Thoughts on Weed Management

Daniel Stephenson, Extension weed specialist

Corn harvest is underway in Louisiana, and I have received questions about controlling morningglory that could decrease harvest efficiency. The best option for this situation is an application of Aim at 1 to 2 ounces/A, paraquat at 0.25 to 0.5 lb/A or 1 gallon/A of a 6 lb/gal sodium chlorate. Tank-mix crop oil concentrate at 1 % v/v with Aim or 0.25% v/v of nonionic surfactant with paraquat. Most likely, these herbicides will be applied with an airplane, but it must be understood that Aim, paraquat and sodium chlorate efficacy is highly influenced by coverage — so the higher the GPA, the better. Also note that corn must have reached black layer (physiological maturity) before any of these products can be applied. Regardless of herbicide used, allow at least seven to 10 days before harvest so that morningglory will become brittle.

Soybean harvest has begun in some areas of Louisiana too. Typically, paraquat is used to desiccate soybeans. Always follow the paraquat label and be sure you have earned your paraquat-certificated applicator license. Additional herbicide choices for desiccation are Aim, Sharpen and sodium chlorate. As with paraquat, read the Aim, Sharpen and sodium chlorate labels prior to use. It has been my experience that tank-mixing paraquat at 0.25 lb and 1 ounce/A of Aim or Sharpen plus nonionic surfactant or crop oil concentrate provides good soybean desiccation and weed control.

Finally, please remember to physically remove any glyphosate-resistant Palmer amaranth or waterhemp still present in your fields. They should be easy to spot because they are most likely 1 to 2 feet taller than cotton and soybean canopies. Practicing a little sanitation now will save you a big dose of heartburn in the future.

Please call or email with questions at 318-308-7225 or dstephenson@agcenter.lsu.edu.

 

Entomology Update August

Sebe Brown and Tyler Towles, LSU AgCenter entomologists

Cotton

With much of Louisiana’s cotton approaching or beyond cutout, insect management decisions should be based on insects present in the field and protecting existing harvestable bolls. Once cutout (average of five nodes above white flower) is reached, growers and consultants can calculate the daily heat units (DD60s) from cutout and terminate insecticide applications accordingly. Fields that have accumulated 325 DD60s are safe from plant bugs while fields that have accumulated 350 DD60s are safe from first and second instar cotton bollworms. Fields accumulating 475 DD60s are protected from stink bugs.

Plant bugs have been persistent in many fields throughout the growing season, with insect numbers often reaching two to three times the threshold. Larger, more mature bolls are typically less susceptible to plant bug injury while smaller, less mature bolls may still be susceptible to adults and large nymphs. Overall, most of the harvestable bolls we now have should be safe from most plant bug injury, although adults and large immature plant bugs may still be a problem in later planted cotton. Therefore, plant bug treatment thresholds can be increased by 2.5, and small first and second instar nymphs can be omitted when determining insecticide applications.

Brown, green and southern green stink bug numbers will often increase as corn is harvested and the cotton crop matures. The Louisiana threshold for stink bugs in cotton is when one adult/nymph are found per 6 row feet, 5% adults/nymphs are in sweep nets or 15% to 20% of 12- to 16-day old bolls have internal injury. Late season applications of acephate plus pyrethroid, ULV malathion and Bidrin XPII give satisfactory control of stink bugs and plant bugs.

Soybeans

As Louisiana progresses into late summer, producers and consultants should look for late-season defoliators such as soybean loopers, velvet bean caterpillars and lingering populations of corn earworms.

Soybean loopers (SBL) have the ability to build large populations quickly and are exaggerated by the use of broad-spectrum insecticides for three-cornered alfalfa hoppers and stink bugs. The threshold for SBL in Louisiana is 150 worms in 100 sweeps or eight worms that are 1/2 inch long or longer per row foot. Because SBL are foliage feeders, adequate insecticide coverage is essential to limiting defoliation and reducing population numbers. Soybean loopers often initiate feeding in the lower portion of the canopy, defoliating soybean plants from the inside out. This cryptic behavior allows SBL to stay protected from some predators and insecticide applications in the dense canopy of soybean plants. Thus, good insecticide coverage is essential for optimal control of SBL. Once soybeans reach R6.5, yield is set and protection from soybean looper defoliation is no longer critical.

