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Late Season Flood/Storm Events in Louisiana Soybeans

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Over the next few days, producers across the state will begin to assess damages to soybeans brought upon by tropical system Harvey.  Unfortunately, there is no cookie cutter answer to how a system like this will affect every grower.  The main distinction of how varying situations will need to be assessed is the growth stage of the soybeans at the time the event occurred.

The lack of available oxygen for plant processes is the main concern in flooded fields. Oxygen is required for many essential plant processes including respiration, water uptake, root growth, and nodulation.  When flood water covers a field, the oxygen concentration drops quickly and can be depleted in as little as 24 hours.  However, depending on additional factors, soybeans can survive flooded conditions for up to 96 hours.

Temperature: Higher temperatures (ambient and water) will accelerate plant respiration, leading to a depletion of oxygen sooner than cool temperatures with cloudy weather.

Water movement: Even moderate water movement can increase aerification and allow oxygen to the plant roots.

Soil type: Flooding is potentially worse on poorly drained clay soils due to the reduction in hydraulic conductivity (the speed at which water can move through and out of the soil) compared to coarse soils.

According to research conducted in Baton Rouge in the late 1990s, the most sensitive growth stages of soybeans to flood stress are the early reproductive stages of R3 to R5 with yield reductions as high as 93% and 67%, respectively, when flood water remained for seven days (Linkemer et al., 1998).  The lack of oxygen associated with flood waters reduces the plants ability to develop additional plant material due to a reduction of photosynthesis and respiration.  At R3, the loss of yield is caused by a reduction in both the number of pods and seed size while the yield reduction at R5 is attributed mainly to seed size.   The same study showed little loss in yield for soybeans flooded after R6 as this rapid seed fill stage is believed to be protected against temporary stresses (Linkemer et al., 1998; Westgate et al., 1989).

R5 Soybeans in standing water. Soybeans are most sensetive to flooding at growth stages R3 to R5. Todd Spivey

R8 soybeans in standing water. Todd Spivey

 

The yield losses discussed in these studies however, only refer to direct reductions of seed number and size by the plant.  The studies presented do not account for yield and quality reductions caused by outside factors associated with these type of weather events.  Late season flooding followed by warm conditions can become conducive to several fungal diseases such as aerial blight, anthracnose, pod and stem blight, and soybean rust.  It is important producers continue to scout fields for an increase in disease incidence in the coming days.

Consideration should also be given to the possibility of seed rot and seed sprouting.  Sprouting can occur in seed that have previously dried down to below 50% moisture before experiencing extremely wet weather.  Additionally, ease of harvest can be reduced with soybeans that received an application of gramoxone just prior to the storm.  As the leaves desiccate and are removed from the plant the stem can still imbibe water.  With no leaves to aid in moving the water out of the stem, the stems will not dry down and producers can see an increase in green stem incidence in many fields.   

 

Linkemer, G, J.E. Board, and M.E. Musgrave. 1998. Waterlogging effects on growth and yield components of late-planted soybean. Crop Sci. 38:1576-1584.

Westgate, M.E., J.R. Schussler, D.C. Reicosky, and M.L. Brenner. 1989. Effect of water deficits on seed development in soybean. II. Conservation of seed growth rate. Plant Physiol. 91:980-985.

Use of Harvest Aids in Louisiana Soybeans

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Todd Spivey, Sebe Brown, Josh Copes, Donnie Miller, Boyd Padgett

Over the last several weeks, we have received numerous calls about soybean harvest aid timing, products, and general recommendations.  The use of harvest aids in Louisiana soybeans is a common practice, with timely applications improving seed quality and harvest efficiency while potentially resulting in a soybean harvest 10 to 14 days earlier when compared to non-treated beans.

