Louisiana Crops News, Recommendations, Analysis

Time 2021-01-05 20:06:36

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How would you manage in-season potassium deficiency in soybean?Dr. Rasel Parvej, LSU AgCenter Soil Fertility Specialist; Dr. David Moseley, LSU AgCenter Soybean Specialist; Dr. Josh Copes, LSU AgCenter Agronomist; and Dr. Syam Dodla, LSU AgCenter Soil ScientistPotassium (K) is the second most yield limiting nutrient in soybean. Even though nitrogen (N) is the most limiting nutrient, soybean plant meets its own N requirement through biological N-fixation. Therefore, soybean is mainly fertilized with K and phosphorus (P) fertilizers in soils that are tested very low to medium K and P levels. Soybean is more responsive to K than P fertilizer and requires a large amount of K to maintain optimum water balance in plants, increase photosynthesis and assimilate translocation from source to sink, reduce transpiration losses of water, and improve uptake of other nutrients. A 55-bushel soybean requires about 160 pounds K2O (potassium oxide) per acre, approximately 2.9 pounds K2O per bushel grain harvested.Potassium deficiency can decrease soybean yield more than 50% across soil types that range from sandy loam to clay loam. In addition, K deficiency decreases P uptake by soybean plants and reduces soybean seed quality by decreasing seed oil and protein content and increasing purple seed stain. Potassium deficiency can occur in any soybean field that is very low to low in soil-test K level and is not fertilized with K. Potassium deficiency, however, often occurs in coarse-textured soils with low cation exchange capacity (CEC 10) such as loamy sand to silt loam soils. Coarse-textured soils are highly prone to K leaching below the root zone. Sometimes, fall application of K fertilizer in coarse-textured soils results in late-season K deficiency due to K leaching from excessive rainfall during winter and/or spring. Coarse-textured soils are also poor in water holding capacity and drought in these soils often causes K deficiency by decreasing K uptake by plant roots.Soybean K deficiency symptoms first appear as irregular yellowing on the edges of K deficient leaves. As growing season progress and the severity of K deficiency increases, the entire leaf edges turn brown and eventually the whole leaf dies. Potassium deficiency symptoms can occur as early as at the V3 vegetative stage (three trifoliolate leaves) mainly on the middle older leaves (Figure 1). But symptoms often occur on the upper younger leaves during the reproductive stages especially under severe K deficiency conditions (Figure 2). Soybean fields with K deficiency symptoms early in the growing season are very easy to diagnose and manage. However, most of the soybean fields often suffer from K deficiency and exhibit yield losses without showing any visible deficiency symptoms at all or at least until the later reproductive stages (beginning seed, R5 to full-seed, R6). This type of phenomenon is called hidden hunger and its most common in soybean fields that are low to medium in soil-test K level, have not received K fertilization, have high leaching potentials due to low CEC and excessive rainfall, or undergo severe drought conditions. Soybean grown in low pH ( 6.0) soils also suffer from hidden K hunger effects because low pH decreases soil K availability even after fertilization.Diagnosing hidden K deficiency early in the soybean growing season is very difficult and requires thorough scouting along with additional information such as fertilization history, soil texture, soil pH, soil-test K level, crop rotation, rainfall amount and distribution after fertilization and during the growing season, drought period, etc. Tissue sampling during the growing season is the best and perhaps the only tool to diagnose hidden K deficiency in soybean. Tissue sampling is predominantly conducted at the full-bloom (R2) stage; but can be done at the later reproductive (early pod, R3 to beginning seed, R5) stages. However, diagnosis at the early growth stages would be more effective and economical in correcting K deficiency and rescuing yield losses than diagnosis at the later growth stages.After tissue sampling, tissue K concentration at a particular growth stage is interpreted to diagnose K deficiency. Many current tissue K interpretations, used by most of the plant diagnostic labs, only allow interpretation of K concentration for soybean leaflet (without petiole) collected at or around the R2 stage. Recently at the University of Arkansas, Parvej et al. (2016) developed critical trifoliolate leaflet and petiole