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Soybean Nutrient Profile: Phosphorus

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We are already on our third installation of the soybean nutrient profile series.  Click on the link below to see this week’s featured nutrient, phosphorus.  This nutrient profile is a part of 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 thirteen 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: Phosphorus

Check back weekly to find the newest soybean nutrient profile.  You can also find the links to the two previous nutrient profiles below.  Contact your local extension agent with any questions you may have. 

Soybean Nutrient Profile: Potassium

Soybean Nutrient Profile: Manganese

 

Soybean Nutrient Profile: Potassium

Soybean Nutrient Profile: Potassium published on No Comments on Soybean Nutrient Profile: Potassium

Click the link below to find the second installation of the soybean nutrient profile series featuring potassium.  This nutrient profile is a part of 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 thirteen 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: Potassium

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

Soybean Nutrient Profile: Manganese

Soybean Nutrient Profile: Manganese published on No Comments on Soybean Nutrient Profile: Manganese

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

Successful soybean production demands proper early-season decisions published on No Comments on Successful soybean production demands proper early-season decisions

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.

 

 

Fall and Spring Burndown Considerations

Fall and Spring Burndown Considerations published on No Comments on Fall and Spring Burndown Considerations

Fall and Spring Burndown Considerations

Josh Copes, Daniel Stephenson, and Donnie Miller

 

This time of year, especially when it is dry, brings about questions concerning a fall burndown application that contains a residual herbicide.  As with any field operation, fall burndown should provide a monetary benefit.  A lot of time and money is spent after harvest preparing fields for planting next spring.  Producers should realize that some fall-applied residual herbicides will provide control of most winter annuals which may result in excess soil and bed erosion.  So, you may be asking why apply a fall burndown?  If glyphosate-resistant Italian ryegrass along with some other winter annual weeds like henbit are an issue, research has shown that a fall residual herbicide application will provide good to excellent control the following spring.  The table below is a glyphosate-resistant Italian ryegrass weed control program developed by Mississippi State University weed scientists that LSU AgCenter weed scientists have adopted.  The fall-applied herbicides listed in the table will provide some control of henbit as well.

To combat glyphosate-resistant Italian ryegrass, many producers tank-mix Select Max/clethodim in their spring burndown.  Unfortunately, several phone calls were received this past spring concerning Italian ryegrass control failures following Select Max/clethodim application.  If controlling Italian ryegrass has been an issue and control failures with clethodim products has occurred, one of the Fall programs in the Table 1 should be utilized.

Regardless of the crop planted in the spring, the LSU AgCenter suggest applying a spring burndown 4 to 6 weeks prior to planting.  This gives plenty of time for weeds to die and break the “green bridge” and for soil residual herbicides, if applied, will keep the fields weed free until planting.  If planting corn, a soil residual herbicide applied in the fall may provide weed control until corn planting time.  Therefore, a fall residual herbicide can pay and keep the field weed free until planting.  If you plan on planting soybeans or cotton however, a fall-applied soil residual herbicide may not be the best choice.  In some years, soil residual herbicides can provide weed control up to 120 days.  If applied in early-December, you could expect good weed control until mid to late March.  We must remember that if a fall-applied residual herbicide is applied in December with the hopes of skipping a spring burndown, you are making a mistake.  Don’t assume a fall-applied residual herbicide will hold through spring.  Fields must be scouted.

When burning down fields near planting, herbicide selection and rate and spray coverage are very important to ensure complete control.  If mare’s-tail, henbit, cutleaf evening-primrose, and sowthistle are present, be sure to apply 2,4-D at 1 lb of acid equivalent per acre.  In many instances, when weed control is unsatisfactory from spring burndown, it is because 2,4-D rates that were too low or 2,4-D was not applied at all.

The main goal of any burndown operation is to be weed free at the time of planting.  Choice of fall or spring burndown will depend on what crop you intend to plant, if Italian ryegrass an issue, and if soil and bed erosion a problem.  Spending more money than necessary is a big concern when deciding on burndown.

