How can we cut down phosphorus and potassium fertilizer rates in 2022?

Rasel Parvej, David Moseley, Syam Dodla, and Rezaul Karim, LSU AgCenter Scientist

Phosphorus (P) and potassium (K) fertilizer recommendations depend on Mehlich-3 soil-test P and K concentrations, respectively from soil sample taken from 0- to 6-inch depth in Louisiana. Fertilization is recommended if soil-test nutrient concentrations fall below the critical level. The critical nutrient level is defined as the range of soil-test nutrient concentration below which crop response to added fertilizer is expected, within which is uncertain, and above which is unlikely. The current LSU AgCenter soil-test-based P recommendations for major row crops (corn, soybean, and cotton) is very straight forward, which offers only one range of critical P concentration (21 to 35 ppm P or 41 to 70 lb. per acre P) for all soil types. However, the soil-test-based K recommendations vary with soil types and soil formation processes and provides 8 to 10 different critical K concentrations across different soil types and geographical locations (e.g., 98 to 141 ppm K for silt loam, 142 to 264 ppm K for clay loam, 142 to 317 ppm K for silty clay soils, and so on). Note that most states have only one critical concentration of a particular nutrient for each row crop. Even some states have only one critical concentration of a particular nutrient for all row crops. Considering these facts, we reevaluated our current P and K recommendations for soybean production in Louisiana across 32 sites with different soil types that ranged from very fine sandy loam to clay from 2020 to 2021. This study will be continued at another 15-20 sites in 2022.

Based on our 2-yr study, we found that regardless of soil types, soybean in soils with less than 15 ppm P at 0- to 6-inch depth always (100%) responded positively to added fertilizer-P, 63% of the time for soil-P concentration between 15 and 20 ppm, 25% of the time for soil-P concentration between 21 and 25 ppm, and responded very seldom when soil-P concentration was more than 25 ppm (Table 1). However, for K fertilization, soybean in soils with less than 101 ppm K always (100%) responded positively to fertilizer-K and never (0%) responded when soil-K concentration was more than 101 ppm regardless of soil types (Table 2). Our results suggest that we need to revise our current P and K recommendations and it may be possible to develop only one single critical K concentration range across all soil types. It also suggests that we must fertilize our soybean fields with soil-test P concentration less than 15 ppm P and soil-test K concentration less than 101 ppm K. Without fertilization soybean yield in these soils can be reduced up to 20% due to no P fertilization, 40% due to K fertilization, and even greater amount if no P and K fertilizers are provided. Although our research was conducted on soybean, this result can also be interpreted fairly well in making fertilization decision for corn and cotton production in Louisiana.

Considering the record high fertilizer price this time of the year, producers may not need to provide all the recommended rates (roughly 60 to 80 lb. P₂O₅/acre and 80 to 100 lb. K₂O/acre, Table 1 and 2) but at least some amounts that they can afford to produce near maximum yield and still be in business. For example, depending on how deficient the soil is, producers can still produce profitable crops by applying around 50-80% of the recommended rates. Note that the lower the soil-test P and K values the higher the chances are for getting return on investment for fertilization. No P and K fertilization would be economically sound for soils with higher than 20 ppm (40 lb. per acre) P and 120 ppm (240 lb. per acre) K for corn, soybean, and cotton production across all soil types in 2022. Overall, soil-testing is very important in making fertilization decision, and producers should invest a little more money on soil-testing rather than fertilizing their field without knowing the magnitude of yield response to added fertilizers. In addition, producers need to interpret the actual soil-test values (ppm or lb. per acre) rather than soil-test level/index (low, medium) in making fertilization decision since the interpretation of soil-test level varies with soil testing labs.

Table 1. The magnitude of yield loss due to no phosphorus (P) fertilization and the probability of yield response to added P under different soil-test P levels for soybean production in Louisiana.

Table 2. The magnitude of yield loss due to no potassium (K) fertilization and the probability of yield response to added K under different soil-test K levels for soybean production in Louisiana.