Velvetbean (VBC) caterpillars, like soybean loopers, can build large populations quickly and defoliate large portions of soybeans in a limited amount of time. The Louisiana threshold for VBC is eight worms that are at least 1/2 inch long per row foot or 300 worms in 100 sweeps. Unlike loopers, VBC are easily controlled with pyrethroids and applications for insects such as stink bugs effectively control this pest.

When making insecticide application decisions for caterpillar pests in soybeans, the insect species and numbers present and defoliation percentage should be taken into consideration. After bloom, soybeans can tolerate no more than 20% defoliation and not experience a significant yield loss.

Lastly, green, southern green, brown and redbanded stinkbugs are currently infesting Louisiana soybean. Action thresholds for brown, green and southern green stinkbugs are 36 per 100 sweeps or one per 6 row feet. However, the action threshold for the redbanded stink bug is 16 per 100 sweeps or one per row foot. The lower action threshold for the redbanded stink bug is due to the increased damaging nature of the pest compared to the other previously mentioned species.

 

Paraquat and Pre-certification Training

Kim Pope Brown, LSU AgCenter pesticide safety coordinator

It’s that time of year when producers are starting to desiccate soybeans to aid in harvesting. The product that many producers use is paraquat. As many of you should be aware, in the fall of 2019, paraquat received a new label requirement. Anyone who handles paraquat products must be a certified applicator and must successfully complete the new paraquat training course. Below are a few reminders for producers and applicators who are using or plan to use paraquat:

• Product may only be mixed, loaded or applied by certified applicators who have successfully completed the paraquat-specific training before use.
• Uncertified applicators can no longer use paraquat under the direct supervision of a certified applicator.
• Training can be accessed online through the “How to Safely Use and Handle Paraquat-Containing Pesticides” paraquat training module.
• Once completed, the training is good for three years.
• The training must be completed by the individual, and the individual must receive a 100% on the final assessment upon completion of the training. Multiple attempts can be made to score 100%.
• A certificate will be generated upon completion. Be sure to maintain this documentation for inspection.

With the changes to some of our product labels, farm crew members that have previously been able to work under the direct supervision of a certified applicator can no longer do so, depending on label requirements. The LSU AgCenter Pesticide Safety Education Program has put together a few opportunities to assist in test prep or pre-certification training for applicators who need a little assistance prior to taking the exam. At this time, we are offering virtual pre-certification training. Please see the dates and times in Table 1.

Table 1. Virtual Pre-Certification training dates and times.

Dates	Times
Sept. 2 to 3, 2020	1 to 3 p.m. each day
Oct. 5 to 6, 2020	1 to 3 p.m. each day
Nov. 2 to 3, 2020	1 to 3 p.m. each day

People who wish to participate must pre-register at the LSU AgCenter online store by 4 p.m. the business day prior to the selected event. Once you have registered, you will be provided a link via email that you will use to connect to the training event on both days of your selected event. This training will be in two parts to help ease the amount of time in front of a computer.

The training will take a total of four hours to complete. The cost of the training is $45. This is the link for registration.

If there is something more specific that you need in your area, please feel to contact Kim Brown or Bryan Gueltig with the LSU AgCenter Pesticide Safety Education Program for more information at kbrown@agcenter.lsu.edu or bgueltig@agcenter.lsu.edu.

 

Harvest Moisture and Its Effect on Corn Price

Michael A. Deliberto, LSU AgCenter economist

Two factors that can influence the price that a corn grower receives are the moisture content of the crop at harvest and the associated drying costs to reach a desired moisture content specified by the local elevator. Collectively, these factors can be considered harvest losses expressed as a portion of the price per bushel. Estimating these two factors against the projected price received are key determinants at to when a corn grower might begin their corn harvest operations.

The first factor associated with corn harvest losses relates to the removal of moisture from grain during the drying process that causes a reduction in grain quality, referred to as moisture shrink. For example, assume that the initial moisture content is 25% and the final desired moisture content is 15%. Using the aforementioned equation, moisture shrink (%) is calculated to be 11.76%. The second factor considers the cost of drying. Grain drying costs are based on either dry or wet grain and can be estimated with the following equations expressed as dollars per dry bushel.