 

Glyphosate-resistant Italian ryegrass control recommendations. Adopted from Mississippi State University.

 Crop Fall Winter Spring
Corn Dual Magnum @ 1.33 pt/A or Zidua @ 2.5 oz/A double disk Select Max @ 12-16 oz/A or equivalent rate of 2 lb clethodim formulation Paraquat @ 0.75-1.0 lb of a.i. or two applications 10-14 days apart
Cotton Dual Magnum @ 1.33 pt/A or

trifluralin @ 3 pt/A or double disk

Select Max @ 12-16 oz/A or equivalent rate of 2 lb clethodim formulation Paraquat @ 0.75-1.0 lb of a.i. or two applications 10-14 days apart
Soybean Dual Magnum @ 1.33 pt/A or

Boundary @ 2 pt/A or trifluralin @ 3 pt/A or double disk

Select Max @ 12-16 oz/A or equivalent rate of 2 lb clethodim formulation Paraquat @ 0.75-1.0 lb of a.i. or two applications 10-14 days apart
Rice Command @ 2 pt/A or double disk Select Max @ 12-16 oz/A or equivalent rate of 2 lb clethodim formulation Paraquat @ 0.75-1.0 lb of a.i. or two applications 10-14 days apart

http://mafes.msstate.edu/publications/information-sheets/i1359.pdf

 

Please feel free to contact us with any concerns or questions.

 

Josh Copes

Cell: 318-334-0401

Office: 318-766-4607

jcopes@agcenter.lsu.edu

 

Donnie Miller

Cell: 318-334-0401

Office: 318-766-4607

dmiller@agcenter.lsu.edu

 

Daniel Stephenson

Cell: 318-308-7225

Office: 318-473-6590

dstephenson@agcenter.lsu.edu

2017 Louisiana Rice Acreage Survey Results

2017 Louisiana Rice Acreage Survey Results published on No Comments on 2017 Louisiana Rice Acreage Survey Results
(click to view results on AgCenter Website)

The results of the 2017 Louisiana Rice Variety Survey are now available online. The survey contains information on:

  • Total rice production in each parish and the percentage of long, medium, and special purpose rice
  • Breakdown of long grain, medium grain, and special purpose varieties and hybrids grown
  • Ratoon and conservation tillage practices used in each parish
  • Planting and water management cultural practices used by farming operations in each parish
  • Various maps and figures summarizing the findings

This survey is done annually by Extension Agents in each of the rice producing parishes by contacting their rice producers and seed distributors. In many cases, the survey results may not exactly match FSA certified planted acres. Deviations can be the result of many factors. The most common factors include large farming operations which farm in multiple parishes only reporting in one parish. Included in the data this year for the first time is water management practices including furrow irrigated rice (row rice) and alternate wetting and drying (AWD). Furrow irrigated rice management was reported in only two Louisiana Parishes with a total of 1,400 acres. The individual survey results, maps and figures are available on the LSU AgCenter Website (individual survey results). All of the data combined into one document can be found here (Complete Results Document). A big thank you to all of the Extension Agents who help make this survey possible!

Weeds: Problematic Year-Round

Weeds: Problematic Year-Round published on No Comments on Weeds: Problematic Year-Round

Weeds: Problematic Year-Round

Josh Copes, Daniel Stephenson, Donnie Miller, and Lauren Lazaro

 

Prolonged rains coupled with the high temperatures during August delayed harvest, caused crop damage, and environmental conditions were optimal for weed growth. We have received several phone calls concerning weeds requiring a herbicide application to better facilitate harvest. Once the crop dries down, weeds will begin to receive adequate sunlight allowing for rapid growth and development. If harvest is delayed for too long weeds, in particular vines, will quickly limit harvest efficiency. Paraquat (1 to 2 pints/acre), Aim (1 to 2 oz/acre), and sodium chlorate (4.8 quarts/acre) are labeled as harvest aids in corn. Labels require 7, 3, and 14 days for paraquat, Aim, and soidium chlorate, respectively, between application and harvest. Seven days or more will be required for adequate weed desiccation. Maximum water volume (gallons of water per acre) should be utilized as large weed size and growth habit within and on top of crop will limit herbicide coverage and desiccation efficacy. After the weeds have dried sufficiently to allow for harvest (and label requirements have passed), harvest as soon as possible to reduce the risk of weed re-growth.