Spring application of phosphorus and potassium fertilizers reduces fertilizer loss and improves yield

Rasel Parvej, David Moseley, Matthew Foster, and Rezaul Karim, LSU AgCenter Scientist

Louisiana producers mostly use triple superphosphate (TSP; 0-46-0) for Phosphorus (P) fertilization and muriate of potash (MoP; 0-0-60) for potassium (K) fertilization and apply both fertilizers mostly in the Fall rather than in the Spring. One of the main reasons for fall application is due to wet soil conditions or limited application time in the Spring. However, lots of producers believe that they must apply both TSP and MoP in the Fall since both fertilizers are rocky materials (TSP is originated from phosphate rock and MoP is from potash ore) and require a long time to dissolve and become available for plant uptake. Practically, both fertilizers are highly water soluble and can rapidly release nutrients, regardless of application time, when dissolve with adequate soil moisture and/or rainfall/irrigation water. Many studies showed that spring application of both TSP and MoP fertilizers is either equal to or better than fall application in increasing crop yield especially in soils that are deficient and highly prone to nutrient losses via leaching, runoff, and/or erosion.

In 2019-2021, we evaluated the effect of P and K fertilizer application timings on soybean and corn yields at the Macon Ridge Research Station (MRRS) in Winnsboro and Northeast Research Station (NERS) in St. Joseph. These trials were conducted in silt loam soils with 38- or 40-inch-wide bed at both locations. The soil-test P and K concentrations were below the critical level at the MRRS sites but were above the critical level at the NERS. As expected, both corn and soybean yields responded positively to both fall and spring fertilization only at the MRRS sites (Fig. 1-2). Our 2-yr results at the MRRS showed that corn and soybean yields were not significantly different between fall and spring application when P and K fertilizers were broadcasted followed by incorporation by rehipping the bed in the Fall but only broadcasted without rehipping the bed in the Spring. However, when rehipping was done in the Spring, corn yield was 12 bu. per acre numerically greater and soybean yield was 5 bu. per acre statistically greater than fall application. Rehipping in the Spring brought most of the P & K broadcasted in the furrow back to the bed, reduced fertilizer-P and K losses from furrow with rainfall and irrigation water over time, increased fertilizer-P and K availability in the bed near plant roots, and improved P and K uptake and crop yield. Although fall application may help save critical time in the planting season, it reduces available quantity of applied nutrients due to losses through leaching, runoff, erosion, or soil fixation through wet and warm winter, common in Louisiana. The following factors need to be considered in making decision regarding fertilizer application time.

• The rapidity of P and K fixation to unavailable forms usually increases with the decrease of soil-test P and K concentrations. For example, soils with deficient P and K will fix applied P and K more rapidly than soils with sufficient P and K. Therefore, fertilizers should be applied in the Spring at or near planting in P and K deficient soils to ensure maximum nutrient availabilities during the time of rapid plant uptake.
• Soil P availability is maximum between soil pH 6.0 and 7.5. Fertilizer-P is fixed to unavailable forms as aluminum phosphate when soil pH falls below 5.5 and as calcium phosphate when soil pH exceeds 7.5. Therefore, fertilizer-P should be applied in the spring as close to planting as possible for fields with low (pH <5.5) or high (pH >7.5) soil pH to ensure maximum fertilizer-P availability for plant uptake.
• Spring application of fertilizer-P and K should be considered for coarse-textured soils with very low cation exchange capacity (CEC <10) such as loamy sand to sandy loam (sometimes silty loam) soils where nutrient leaching and soil erosion are common, and nutrient deficiencies are often observed.
• For soils that are very prone to waterlogged/flooded conditions, fertilizer-P and K should be applied in the spring at or near planting since the alternating flooding (anaerobic) and non-flooding (aerobic) conditions decreases soil nutrient availability and increases losses.
• Fall application of P and K should be considered for soils with nutrient concentration within (medium) or above (sufficient) the critical level, where fertilizers are mainly applied to replace soil nutrients that are removed by harvested grains to maintain the same level of soil nutrient level. In addition, fertilizer-K should be applied in the fall in fields that have long history of chloride (Cl) toxicity problems and are poorly drained. Since K fertilizer (MoP) mainly consists of KCI, fall application will allow enough time to decrease Cl toxicity by reducing Cl accumulation from fertilizer KCl through winter and early spring rainfall.