K concentrations from the R2 to R6 reproductive stages (Figure 3). These critical K concentrations would allow soybean producers, agronomists, and crop consultants to sample either leaflet or petiole or both to diagnose K deficiency across the reproductive growth stages of soybean.For proper tissue sampling, 15 to 20 recently mature trifoliolate leaves including petioles from the 3rd node from the top of the soybean plant should be collected and the date and soybean growth stage should be recorded (Figure 4). Then the leaflet of each trifoliolate leaf should be separated from the petiole and both the leaflet and the petiole or the leaflet only should be sent immediately to the plant diagnostic lab for K concentration. After receiving the results, tissue K concentrations for both the leaflet and the petiole at the specific growth stage can be interpreted using Figure 3. For example, the critical K concentration at the R2 stage ranges from 1.46 to 1.90% for leaflet and 3.01 to 3.83% for petiole and any K concentration below the critical level would be deficient and above the critical level would be sufficient. From the R2 stage, critical tissue K concentration declines linearly with the advancement of growth stage due to K translocation from vegetative to reproductive plant parts (pods and eventually seeds). Therefore, the growth stage at the time of tissue sampling should be recorded to properly interpret the tissue K concentration.For maximum soybean growth and yield, tissue K concentration should be above the critical level across the growth stages. If the tissue K concentration falls below the critical level, especially during the early reproductive stages, soybean should be fertilized with K to make sure K is not yield liming. Soybean K deficiency can easily be corrected by applying K fertilizer during the growing season. However, the effectiveness and economics of applying K fertilizer to rescue yield loss depends on soybean growth stage and the severity of K deficiency. The earlier the growth stage for K application the more effective and economic it would be in recovering yield loss. Recently, research conducted at the University of Arkansas suggests that soybean K deficiency can be effectively and economically corrected by applying 60 pounds K2O per acre until the R5 stage or about 5-weeks past the R2 stage. This is because soybean uptakes more than 70% of the total K after blooming and maximizes (100%) K uptake near the R6 stage. Therefore, diagnosis of K deficiency followed by an immediate K application early in the growing season would allow soybean plant enough time to actively uptake K from soils or through leaves and recover significant yield losses. However, pre-plant K application is the best way to maximize soybean yield.Both dry and liquid fertilizers can be used in correcting soybean K deficiency during the growing season. However, dry fertilizer would be more effective and economical for correcting severe K deficiency since a high amount of K would be required. Foliar application of liquid K may be effective for small amount of K requirement since K fertilizer has a high salt index that can burn soybean foliage if applied in high concentrations (Figure 5). Therefore, foliar method requires several applications to correct a severe K deficiency that would increase application cost. Also, foliar K fertilizer is more expensive than dry K fertilizer. The most effective and economical method is either by top-dressing or flying 100 pounds Muriate of Potash (0-0-60; 60 pounds K2O) per acre.Figure 1. Potassium deficiency symptoms during the early vegetative growth stages of soybean. Figure 2. Potassium deficiency symptoms during the reproductive growth stages of soybean.          Figure 3. Critical soybean leaflet and petiole K concentration from the R2 to R6 stages. (Source: Parvej, M.R., N.A. Slaton, L.C. Purcell, and T.L. Roberts. 2016. Critical trifoliolate leaf and petiole potassium concentrations during the reproductive stages of soybean. Agronomy Journal 108:2502-2518. doi:10.2134/agronj2016.04.0234; Y-axis is changed to English unit)    Figure 4. Steps of soybean tissue sampling during the R2 reproductive stage. Pencil in the picture indicates 3rd node from the top of the plant.  