Calls have also been received regarding control options for weeds post-harvest. Earlier harvest trends have resulted in adequate time for weeds to set seed between harvest and a killing frost. This time period can range from 1 to 4 months. The average first frost date in North and Central Louisiana is November 15 and 25, respectively. Since a lot of money and effort is spent in controlling weeds during the growing season to negate yield loss, timely weed control practices following harvest is important. These practices can reduce weed seed return to the soil seedbank, thus ensuring fewer weeds to fight in future cropping seasons. Post-harvest weed control is especially important in fields containing herbicide resistant weeds. A good example to illustrate the importance of post-harvest weed management is the ability of glyphosate-resistant Palmer amaranth to produce mature seed in as little as 30 days after emergence during late summer and early fall. Many other grass and broadleaf weeds are capable of setting viable seed in a similar time frame.

For weeds that are present in the field at harvest time, mowing and/or tillage should be conducted as soon as possible upon harvest to ensure viable seed set is reduced. Rainfall will influence subsequent germination of weed seed and therefore the need for additional weed control. Furthermore, rainfall following cultivation could increase weed seed germination, however, if the weeds are controlled, the soil seedbank would be reduced. Producers in no-till systems will have to rely on mowing and herbicides to prevent weed seed production.

Other methods of weed control include the use of herbicides. Herbicide applications should be targeted from late-September through October when the time period from application to first killing frost is shortened. Multiple herbicide applications for post-harvest control of summer annual weeds should be avoided. Residual herbicides such as S-metolachlor, pyroxasulfone, linuron, and diuron, among others, can be applied in the fall following harvest. However, rotation interval restrictions must be followed and length of residual control will be influenced by soil temperature and saturation. Glyphosate plus 2,4-D and/or dicamba or paraquat plus diuron and/or linuron are some choices for late-fall post-harvest applications. Diuron and linuron will offer soil residual; however, if soil temperatures are warm and rainfall frequent, do not expect long residual from these products. Likewise the lack of rainfall to properly activate residual herbicides to minimize weed germination can negatively impact treatment effectiveness.  Maximize water volume to ensure good weed coverage, as this is critical for good weed control, especially for paraquat plus diuron and/or linuron.

To reiterate, some weeds are capable of setting viable seed within 30 days after emergence during late summer and early fall. Post-harvest weed control is especially important when combatting glyphosate-resistant weeds such as Palmer amaranth, waterhemp, or johnsongrass. Problem fields should be identified and receive top priority for preventing seed return to the soil seedbank. Once harvested these problem fields should be mowed or tilled shortly after harvest to prevent and/or reduce seed set. Fields should then be regularly scouted for emerging weeds and additional control tactics applied prior to seed set. This will require close inspection of weed species to determine when they are flowering. Once a weed species is observed flowering a weed control operation should be implemented. Depending on weather conditions following harvest, weed control tactics may need to be implemented approximately every 3 to 4 weeks until a killing frost has occurred. If glyphosate-resistant Palmer amaranth or waterhemp is an issue, a management tactic (i.e. mowing, tillage, herbicide application) should be employed every 3 to 4 weeks.

Fall herbicide applications can be made for control of perennial weed species such as johnsongrass, bermudagrass, alligatorweed, and redvine. Studies conducted by LSU AgCenter weed scientist have determined that fall applications should be made from September 15 to October 15 when environmental conditions favor weed growth (http://www.lsuagcenter.com/portals/communications/publications/agmag/archive/2006/summer/longterm-management-of-perennial-weeds-starts-in-the-fall). For johnsongrass, bermudagrass and alligatorweed control, 1.0 lb ai/acre of glyphosate should be applied. Two lb ai/acre of glyphosate or dicamba are effective control options for redvine. Glyphosate (2.0 lb ai/acre) plus dicamba (1.0 lb ai/acre) can also be an effective control option. Fields should be scouted the fall following herbicide application to determine whether an additional application is needed. Do not mow or till fields for several weeks following herbicide application.