Grain elevators often charge corn growers a per bushel fee to dry grain based on the moisture level and their costs of running and maintaining drying equipment at the facility.

Figure 1 shows a chart of the shrinkage and drying costs based on the corn moisture at harvest and the costs of propane and electricity for corn priced at $4.00 per bushel. Assuming that an elevator prefers to accept corn at 15% moisture, a grower can infer from the figure that harvesting corn at higher moisture content can negatively affect the price received. Meaning that a moisture levels of 23%, the price risk is $0.83 cents ($0.38 from shrink and $0.46 for drying charges) from the original $4.00 corn price (net price of $3.17). As the moisture content of the corn is reduced, the grower price increases.

Figure 1. The shrinkage and drying costs based on the corn moisture at harvest and the costs of propane and electricity for corn priced at $4.00 per bushel.

A decision tool was developed by the LSU AgCenter through which a producer — by inputting their initial expected price of corn at harvest, estimated harvest and target moisture levels, price per gallon for liquified petroleum gas and price per KWH for electricity — can calculate their risks or potential losses associated with moisture, shrinkage and price risk. The user-specified decision tool also contains a worksheet that allows a corn grower to enter their initial moisture content and the final desired moisture content so that the total cost per bushel can be calculated on a per bushel basis. Cells containing blue font in both worksheets can be changed by the grower. The grower can also enter their gas and electricity costs to estimate the drying costs. To access a detailed report and accompanying decision tool, please visit the LSU AgCenter corn production webpage.

Appreciation is extended to the Louisiana Soybean and Feed Grain Research and Promotion Board for their support of this applied farm management research.

This graph shows the effect that higher moisture content has on the corn price a grower received. At higher moisture levels, greater that 15%, the corn price is reduced, assuming the desired level is 15%.

The effect that higher moisture content has on corn price. At higher moisture levels, greater than 15%, the corn price is reduced, assuming the desired level is 15%.

 

Update on the Current Status of the Invasive Guava Root-knot Nematode in Louisiana

Tristan Watson, LSU AgCenter nematologist, and Josielle Rezende, LSU AgCenter plant pathologist

Plant-parasitic nematodes are semi-microscopic roundworms that feed on plants, often resulting in yield loss on economically important crops. There are a range of plant-parasitic nematode species that are fairly common in Louisiana production fields, including southern root-knot nematode (Meloidogyne incognita) and reniform nematode (Rotylenchulus reniformis). Recently, a new nematode, known commonly as guava root-knot nematode (Meloidogyne enterolobii), has been introduced into Louisiana on contaminated sweetpotato planting material from North Carolina. Guava root-knot nematode is potentially a serious problem for Louisiana growers due to the wide host range of this pest, the devastating crop damage it can cause, and the ability of this pest to overcome nematode resistance in commercially available crop varieties. On many crops, feeding by guava root-knot nematode results in the formation of large, spherical growths (i.e. root knots) on the root system. Unlike the more common southern root-knot nematode, infestation by the invasive guava root-knot nematode can cause total crop loss in a production field.

The guava root-knot nematode was introduced into Morehouse Parish, Louisiana, in 2018 via contaminated storage roots imported from North Carolina. In 2020, the guava root-knot nematode was detected in Franklin Parish, Louisiana, in a shipment of certified pest-free sweet potato planting material. Although this nematode has been introduced twice, our 2019 soil survey results indicated that this nematode has not yet established in Louisiana soils. The guava root-knot nematode soil survey will continue during the 2020 growing season. We encourage any growers experiencing nematode symptoms characteristic of root-knot nematode (i.e. knots in root tissue; Figure 1) to submit soil and root samples collected from symptomatic fields (Figure 2) to the LSU AgCenter Nematode Advisory Service for free diagnosis. Included here is a link to the LSU AgCenter Nematode Advisory Service root-knot nematode survey form and a guide on how to sample fields for nematodes.

Figure 1. Root damage caused by root-knot nematode (Meloidogyne spp.) on (A) cotton, (B) soybean, (C) cucumber and (D) sweet potato. Notice the presence of numerous large, spherical knots or bumps on root tissue characteristic of nematode parasitism.

Figure 2. Crop damage caused by root-knot nematode ( Meloidogyne spp.) on (A) cotton and (B) soybeans. Notice the circular patch of stunted plant growth characteristic of a field infested with this nematode.