Timing

If the goal of harvest aid use in soybeans is to promote early harvest and improved harvest efficiency, harvest aids must be applied as timely as possible.  Once seed have separated from the white membrane inside the pod, they have reached physiological maturity and will no longer increase in size.  Any use of a harvest aid prior to the majority of seed reaching physiological maturity will result in a loss in yield.  Table 1 gives the paraquat label requirements for harvest aid application timing in soybean. Research conducted in Louisiana by Dr. Jim Griffin and Joey Boudreaux established that a harvest aid application could be made to soybean without yield penalty as long as soybeans are at reproductive growth stage R6.5 (physiological maturity). They provided a list of procedures to help determine when harvest aids can be safely applied to soybeans:

  1. Begin to scout fields for harvest aid timing when leaves begin to yellow
  2. Collect pods from the top four nodes of the plant at multiple, random locations within a field
  3. Open soybeans from pod, they should shell easily, and look for soybean separation from the white membrane
  4. If soybean separation from the white membrane has occurred for all pods collected, the seed has reached maximum dry weight and harvest aid application can be made without yield penalty

Plant appearance at growth stage R6.5 will vary by variety so close attention should be made to pods collected from the field and if seed have separated from the white membrane (Griffin and Boudreaux 2011 Louisiana Agriculture magazine Vol. 54, No. 2, Spring 2011).

Table 1. Proper application timing of harvest aid in indeterminate and determinate soybean varieties.
Indeterminate Varieties 65% of pods have reached a mature brown color or seed moisture is less than 30%
Determinate Varieties Plants are mature; beans are fully developed, 50% of leaves have dropped and remaining leaves are yellowing.

Products

Producers have several harvest aid options, though the typical harvest aid application consists of paraquat with an additional nonionic surfactant.  With excessive morningglory pressure, growers might consider including carfentrazone (Aim) or saflufenacil (Sharpen) with paraquat to improve desiccation of vines and in situations with high grass pressure, a tank-mix of paraquat with sodium chlorate may be warranted to improve the desiccation of grassy weeds prior to harvest.  Questions have also been received in regards to the use of sodium chlorate to aid in drift reduction of paraquat applications made by air.  The LSU AgCenter has no data to support this claim and only recommends the use of these products together for improved desiccation of weeds and soybeans present in the field.

It is also imperative that producers consider the required preharvest interval (PHI) associated with each product label.  When using multiple products, the longest PHI must be adhered to.  Labeled rates and comments are presented below in the excerpt from the 2017 Louisiana Suggested Weed Management Guide.

Labeled rates and comments of soybean harvest aid products from the 2017 Louisiana Suggested Weed Management Guide

Redbanded Stink Bug Considerations

Producers should also continue monitoring redbanded stink bug (RBSB) populations and should not rule out the inclusion of an insecticide with the application of a harvest aid.  LSU AgCenter entomologists recommend the control of threshold populations of RBSB until the soybeans are out of the field.  This means that many producers could, and should, include an insecticide for the control of RBSB with their harvest aid application (sodium chlorate cannot be tank-mixed with any insecticide).  It is important to keep in mind the restrictions placed upon many of the products at this point in the season.  These restrictions may include total active ingredient restrictions and PHIs.  Acephate, a common recommendation for RBSB control, can only be applied up to 2 lb ai A-1 year-1 in Louisiana.  Other insecticides also have increased PHI such as the pre-mix product Endigo, with a PHI of 30 days.  It is important to read and adhere to the label of all labeled materials prior to use.  When label restrictions prevent the inclusion of an insecticide with the harvest aid, producers should not delay the harvest of soybean so that the seed can be removed from the field as quick as the label allows.

Redbanded Stink Bug Numbers Increasing in Soybeans

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Reports from the field indicate redbanded stink bug (RBSB) numbers are beginning to build in soybeans at the R5 development stage and beyond. Once RBSB colonize a field, native stink bugs often are forced out or are outcompeted, leaving only RBSB behind. The Louisiana threshold for RBSB is four insects per 25 sweeps. RBSB are strong fliers, and routine scouting is essential to detecting an influx of these insects. Furthermore, the presence of immatures signals that RBSBs are reproducing, meaning previously applied insecticidal controls may no longer be active. Recommended insecticides include pyrethroids, neonicotonoids and organophosphates.

