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Louisiana Soybean Extension Specialist

Soybean Nutrient Profile: Manganese

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Click the link below to find the first soybean nutrient profile on manganese.  This nutrient profile is the first in what will be a weekly series dedicated to the function of the 16 essential nutrients in soybean.  After excluding carbon, hydrogen, and oxygen, we are left with a 13 part series in which we will explore how nutrients are used throughout the plant as well as how to identify deficiency symptoms and develop nutrient management decisions.   

Soybean Nutrient Profile: Manganese

Check back weekly to find the newest soybean nutrient profile.  Contact your local extension agent with any questions you may have.

Successful soybean production demands proper early-season decisions

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Todd Spivey, Sebe Brown, Trey Price, and Daniel Stephenson

 

Soybean Planting Decisions

In Louisiana, soybean planting practices vary across the state, due in large part to varying environments and cropping systems.  Planting date, seeding rate, and seeding depth decisions should all be based on local conditions and factors affecting your farm such as soil moisture and temperature, soil type, and cropping rotation.

Planting Date

Regardless of location and cropping system, our optimal planting window will typically fall between April 10 and May 10.  Although it is possible to produce high yields outside of this window, research from the LSU AgCenter has shown yields are most consistent when planted timely.  Due to recent mild winters observed across Louisiana, it is not uncommon for soybeans to be planted as early as late March, though additional considerations should be taken to account for potential cool, wet conditions that are often observed.  If growers intend to plant early, soil temperatures should be monitored so that soils reach at least 55 to 60°F by 10AM.  The forecast for up to seven days after planting should also be considered for early planted soybean as emergence will also be affected by cool soil temperatures after planting.

Planting date of soybean, being a photoperiod sensitive crop, also directly influences the number of days to flowering.  Timely planted soybean has more time and a greater potential to develop adequate vegetative infrastructure to support maximum yields than do late plantings.  The goal of vegetative growth, as it concerns planting date decisions, is to close the canopy before R1 (first flower).

Seeding Rate

Because seed size can vary by variety and even by seed lot within a variety, pounds of seed per acre should never be used in determining seeding rates.  Growers should calibrate seeding rates based on seed per foot (Table 1).  Seeding rates that are too low do not allow for adequate vegetative infrastructure for optimal yields.  On the other hand, seeding rates that are too dense can reduce yields, encourage disease proliferation and lodging, and increase seed cost.  Research conducted by the LSU AgCenter has shown that soybean yields are not reduced with populations as low as 70,000 plants per acre as long as plants are uniformly distributed through the field (Figure 1).  These same studies show that yields are not increased by increasing seeding rates as high as 175,000 seed per acre.

The LSU AgCenter recommendation for soybean seeding rates on sugarcane beds is 140,000 seed per acre.  In most other systems, seeding rates should range from 115,000 to 130,000 seed per acre in optimal planting conditions down to 30 inch rows.  Seeding rates should be increased to a range of 125,000 to 140,000 on 20 inch rows or less.  Regardless of row spacing, these values should be adjusted up to a maximum of 150,000 seed when environmental conditions before or after seeding are not conducive to seedling development.  These environmental conditions are often encountered with early plantings and include current or forecasted cool soil temperatures or excessive soil moisture.  Late planted soybean seeding rates should also be adjusted up to account for the lack of time available for vegetative growth before flowering, as discussed previously.  With few exceptions, soybean seeding rates in Louisiana should not exceed 150,000.

 

Table 1. Seeding rates expressed as seed per foot of row.
Row Spacing 6 ft Sugarcane Bed 38” 36” 20” 15” 7”
  3 drills 2 drills
  —————————————- seed / foot —————————————-
150,000 seed 6.9 10.3 10.9 10.3 5.7 4.3 2.0
140,000 seed 6.4 9.6 10.2 9.6 5.4 4.0 1.9
130,000 seed 6.0 9.0 9.5 9.0 5.0 3.7 1.7
120,000 seed 5.5 8.3 8.7 8.3 4.6 3.4 1.6
115,000 seed 5.3 7.9 8.4 7.9 4.4 3.3 1.5

Figure 1. LSU AgCenter studies have shown seeding rates as low as 75,000 seed per acre are able to maintain optimal yield.