 

LSU AgCenter Variety Testing

David Moseley, LSU AgCenter soybean specialist, and Trey Price, LSU AgCenter plant pathologist

Selecting a soybean variety is one of the most important decisions a producer can make to have a successful season. To help Louisiana soybean producers select the most suitable variety, the LSU AgCenter conducts an Official Variety Trial (OVT) and Core-block Demonstration Plots.

Official Variety Trial

The 2020 OVT consist of 158 varieties entered by 16 seed companies and three university soybean breeding programs. The varieties consist of several different herbicide technologies, and the maturity groups range from 3.7 to 6.0. The trial is replicated at seven research stations across the state in different soil types including fine sandy loam, silt loam, silty clay and clay. At each location, the varieties are replicated four times (Figure 1). Because of varying disease pressure across the state, the trial is over-treated with fungicides at some locations while at others, it is not.

Figure 1. The 2020 official variety trial at the Dean Lee Research Station showing growth differences between varieties.

Core-Block Demonstration Plots

In addition to the OVT, the LSU AgCenter collaborates with soybean producers to evaluate soybean varieties directly on farms. For these core-block demonstration plots, LSU AgCenter parish agents cooperate with the producers to plant, maintain and harvest strip trials submitted by seed companies and university soybean breeding programs. These demonstrations provide valuable yield data from local growing conditions and agronomic practices. In some cases, observations from these large plots can result in identification of varieties that are resistant to a number of soilborne maladies.

In 2020, 11 seed companies and two university soybean breeding programs submitted varieties to be evaluated in the core-block demonstrations. Eighteen demonstrations were planted across 11 parishes (Figure 2). The demonstrations were divided by maturity group (MG). A demonstration consisted of varieties with a MG of 3.7 to 4.4; 4.5 to 4.9; or 5.0 to 5.6. The number of varieties submitted for each MG were seven (MG 3.7 to 4.4), 20 (MG 4.5 to 4.9) and 16 (MG 5.0 to 5.6).

Figure 2. Eighteen core-block demonstrations across 11 parishes.

Variety Testing Results

The yield data from the OVT and core-block demonstrations will be published by the LSU AgCenter in the annual soybean variety testing summary. Maturity date, height, lodging and disease reaction information from the OVT also will be included. The 2020 OVT results will be published following harvest to assist with 2021 variety selections and planting decisions. The variety publication for the 2020 growing season can be found at 2020 Soybean Variety Yields and Production Practices.

When choosing a variety, it is important to consider performance and stability. A producer should evaluate varieties that perform the best in an environment similar to their own and varieties that perform well over multiple environments. When possible, variety performance over multiple environments and multiple years should be considered.

Specialized Variety Trials

When time and resources allow, specialists may plant specialized variety trials aimed at solving one specific problem. Such a trial was planted this year at the Macon Ridge Research Station in Winnsboro. Essentially, a scaled-down (two-row plots) copy of the official variety trial was inoculated with Xylaria sp. in an effort to identify commercial sources of resistance to taproot decline (Figure 3). Row 1 was inoculated at planting, while Row 2 was subjected to natural infestation (Figure 4). The location has not been tilled and has been in soybean monoculture for many years. The trial has been rated multiple times, and results will be provided to stakeholders in a timely manner this fall and winter.

Figure 3. Soybean plants showing interveinal chlorosis caused by taproot decline at the R2 growth stage after inoculation at planting.

Figure 4. A soybean plot (R5.5) from the 2020 Specialized Variety Trial (taproot decline) at the Macon Ridge Research Station. Non-inoculated (left) vs. inoculated (right).