Figure 5. Soybean foliage damage due to sidedressing of high rate of liquid potassium. Should You Apply Pre-Tassel Nitrogen in Corn?Rasel Parvej, Dan Fromme, Josh Copes, and Syam DodlaNitrogen is the most yield liming nutrient for corn production. Corn requires nitrogen for amino acids, protein, and chlorophyll production. Chlorophyll is the key component for photosynthesis. Chlorophyll deficiency results in reduced yield potential. A 200-bushel corn requires about 200 to 240 lb nitrogen per acre i.e. roughly 1 to 1.2 lb nitrogen per bushel corn harvested. Applying all the nitrogen at or before planting may subject to loss to the environment through volatilization (if nor incorporated, mainly for urea), denitrification (due to water-logged anaerobic conditions), and leaching (due to excessive rainfall for coarse-textured low cation exchange capacity soils). Therefore, nitrogen management in corn is one of the biggest concerns each producer has every year. It is recommended to apply nitrogen in at least 2 splits during the growing season with 1/3 at planting and 2/3 around V5-V6 stage (5-6 leaves with visible collars and plant is about 12-inch tall). Providing adequate nitrogen plus other deficient nutrients (mainly phosphorus and potassium based on soil-test level) around V5-V6 stage is very important because corn initiates ear shoots and tassel and sets yield components at or little after V6 stage.Although most of the researchers showed that two applications are good enough to maximize corn yield under ideal conditions for most soils having medium to high cation exchange capacity (CEC 10), sometimes it is advised to apply nitrogen in 3 splits with 1/4 at planting, 2/4 around V5-V6 stage, and 1/4 before tasseling especially for coarse-textured soils with low CEC ( 10) and for years with lots of rainfall during the early corn growing season. Including pre-tassel application in nitrogen fertilization program can help reduce nitrogen loss and ensure adequate nitrogen supply during the maximum nitrogen uptake period from V10 to tasseling. It also helps adjust nitrogen rate based on crop growth, environmental forecasts, crop sensing, and tissue testing. Many land-grant university trials showed that pre-tassel nitrogen application can increase corn yield if some pre-plant and sidedress nitrogen are lost due to excessive rainfall during early growing season (Figure 1).Corn tissue testing is one of the important tools that guides whether pre-tassel nitrogen is required. For tissue testing, about 15-20 fully developed entire leaf below the whorl should be collected around V12 stage and sent immediately to the lab for analysis. This would allow producer enough time to get the results back and make decision. The critical (normal) corn leaf nitrogen concentration around pre-tassel stage ranges from 2.75 to 3.5%. So, leaf nitrogen concentration below 2.75% would be considered low and above 3.5% would be high. One caveat about tissue testing is, nitrogen concentration in corn leaf is highly influenced by crop growth and dilution factor; so, it may not always accurately diagnose nitrogen deficiency and indicate pre-tassel nitrogen need.Considering excessive rainfall, crop growth, and/or tissue-testing, once producer decided to apply pre-tassel nitrogen, the application rate should not be too high at this stage especially as foliar application. Broadcasting high rate of nitrogen would burn foliage (Figure 2). The pre-tassel nitrogen rate should be 15 to 25% of the total nitrogen applied i.e. roughly 50 lb nitrogen per acre. Producer can choose dry (urea) or liquid (UAN) nitrogen source. Both dry and liquid nitrogen can be flown by airplane; but it would be better to place nitrogen close to plant base, if possible, with high clearance applicator using “360 Y-drop” to facilitate rapid uptake and avoid foliage damage. A little rainfall should be expected after aerial application, which would help incorporate nitrogen fertilizer and reduce foliage burn.Figure 1. Nitrogen deficient corn in saturated soils due to excessive rainfall. (Source: https://www.pioneer.com/us/agronomy/nitrogen_application_timing.html)Figure 2. Corn leaf burn due to broadcasting 100 lb nitrogen per acre as UAN. Photo courtesy: John E. Sawyer, Extension Soil Fertility Specialist, Iowa State University. A new Louisiana Rice Field Notes is now available. This edition covers the Rice Station virtual field day, crop progress, DD50 heat units and expected harvest time, how to id GR and PD, smut control, and chinch bugs.