safe to say that estimating yield loss at or shortly after a flood is very difficult since to a great degree the environmental conditions for the rest of the growing season can have a great impact on the final yield loss.