 

Soil Sampling and Testing

Rasel Parvej, Brenda Tubana, and Jim Wang, LSU AgCenter soil scientists

Soil Sampling:

• Soil should be tested at least once in every two to four years, or once in a complete crop rotation.
• Soil sample should be taken at the same time of each sampling year and at a constant depth of 0 to 6 inches with a soil probe or auger.
• At least one sample should be taken for every 10 acres of land for zone sampling and 2.5 acres of land for grid sampling. Both zone and grid sizes depend on spatial variability of the field. More soil samples are needed per unit area for highly variable fields. Therefore, soil type and color, past management, and yield map should be considered to determine the actual zone and grid sizes.
• Each soil sample should consist of 15 to 20 subsamples (i.e. two to three subsamples per acre for zone sampling and six to eight subsamples per acre for grid sampling). However, more subsamples are needed for fields that received fertilizer banding and/or manure spreading in the past. Subsamples should be taken in a zigzag pattern within each zone or grid.
• Scrape the thin layer of soil surface before inserting the soil probe or auger. When furrow irrigation is used, samples should be taken from the shoulder of the bed. However, for any field, samples should not be taken from fertilizer bands, manure or lime stockpiles, wet spots, fence rows and areas that are too small to manage separately. All subsamples should be mixed thoroughly in a clean plastic bucket. Remove stones, roots, stems, trash and other debris from the mixed soil samples.
• Each soil sample should be placed in a separate plastic bag with clear labelling that includes farm name and location, sampling date and depth, previous crop, and expected crop to be grown and sent immediately to the soil testing lab for routine soil analysis.

Soil Testing:

• Soil samples should be tested in a certified soil lab, such as the LSU AgCenter Soil Testing and Plant Analysis Lab in Baton Rouge, that uses appropriate soil extraction methods. Soil-test-based fertilizer recommendations in Louisiana are based on Mehlich-3 soil exaction method.
• Soil samples should be analyzed in the same lab each year to create a historical record.

 

Soil-test-based Fertilizer Recommendations and Fertilizer Application

Rasel Parvej, Brenda Tubana, and Jim Wang, LSU AgCenter soil scientists

Soil Test Interpretation and Fertilizer Recommendations:

• Soil test results should be interpreted based on the critical soil test nutrient concentration. The critical concentration is defined as the soil test nutrient concentration below which crop response to added fertilizer is expected and above which is unlikely. Critical nutrient concentration varies with crops, soil types and U.S. state. Therefore, soil test results should be interpreted with crop, soil type and state-specific critical nutrient concentrations that are derived from correlation and calibration research. Usually, soil scientists from every land-grant university develop their own critical soil test nutrient concentrations for each crop of that state. So, it is best to analyze soil samples in the state soil testing lab.
• Fertilizer recommendations should be based on critical soil test nutrient concentration and fertilization philosophies. There are three main fertilization philosophies: sufficiency, buildup and maintenance, and cation saturation ratio. In the sufficiency approach — used by most land-grant universities, including the LSU AgCenter — fertilization is only recommended if the soil test nutrient level is at or below the critical level and the fertilizer rate is determined based on expected crop yield increase. This approach is called “fertilize the crop.”
  • In the buildup and maintenance approach, also known as “fertilize the soil,” fertilization is almost always recommended unless the soil test level is very high. The buildup part of this approach is used for soils with nutrient concentration below the critical level and the fertilizer rate is determined based on sufficiency rate plus some extra fertilizer to raise the soil test nutrient concentration above the critical level. The maintenance part is used for soils with nutrient concentration above the critical level and the fertilizer rate is determined based on the expected nutrient removal rate by the crop to maintain soil test nutrient concentration at the same level.
  • The cation saturation ratio is not very accurate or economic for fertilizer recommends. In this approach, fertilizer is recommended based on the cation ratio of mainly calcium (Ca), magnesium (Mg) and potassium (K) on the cation exchange site. The most used ratio is 65% Ca, 10% Mg, 5% K and 20% others.
• Care should be taken using the buildup and maintenance philosophy for K fertilization in coarse-textured soils with low cation exchange capacity (CEC <10) such as loamy sand and silt loam soils. Potassium is highly prone to leach down in the soil profile with excessive rainfall in low CEC soils. Building up the soil test K level in coarse-textured, low-CEC soils may not be feasible or economical. Please visit the LSU AgCenter website for detailed soil-test-based fertilizer recommendations for each crop.