Fig. 1. Corn grain yield response to fall vs. spring P and K fertilizer application time and application method for research trial conducted at the Macon Ridge Research Station in Winnsboro from 2020 to 2021.

Fig. 2. Soybean yield response to fall vs. spring P and K fertilizer application time and application method for research trial conducted at the Macon Ridge Research Station in Winnsboro from 2020 to 2021.

Which fertilizer (10-34-0 or 32-0-0) should we use as starter in corn production?

Rasel Parvej, Syam Dodla, Matthew Foster, and Rezaul Karim, LSU AgCenter Scientist

Louisiana corn producers mostly use ammonium polyphosphate such as 10-34-0 or 11-37-0 as starter fertilizer and apply it in the seed furrow or in a 2 by 2 band. These starter fertilizers mostly contain small amounts (10 or 11%) of nitrogen (N) and high amounts (34 or 37%) of phosphorus (P₂O₅). Starter fertilizer is usually applied at a lower rate (5 gal/acre) for in-furrow application due to the chance of salt injury from ammonium-N (NH₄⁺-N) but can be applied at a higher rate (up to 10 gal. per acre) for 2 by 2 banding or dribbling. Although adequate soil moisture at planting can reduce the likelihood of salt injury, high rates of N fertilizer is still not recommended for in-furrow application.

The main purpose of applying starter fertilizer is to help the germinating corn seedling boost up early-season growth by easily accessible nutrients placed near the seeds, resulting in increased yield potential. However, corn grain yield response to 5 gal/acre of 10-34-0 or 11-37-0 starter fertilizer is very inconsistent with no yield response being common across the mid-South and Midwest. Mascagni et al. (2007) conducted 15 site-years research trials from 1991 to 2005 on starter fertilizer (10-34-0 or 11-37-0) for corn production in northeast Louisiana and found that starter fertilizer increased corn yield by 8 to 25 bu/acre in only 5 out of 15 trials (i.e., 33% accuracy; Fig. 1). They reported that the positive yield response to starter fertilizer came only from phosphorus (P) but not from nitrogen (N). This was because 5 gal. per acre of in-furrow starter fertilizer (10-34-0) contains 19.8 lbs P (P₂O₅) but only 5.8 lbs N per acre, which is a very small amount to make any yield difference by N at planting.

The most important point that Mascagni et al. (2007) reported was that starter fertilizer increased corn yield only in coarse-textured soils such as sandy loam soils. They also reported that sandy loam soils were cold-natured soils with low organic matter content and nutrient holding capacity, where P deficiency symptoms were common early in the growing season (mid-March to mid-April). Cold soil temperature often causes reduced P uptake by young corn plants, due to slow root growth, resulting in temporary P deficiency especially in sandy loam soils, even though soil-test P levels are adequate (21 to 35 ppm P or 42 to 70 lb./acre P at 0- to 6-inch soil depth). However, this early season P deficiency especially in soils with sufficient P level can go away with warmer soil temperature and adequate moisture and usually does not negatively affect corn yield. If the Mehlich-3 soil-test P level is low (less than 21 ppm or 42 lb. per acre) and producers already applied P fertilizer either in the Fall or Spring, there may be no benefit of using 10-34-0 or 11-37-0 starter fertilizer. This is also true for high P testing soils (more than 35 ppm or 70 lb. per acre P) where additional P is not needed. For medium testing soils (21 to 35 ppm or 42 to 70 lb. per acre), if producers do not apply any P fertilizer, there may or may not be a benefit of using 10-34-0 or 11-37-0 starter fertilizer especially in coarse-textured soils with early planting. No yield benefit from these starter fertilizers is common for corn production in fine-textured soils such as clayey soils. Since fertilizer-P when applied in soils can be fixed to unavailable forms as aluminum phosphate when soil pH falls below 5.5 and as calcium phosphate when soil pH exceeds 7.5, starter fertilizer 10-34-0 or 11-37-0 that mostly contains P, may be beneficial to ensure maximum fertilizer-P availability for early-season plant uptake especially for soils with pH outside of this range (6.0 to 7.5), deficient in soil-P, and no P fertilizer is applied during the Fall or Spring.