If you have any questions please contact us.

Josh Copes

Cell: 318-334-0401

Office: 318-766-4607

jcopes@agcenter.lsu.edu

 

Donnie Miller

Cell: 318-614-4044

Office: 318-766-4607

dmiller@agcenter.lsu.edu

 

Daniel Stephenson

Cell: 318-308-7225

Office: 318-473-6590

dstephenson@agcenter.lsu.edu

 

Lauren Lazaro

Cell: 210-562-0878

Office: 225-578-2724

llazaro@agcenter.lsu.edu

Louisiana Rice Notes – March 30, 2017

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This is the third installment of the Louisiana Rice Notes newsletter for  2017.  This edition covers planting progress and the quick start to the rice season in southwest Louisiana, accumulated DD50 heat units so far, rice seedling development, the importance of Clearfield Stewardship Guidelines, starter N fertilizer guidelines, and planning your 2017 disease management program. This edition can also be found on the LSU AgCenter’r rice website (click here to view).

Mitigating and/or Managing Herbicide-Resistant Weeds

Mitigating and/or Managing Herbicide-Resistant Weeds published on No Comments on Mitigating and/or Managing Herbicide-Resistant Weeds

Drs. Daniel Stephenson and Josh Copes

LSU AgCenter

 

Herbicide-resistant weeds, especially glyphosate-resistance, is not a new topic.  Glyphosate-resistant (GR) Palmer amaranth was documented in Louisiana in 2010.  GR waterhemp was documented in 2015.  As of today, glyphosate-resistant Palmer amaranth can be found in virtually every row crop parish in Louisiana.  I’m not saying it has infested every field in every parish, but those fields having infestations range from a few plants to an extreme number of plants.  GR johnsongrass and Italian ryegrass have been documented too.  Although we haven’t officially documented GR horseweed (mare’s-tail) in Louisiana, I am certain it infests many acres in northern Louisiana.  Therefore, Louisiana producers must implement strategies to mitigate and/or manage this extremely troublesome pest.

 

In 2015, the LSU AgCenter published an extension publication entitled “Herbicide Programs for Managing Glyphosate-Resistant Palmer Amaranth and Common Waterhemp in Louisiana Corn, Cotton, and Soybean”.  It can be found at http://www.lsuagcenter.com/~/media/system/c/7/5/a/c75a63bba3f758d391b8c91871076ba6/pub3522herbicideprogramsformanagingglyphosateresis.pdf.  This publication provides suggested programs that can help mitigate and/or manage glyphosate-resistant pigweeds.  In addition, if these programs are implemented, they offer control of many if not all of the other grass and broadleaf weeds Louisiana crop producers deal with every year.

 

I’m not going to discuss the programs in this article in depth, so I ask that you view the document.  If you have questions, please call.  However, I will highlight the main focus of all programs.  The main, primary, essential thing to remember in designing a program to manage glyphosate-resistant weeds is residual herbicides.  Let’s break it down.

Step 1:  It is crucial for producers to apply a residual herbicide just prior to planting, at planting, or preemergence.  Paraquat at 0.5 to 1 lb ai/A (i.e 1 to 2 quarts/A of Gramoxome SL or 0.67 to 1.33 quarts of a generic 3 lb ai/gal paraquat) needs to be tank-mixed with this preemergence residual herbicide to kill any emerged weeds to ensure that the crop emerges in a weed-free seedbed.  All the residual herbicides listed in the preemergence section of the publication will provide residual control of pigweed following proper activation.  The choice of preemergence herbicide depends upon other weed species found in the field.  I won’t go into each different situation, so please call us to discuss if needed.