(click to view) In the past week, I have looked at a few central Louisiana cotton fields that appeared to have severe thrips injury, yet no adult or immature thrips were present. Thrips are often one of the first factors people attribute to seedling cotton injury. However, several factors can contribute to early-season cotton injury. These include cold temperatures, insect feeding, preemergence herbicides, sand blasting, seedling disease and water stress. Fields planted in April often will experience some form of environmental stress that delays seedling growth and vigor. Severe issues often arise when these factors become additive, such as chilling injury coupled with use of preemergence herbicides. Much of this injury will look very similar to thrips and is easily mistaken as such. This type of injury can lead automatic thrips sprays when they are not warranted.The key to making thrips rescue sprays is the presence of immatures. When immatures begin to appear, this means the seed treatment has broken and reproduction is occurring. Luckily, thrips numbers appear to be low thus far in 2020 and are primarily composed of tobacco thrips; however, this can quickly change and may differ across the state.  If a rescue spray is deemed necessary, the decision should be made based on the presence of immature thrips and not old thrips damage or other non-insect related damage.Below are some considerations when deciding what foliar insecticide to use.Dimethoate:Positives: Relatively inexpensive, decent efficacy at high rates, less likely to flare spider mites and aphids than acephate.Negatives: Less effective on western flower thrips, less effective than acephate or bidrin when applied at lower rates.AcephatePositives: Relatively inexpensive, effective towards western flower and tobacco thrips.Negatives: May flare spider mites and aphids if present.Bidrin Positives: Effective, less likely to flare spider mites and aphids than acephate.Negatives: More expensive, less flexibility with applications early season.Intrepid EdgePositives: Effective, unlikely to flare spider mites and aphids. Intrepid Edge is a mix of Radiant and Intrepid. Activity is similar to Radiant.Negatives: Requires the application of two modes of action but only gets the benefit of one.If you have any questions or concerns please contact your local county agent or AgCenter specialist. How many of you had an issue with glyphosate-resistant Italian ryegrass this spring?  Did you expect clethodim to solve the problem and then found it did not?  Did you apply paraquat and were not satisfied?  Many farmers, consultants, and dealers commented to me since late January that the Italian ryegrass problem has exploded in Louisiana.  Honestly, this is not surprising because we have not been addressing this pest properly.  Mississippi has had this issue for longer than Louisiana has.  Mississippi State University weed scientists determined a good strategy to manage glyphosate-resistant Italian ryegrass five or six years ago.  LSU AgCenter weed scientists adopted their strategies and began disseminating that plan.  It starts with tillage or a residual herbicide application in the fall, which has not been adopted by many producers in Louisiana.  This article will not go into detail about Mississippi State University’s glyphosate-resistant Italian ryegrass management plan in this article, BUT it will be covered at length later this year.I am writing this article because I would like for Louisiana farmers, consultants, dealers, and ag lenders to notice that glyphosate-resistant Italian ryegrass is still present in corn, cotton, and soybean fields on May 1st.  It may be brown following herbicide applications, but it is still competing with crops as you can see in the photo (Figure 1).Do not take glyphosate-resistant Italian ryegrass lightly.  Remember what crop fields look like in the spring so that you will be motivated to implement good management strategies in the future.  More to come later.  If you have questions, please contact your LSU AgCenter parish agent.  Feel free to contact me at 318-308-7225.  Have a great day.Figure 1. Italian ryegrass competing with seedling soybean To manage weeds preplant, meaning two to three weeks prior, or preemergence, an application of paraquat at 0.5 to 0.75 lb/A plus a residual herbicide is needed to remove existing weeds and maintain fields weed-free.  If paraquat is applied preplant, a second application may need to be applied at planting to remove any remaining green vegetation.If 2,4-D, dicamba, Elevore, and others may be applied, read the label because there are planting restrictions for cotton and soybean.  However, there are no planting restrictions for Enlist Duo and Enlist One in Enlist crops or Engenia, FeXapan, Tavium, and XtendiMax in Xtend crops.Choosing a residual herbicide, whether applied preplant and/or preemergence, depends on the crop to be planted and the weed spectrum.  There are numerous choices of residual herbicides labeled preplant/preemergence in cotton, but research has shown that Cotoran at 2 pints/A is a good choice for control of numerous grass and broadleaf weeds.  