Corn requires 30 to 45 lbs N from planting to V6 stage (6 visible collar leaves and plant is about 12-18 inches tall) and this N requirement can only be fulfilled by either applying 65 to 98 lbs. of urea (46-0-0) or 8.5 to 12.7 gals of UAN (32-0-0) per acre at planting. Urea (46-0-0) should be broadcasted followed by incorporation at or before planting. UAN (32-0-0) can be applied as 2 by 2 banding or dribbling at planting. If 10-34-0 or 11-37-0 starter fertilizer is used, producers need to apply 26 to 39 gallons per acre to fulfill the 30 to 45 lbs. N requirement at planting, which would not be economically sound since fertilizer price is at a record high. In addition, when corn is planted in high plant residue with or without cover crops, this high plant residue sometimes results in early-season N deficiency due to N immobilization by soil microbes. Providing 30 to 45 lbs. of N using 32-0-0 during planting as a starter would offer more benefits in these soil conditions than 5.8 lbs. of N using 10-34-0 or 11-37-0.

Overall, producers should use 32-0-0 (30-0-0-2S or 28-0-0-5S can also be used if 32-0-0 is not available but the rates need to be adjusted) as a starter fertilizer during corn planting to provide the need of 30 to 45 lbs. N from planting to V6 stage and come back with the rest of the N rate as a sidedress during V6-8 stages. Note that producers should subtract the starter N rate from the total N rate during sidedressing. The total N rate depends on corn yield goal and soil type. Corn requires 1 lb. of N per bushel of yield for sandy to silty loam soils and 1.25 lbs. of N per bushel of yield for clayey soils. Therefore, producers should apply 30 lbs. of N for sandy to silty loam soils and 45 lbs. of N for clayey soils during planting (Please read the article “Optimum nitrogen fertilizer rate and timing for corn production in Louisiana” for more information regarding optimum N rate and timing).

Figure 1. Corn yield response to starter fertilizer in research trials conducted by Mascagni et al. (2007) on Mississippi River alluvial sandy loam/silt soils at the Northeast Research Station in St. Joseph, Louisiana. [NS, not significant at the 0.05 probability level; Source: Mascagni (Rick), H.J., D. Boquet, and B. Bell. 2007. Influence of starter fertilizer on corn yield and plant development on Mississippi River alluvial soils. Better Crops. Vol. 91(2)]

Optimum nitrogen fertilizer rate and timing for corn production in Louisiana

Rasel Parvej, Syam Dodla, Matthew Foster, and Rezaul Karim, LSU AgCenter Scientist

Nitrogen (N) is the most yield limiting nutrient for corn production. The LSU AgCenter recommends 1 to 1.25 lbs of N per bushel of corn harvested i.e., a 200-bushel corn crop requires about 200 to 250 lbs N/acre. Corn in clayey soils requires more N for the same yield goal than corn in sandy to silty loam soils. This is mainly because clay fixes more fertilizer-N (ammonium ion, NH₄⁺) to a biologically unavailable form and has complex N uptake route from fertilizer-N to plant roots due to the presence of a huge amount of micropores compared to sandy or silty loam soils. The upper range of recommendations i.e., 1.25 lb N/bu. corn yield is, therefore, for clayey soils and the lower range i.e., 1 lb N/bu. corn yield is for sandy to silty loam soils.