Seeding Depth

Plant only deep enough to place the seed in moist soil.  Dependent on soil moisture, seed should be planted from 0.75 to 1.5 inches on sandy or silt loam soils and 1 to 2 inches on clay soils.  Good seed to soil contact is imperative and must be a strong focus, especially when planting into residue from the previous crop or cover crop.  Although planting deeper often results in reduced vigor, many of our varieties can emerge from depths below what is recommended and growers can err on the deep side if soil coverage is a concern.

 

Seedling Disease and Fungicide Considerations

Early season soybean disease concerns can include Pythium or Phytophthora species causing seed rot, damping off, or root rot in areas that are not well-drained.  Group 4 seed treatment fungicides will provide some protection against these species.  If soils are well-drained and planting conditions are optimal, disease caused by these pathogens is unlikely.

Pre-emergence seedling disease or post-emergence damping-off caused by Rhizoctonia solani is the most-commonly observed seedling disease in soybean in Louisiana (Figures 2 & 3).  Plants surviving the seedling stage may develop a root rot resulting in delayed development and stunting.  Stresses such as cold weather, nematode/insect infestation, or herbicide damage may exacerbate Rhizoctonia damping off.  In recent years, significant stand losses have been observed in Louisiana due to less-than-ideal planting conditions.  Seed treatments containing a strobilurin (Group 11) or SDHI (Group 7) compound are very effective at reducing incidence and severity of Rhizoctonia damping off.  The pathogen population, which is soilborne, may be reduced during long periods of flooding, high soil temperatures, or fallowing fields.  Potential for disease is greater in lighter soils, and optimal conditions for disease development are 75 to 90°F with 30 to 60% soil moisture, although the pathogen is capable of causing disease at lower temperatures and in any soil type.

Figure 2. Thin soybean stand as a result of Rhizoctonia solani.

Figure 3. Soybean seedling infected by Rhizoctonia solani.

In recent years, “base” fungicide seed treatments (usually consisting of metalaxyl/mefenoxam + at least one broad spectrum fungicide) are more-commonly found on soybean than in previous years.  In most cases “base” fungicide seed treatments are adequate at protecting seedlings under adverse growing conditions that are often encountered early during the planting window.  Results from many years of field research trials at multiple research stations in the state indicate that fungicide seed treatments will result in increased stand under moderate to severe disease pressure; however, realizing significant yield preservation and economic benefit in soybeans is the exception rather than the rule.  If your seed company does not offer a choice of seed treatments, the “base” offering likely will be sufficient for establishing a stand under tough conditions.  It is not necessary to over-treat base fungicides with additional fungicides in soybeans unless you are targeting a specific problem on your farm.  Also, it is important to specifically know which fungicides come on the seed as it is redundant to over treat with a fungicide having the same mode-of-action.  If seed companies offer “naked” seed, soybeans may be planted without fungicide seed treatment as long as you have no history of seedling disease issues, plant during the recommended window, achieve appropriate soil temperature and soil moisture, and schedule planting when the long term weather forecast is ideal for soybean development.  If you prefer to plant fungicide-treated seed, significant cost savings may be attainable by allowing distributors to over-treat or treating naked soybean seed yourself with a product of choice.

 

Early-Season Insect Pests and Insecticide Seed Treatment Decisions

One of the most important decisions producers must make when planting soybeans in Louisiana is planting date. Soybeans have the utility to be planted in early March to late June. This wide variation in planting dates exposes seedling soybeans to a multitude of insect pests that affect both above and below ground plant structures.