Fertilizer Application:

• Fertilizer recommendations based on soil tests mainly include phosphorus (P) and K, which can be applied in the fall, especially for fine-textured soils with a high CEC (>20). For coarse-textured, low-CEC soils, it is better to apply all fertilizers in the spring or at planting. There is a misconception about spring application of P (triple super phosphate) and K (potash) that both fertilizers require a long time to dissolve and become available for plant uptake. Many studies have shown that a spring application of both fertilizers is better than a fall application in increasing crop yield, especially for soils that are highly prone to nutrient loss via leaching, runoff and erosion.
• Because both P and K are highly immobile in soils, both fertilizers should be placed near the root zone (on the top of the bed for furrow irrigation systems) and incorporated with shallow tillage for tilth fields. Banding of P fertilizer is very effective for both acidic (pH <5.5) and alkaline (pH >7.5) soils because P availability is greatly affected by soil pH.

 

Soil pH, Liming, and Liming Materials

Rasel Parvej, Brenda Tubana, and Jim Wang, LSU AgCenter soil scientists

Soil pH and Liming:

• Soil pH is the most important soil quality component that greatly influences soil nutrient availability. Most nutrients are highly available at a soil pH of 6.5. Therefore, soil pH needs to be adjusted to the target pH either by applying lime for low pH (<6.0) soils or by elemental sulfur for high pH (>7.5) soils. Increasing soil pH by liming is a more common practice than decreasing soil pH with elemental sulfur.
• The rate of lime depends on the initial and target soil pH and the buffering capacity of the soil (buffer pH, ability of a soil to resist the change of pH). If the soil buffering capacity and the difference between initial and target soil pH are low, lime rate would be low. However, for soils with high buffering capacity (low buffer pH), lime rate would be high even for a small change of soil pH. Clay soils have higher buffering capacity and require a greater amount of lime for each unit increase of soil pH than silt loam soils. Note that LSU AgCenter Soil Testing and Plant Analysis Lab does not run buffer pH but does indicate the unit change of soil pH with the addition of maximum 3 tons of lime, thus allowing farmers decide how much they would like to spend, assuming higher than 3 tons lime may be too expensive.
• The target soil pH should be determined based on the crop to be grown. For example, soybeans are more sensitive to low soil pH than corn and cotton. The target soil pH should be set at 6.3 for soybeans and 6.0 for corn and cotton. Lime is required if the target soil pH is 0.2 unit more than actual soil pH.
• Lime takes at least six to nine months, depending on liming materials, to react with the soils and raise soil pH. Therefore, lime should be applied uniformly and incorporated by tillage in the fall.

Liming Materials:

• The quality of liming materials is very important to raise soil pH. There are two qualities of liming materials: purity and particle size. The purity of a liming material is determined in relation to pure calcium carbonate (CaCO₃), calcitic limestone, which is rated as 100% (molecular weight of pure calcium carbonate is 100) and this rating is called calcium carbonate equivalent (CCE). The rate of lime recommended by soil testing labs is based on pure calcitic limestone with 100% CCE. So, the actual lime application rate should be adjusted based on the CCE of the liming materials. For example, if the CCE of the liming material is 80% and the recommendation is 2 tons of lime per acre, 2.5 tons of lime (2 tons/0.80) per acre should be applied.
• Particle size is the fineness factor of liming material and is expressed as the percentage of liming material passes through various sized screens. The higher the percentage of liming material passes through the larger size screen (i.e. smaller hole), the greater the fineness factor would be. Finer particles are more efficient in neutralizing soil acidity (increasing soil pH) by reacting quickly with soils due to greater surface area or soil contact. However, the liming materials should have a good distribution of particle sizes with both smaller and larger particles so that smaller particles can raise the soil pH quickly and larger particles can have a long-term control in neutralizing soil acidity. According to current Louisiana recommendations for ground lime, 90% of liming materials should pass through a 10-mesh sieve, 50% should pass through a 60-mesh sieve, and 20% should pass through a 100-mesh sieve.
• Both purity (CCE) and particle size (fineness factor) of the liming material are expressed together as effective CCE (ECCE) or effective neutralizing value (ENV). The higher the ECCE or EVN of the liming material the more efficient it is in increasing soil pH. Like CCE, the actual lime rate also needs to be adjusted with the ENV of the liming material if the recommendations are based on ENV. For example, if the ENV of the liming material is 60%, but the recommended lime rate is based on standard calcium carbonate with 90% ENV, 1.5 tons (0.9/0.6) of lime should be applied for every 1 ton of lime recommended. Note that the lime recommendations from LSU AgCenter Soil Testing and Plant Analysis Lab is based on 50% ECCE or ENV.