If glyphosate-resistant Palmer amaranth and waterhemp are a major concern, Brake plus Cotoran, both at 1 pint/A, is an excellent choice.In soybean, there are numerous residual herbicide options.  Many will provide control of glyphosate-resistant pigweeds; however, they differ in the other weeds they will control.  For example, if pigweed, yellow nutsedge, and grasses are the targets, Boundary at 1.5 to 2 pints/A is a good choice.  But, if morningglory or smellmelon are also an issue, herbicide formulations that contain sulfentrazone (Authority formulations, Sonic, BroadAxe, etc.) or Canopy DF at 4 to 6 oz/A plus S-metolachlor at 0.95 lb/A would provide control.  Please contact your local LSU AgCenter agent to discuss your specific weed spectrum and residual herbicide options.The length of maximum control provided by a residual herbicide is usually 3 to 4 weeks when properly activated.  So, if applied 2 weeks prior to planting, one may only expect 1 to 2 weeks of residual control in-crop.  In contrast, if the residual is applied preemergence, 3 to 4 weeks of control may be expected in-crop.  So, the choice of preplant or preemergence residual herbicide application will influence when the first in-crop postemergence application should occur.  Remember, seedling cotton and soybean must be protected from weed competition to help maximize yield potential, so plan accordingly. In the past two weeks, instances of true armyworms (TAW) in wheat, corn and pastures have increased across the state. TAW are similar in appearance and size to fall armyworm (FAW). TAW possess a mottled brown head capsule (Figure 1) while FAW have an inverted “Y” on their head capsule. TAW develop into six instars, with larval development taking roughly 20 days and generational turnover occurring in 30 days. This insect is not well adapted to hot temperatures, and survival decreases significantly when air temperature is above 86 degrees F. TAW prefer grass hosts but will feed on broadleaves. TAW primarily feed at night, making observation during the day difficult. Larva consume 80% of the total foliage required for development in the last three to five days as larva. Larva congregate at the base of plants and on the soil surface to avoid midday temperatures. There are several natural enemies of TAW in Louisiana field crops. Predacious insects, parasitoids and pathogens occasionally will control TAW populations before a foliar overspray is requiredFig 1. True armyworm head capsuleTAW infesting Bt corn rarely causes economic injury, and Bt proteins available in field corn work very well controlling TAW. Non-Bt corn can experience significant injury from TAW, and fields should be scouted regularly to avoid defoliation. TAW can graze non-Bt corn to the ground; however, if the growing point is still beneath the soil (up to roughly V5), corn seedlings will recover quickly.TAW can significantly injure wheat if worms are allowed to defoliate the flag leaf before soft dough or clip wheat heads at any stage. The LSU AgCenter threshold for TAW in wheat is when five worms per square foot are found and foliage loss is occurring.In hayfields and pastures, TAW can cause significant injury to grass crops if left uncontrolled. TAW injury is identical to FAW, and routine scouting in the spring is recommended. The LSU AgCenter threshold is one worm per sweep.Pyrethroid insecticides control TAW very well in corn, wheat and pastures. As a general rule, large worms are harder to control than small worms.If you have any questions or concerns, please contact your local AgCenter agent for more information. Many producers are having to replant corn due to poor stands. There are three main ways to remove a failed corn stand.Use tillage equipment to physically remove the existing corn.Apply 0.0469 lb clethodim/acre and wait six days before planting the second corn crop. That equals 6 ounces of a 1 lb/gal clethodim, 3 ounces of a 2 lb/gal clethodim or 2 ounces of a 3 lb/gal clethodim. Waiting six days before planting is critical to prevent injury.Apply 0.625 lb paraquat/acre plus atrazine at 1 pint/A or diuron at 1 pint/A or metribuzin at 3 oz/A. Good coverage is essential. Even then, don’t expect outstanding control with this choice.Call 318-308-7225 with any questions. The first edition of Louisiana Rice Notes is now available. This edition covers planting progress, effects of warmer than average March weather on current crop, young rice farmers helping out, and new AgCenter rice publications which are available online. (click to view)

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