 

Optimal Seeding Dates in Louisiana by Maturity Group
Group III April 15 – May 10
Group IV April 15 – May 10
Group V March 25 – May 5
Group VI March 25 – April 30

 

Soybean seedlings possess an exceptional amount of vigor and can tolerate a substantial amount of insect injury during the seedling stage. However, early planted soybeans may also encounter greater amounts of environmental fluctuations that affect air and soil temperature. Cool conditions can negatively affect vigor and under the right conditions stall plant growth and development. The addition of insect injury, to the aforementioned mentioned environmental conditions, increases stress the plant encounters resulting in loss of stand and yield potential. Therefore, the inclusion of an insecticide seed treatment (IST) provides growers a risk management tool when soybeans are planted early.  The primary insect pests of early planted soybeans are bean leaf beetles, wireworms and grape colaspis.

On the opposite end of the spectrum are soybeans planted late, i.e. behind wheat or are late due to unforeseen circumstances such as inadequate or excessive soil moisture. These beans are more at risk for insect injury due to the potential for large insect populations to build in neighboring fields and generally more insects present in the environment. As a general rule with all agronomic crops, the later the crop the more insect pressure that will be encountered throughout the season.  This is particularly evident when soybeans are planted into wheat stubble. Wheat stubble is favorable for the development of threecornered alfalfa hoppers and thrips. Thus, an IST is a sound investment when soybeans are planted late.

However, soybeans planted in a timely manner, that being within the recommended planting window, under optimal soil conditions, and low pest densities will often not benefit from the addition of an IST.  Insecticide seed treatments typically produce the most benefits when environmental conditions are sub optimal as outlined in the prior paragraphs. With the current economic climate and many ag-professionals looking at areas to cut inputs, justifying the use of IST on soybeans when planted under optimal conditions becomes harder to support. Saving the cost of an IST can go to making a stink bug application later season that may provide a greater economic return.

In addition to early or late-plantings, there are other situations in which ISTs are justifiable. These include weedy fields with incomplete burn down applications, reduced tillage field arrangements, fields with historically problematic early insect pests (wireworms and/or threecornered alfalfa hoppers) and continuous plantings of one crop.  Each field is unique and the use of ISTs as a blanket treatment over every acre may not be justifiable with $8 soybeans.

 

Early-Season Soybean Weed Control Decisions

Data has shown that maintaining soybean weed-free for the first 5 weeks after emergence is required to maximize yield.  The best program to maintain soybean weed-free for 5 weeks is

 

  1. Apply a residual herbicide to the soil after planting before soybean emergence. This application is commonly called a preemergence or PRE application.  The choice of PRE herbicide depends upon the weed spectrum in the field, so call your LSU AgCenter parish agent or a weed scientist for help.  If small weeds are present at planting, tank-mix paraquat at 0.5 lb ai/A (32 oz/A of a 2 lb/gallon formulation or 21.3 oz/A of a 3 lb/gallon formulation) with the PRE residual herbicide to provide control.

 

  1. Apply a residual herbicide, such as Dual Magnum, Zidua, Zidua SC, Warrant, Prefix, or Warrant Ultra, at labeled rates tank-mixed with glyphosate or Liberty postemergence (POST) 2 to 3 ½ weeks after PRE application.

 

Farmers, consultants, and pesticide dealers often worry about injury to seedling soybean by a PRE herbicide and don’t want to use them.  I have evaluated PRE herbicides in soybean for the past 8 years and rarely have I observed a reduction soybean yield due to early-season herbicide injury.  Did some of these herbicides reduce soybean growth?  Yes, but when growing conditions are proper, yield most likely won’t be reduced.

Glyphosate-resistant Palmer amaranth and waterhemp, both pigweed species, can be found in virtually all Louisiana parishes where soybean is grown.  To manage resistant pigweeds, a herbicide program must contain residual herbicides.  Also, a herbicide program for resistance management must contain multiple modes of actions, meaning every herbicide applied is killing the weed in a different way.  If a weed isn’t killed by a herbicide application, the first thing to do is remove it from the field by pulling it up, then call us to help you figure out why the herbicide application didn’t work.

 

 

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.

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