The use of premix insecticides, including Endigo ZC and Leverage 360, may offer a degree of repellency not observed with other insecticides. Insecticide efficacy tests conducted at the Macon Ridge Research Station in Winnsboro demonstrated satisfactory control of RBSB while also having a possible added benefit of repellency. However, these insecticides perform best when populations of RBSB have not exceeded threshold. Once RBSB populations have exceeded threshold, the use of tank mixes of either acephate (0.75 to 1.0 pounds per acre) plus bifenthrin (6.4 ounces per acre) or Belay (4.0 ounces per acre) plus bifenthrin (4.0 ounces per acre) may be required to get them under control.

As with most insects, staying ahead of RBSB populations will make season-long control much easier while also reducing injury. Please contact your county agent or me for more information.

Bt Cotton Situation

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For the past two weeks, most of Louisiana has been in the midst of a very large bollworm moth flight. Our moth trap catches were averaging about 10 moths per day and moved to more than 100 late last week. I have received numerous phone calls on how the technology is holding up and what insecticide should be used to over-spray. Another issue to consider is how much these worms were pre-selected in Bt corn. My colleagues around the Midsouth and Texas have seen a very large number of worms coming through Bt corn and Louisiana is no exception. Further, LSU AgCenter entomologists discovered a change in susceptibility of bollworm to Cry1Ac and Cry2Ab. The resistance does not appear to be complete and some fitness costs may be associated. If these results are any indication of Louisiana’s bollworm population this year, we may experience more escapes in Bt cotton.

Results from our Bt technology tests and reports from the field indicate that Widestrike cottons (including 499, 312 and 333) are experiencing large amounts of injury. Our small plot work at the Macon Ridge Research Station in Winnsboro is averaging 10 percent fruit injury in Widestrike (WS) and 6 percent in Widestrike 3 (WS3). Based on our work we conducted with the mid-South entomology group last year, we validated a 6 percent fruit injury threshold in Bt cotton. Therefore, WS3 is better than WS, but both technologies would need to be over-sprayed to preserve yield in this situation.

Furthermore, Bollgard 2 (BG2) and Twinlink (TL) have a more robust Bt package than WS. However, I have seen these technologies fail under severe pressure. As of this week, reports from the field and results from our trial work indicate BG2 is still performing well — but this can change quickly. TwinLink’s performance has been inconsistent, with a number escapes being reported. This seems to be dependent on the environment and insect pressure. Keep in mind that stress can negatively affect Bt expression in cotton. Stressed plants may not express a high enough level of toxin to control bollworms.

Independent of environmental factors, if bollworm escapes are detected, a rescue spray may be warranted. The use of pyrethroids is strongly discouraged. Louisiana bollworm populations have the highest level of pyrethroid resistance in the United States, and pyrethroid applications may not provide adequate control. They may even flare secondary pests such as spider mites. The LSU AgCenter recommends the diamide chemistry (Prevathon, Besiege) for control of bollworms in cotton. Beware that Besiege contains a pyrethroid and use may inadvertently flare secondary pests. Keep in mind that bollworms are cryptic feeders, and worms that have established in squares and bolls may not be controlled by diamides. If you have any questions or comments, please feel free to contact your county agent or me.

Estimating Yield Potential of Corn

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This article covers how to estimate the yield potential of field corn.  Please contact Drs. Dan Fromme, cellphone: (318)-880-8079 office: (318) 427-4424 or Josh Copes, cellphone (318) 334-0401, office (318) 766-3769 for more information.

Louisiana Rice Notes #8

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This edition covers the current rice market outlook, crop progress, a potential tropical depression, mid-season fertilization, GA use for ratoon crop, and drain timing.

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Louisiana Rice Notes #7

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This edition covers current crop issues including the rise in sheath blight in south Louisiana, effect of midday rain on flowering rice, leaf miners, the black rice bug.

Louisiana Rice Notes #6

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This edition covers current crop progress and issues, recommended fungicide rates and timings, shortage of Sercadis, and stink bug BMP’s.

Louisiana Rice Field Notes #5

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This edition covers current crop progress and issues, effect of time of flooding on rice at PD, how to identify Cercospora, and proper fungicide timing.

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