Step 2:  The next crucial step is to apply a residual herbicide tank-mixed with a non-selective herbicide 3 to 4 weeks after planting.  Examples of herbicides that offer residual control when applied POST are Dual Magnum (or many generics at proper rates), Prefix, Warrant, or Zidua.  Tank-mix one of them with glyphosate in Roundup Ready crops or with Liberty in Liberty Link crops.

 

Implementing steps 1 and 2 overlays residual herbicides during the early growing season, which protects the crop from early season competition.  The best time to kill a pigweed is when it is emerging or when it is very small (less than 3-inches).  Residual herbicides will kill the pigweed as it germinates or while it is emerging.  In addition, research has shown that maintaining soybean weed-free for the first five weeks after emergence maximizes yield, assuming proper growing conditions and insects/diseases are managed.

 

Between burndown and planting, pigweed and other weeds could emerge and reach heights too large to kill with an at-planting application of any labeled non-selective herbicide, specifically paraquat.  This situation usually occurs in fields that received a burndown application greater than 4 weeks prior to planting or when a burndown application didn’t include a residual herbicide.  Remember, Palmer amaranth has the potential to grow one-inch in height per day.  Therefore, it is critical that emerged Palmer amaranth or any other weed species be controlled when they are small with either tillage or a non-selective herbicide before planting.  Tank-mixing a residual herbicide with this preplant application will help to maintain your field weed-free up to planting.  However, do not think that applying a residual herbicide weeks prior to planting will be sufficient for residual control in-crop.  A preemergence residual herbicide will still be needed to maintain the crop weed-free until the first postemergence application.

 

In many states to our north, PPO-resistant Palmer amaranth and waterhemp have been documented.  To date, the LSU AgCenter has not documented any PPO-resistant pigweed in Louisiana.  However, we are screening some populations, so the potential for this is there.  You are probably wondering what are PPO’s?  PPO-inhibiting herbicides include Valor, Envive, Enlite, Valor XLT, Rowel, Rowel FX, all the Authority products, BroadAxe, Prefix, Flexstar, Flexstar GT, Reflex, Cobra, Ultra Blazer, Resource, ET, Cadet, and many more.  Honestly, this worries me as a weed scientist more than glyphosate resistance!  In Louisiana, the most common weed is morningglory.  Producers historically rely upon one of these herbicides to control morningglory.  It was always a big positive that they controlled pigweed and other weeds such as hemp sesbania, sicklepod, Texasweed, smellmelon, and others too.  We all should remember the articles in popular press articles showing the devastating effects of uncontrolled Palmer amaranth on a crop.  Imagine spraying a PPO-inhibiting herbicide for morningglory, hemp sesbania, AND Palmer amaranth control and you get little to no control of pigweed.  In this situation, I would have no suggestion for a herbicide application to help you.  In the presence of glyphosate and PPO-resistance Palmer amaranth, we will still have products that contain Dual Magum and other metolachlor products, metribuzin and products that contain it, Zidua, Warrant, Classic, and Liberty, but use of only these products would severely limit a producers ability to effectively manage herbicide-resistant weeds and all the numerous weed species Louisiana producers struggle with.  I’m not trying to be “chicken little” and claim the sky is falling.  I just want the reader to understand that this isn’t something to play with and a plan should be developed and implemented to prevent it.

 

Use of residual herbicides before crop emergence and in the first postemergence application is vital for weed management in Louisiana corn, cotton, and soybean.  In cotton, a residual herbicide is most likely needed in the second postemergence application too.  To mitigate and/or manage glyphosate-resistance and/or PPO-resistance, we have to use residual herbicides, rotate crops, tank-mix multiple herbicidal modes of action in a single application, don’t use similar herbicidal modes of action every year, and if you see a weed that should have died after application, go pull it up and burn it.  Those steps will help in the fight against herbicide resistance.  If you have any questions, please call your local county agent.  Good luck.

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