Nitrogen management in corn production is one of the biggest concerns for corn producers every year. Nitrogen is recommended to apply in 2 to 3 splits from planting to tasseling since it is very prone to loss in the environment via different loss mechanisms. Unfortunately, many corn producers in Louisiana apply the total N fertilizer in a single application as sidedress at or few weeks after corn emergence. A significant amount of this early N in most years can be lost during the growing season through volatilization, denitrification, leaching, and/or runoff, resulting in corn yield loss. Volatilization loss is very high in hot and humid climates, common in Louisiana, and in alkaline soils (pH more than 7.0) if N fertilizer (especially urea but can be UAN as it contains 50% urea) is not incorporated within a few days after application. Denitrification loss is the main concern in poorly drained soils but can occur in any soil with excessive rainfall that creates water-logged anaerobic conditions. Leaching loss is high in high rainfall areas especially in sandy soils with low cation exchange capacity (CEC). In most years in Louisiana, excessive rainfall often occurs in the lower Mississippi Delta early in the growing season, resulting in saturated soils for several days, which accelerates N losses via denitrification, leaching, and/or runoff and reduces corn yield potential. Although researchers from the mid-South states have showed that it is possible to maximize corn yield by a single N application during the growing season in both silt loam and clay soils, for this to occur, the growing season must be ideal with moderate temperature and adequate and evenly distributed rainfall, which seldom occurs in Louisiana. Since we cannot predict weather conditions during the growing season, a single application is, therefore, a risky N management plan for corn production in most years in Louisiana.

In general, a 200-bushel corn requires about 15% (~35 lbs) of the total N from planting to V6 stage (6 visible collar leaves and plant is about 12-18 inches tall), 60% (~150 lbs) from V6 to R1 (silking), and 25% (~65 lbs) from R1 to R6 (maturity; Figure 1). Corn sets yield components such as kernel rows/ear from V1 to V7 stages, potential kernels/row from V7 to tasseling, and harvestable kernels/row from tasseling to R3 stages (Figure 2). Considering both N requirement and yield component development at different growth stages of corn, N should be applied in 3-splits with a small amount of N at planting, most at around V6 stage, and another small amount at pre-tassel stage to achieve maximum N use efficiency and corn grain yield.

Applying a small amount of N (30 to 45 lbs) at or before planting would provide the corn plant enough N for setting the most important yield components of kernel rows/ear from V1 to V7 stages and potential kernels/row from V7 to V15 stages (Figure 2). It would also provide a wide window of opportunity to the sidedress N application from V6 to V8 stages. For instance, having a pre- or at-planting N application would allow the producers to delay their sidedress application if missed at V6 stage due to rainfall and wet soil conditions. Unfortunately, pre- or at-planting N application is not very common in Louisiana corn production. Rather, most corn producers in Louisiana often use in-furrow starter fertilizer (ammonium polyphosphate 10-34-0 or 11-37-0 @ 5 gal/acre) that provides 19.8 lbs phosphorus (P₂O₅) and only 5.8 lbs N and would not be able to compensate early season corn N requirements. Corn early season N demand (30 to 45 lbs/acre) can only be met by either applying 65 to 98 lbs of urea (46-0-0) or 8.5 to 12.7 gals of UAN (32-0-0) per acre at planting. If 10-34-0 or 11-37-0 starter fertilizer is used, producers need to apply 26 to 39 gals/acre to fulfill the 30 to 45 lbs N need at planting, which would not be economically sound since fertilizer price is at a record high (Please read the article “Which fertilizer (10-34-0 or 32-0-0) should we use as starter in corn production?” for more information about starter fertilizer).

Although most of the research showed that N application in 2-splits (planting and V6-8) is good enough to maximize corn yield under normal conditions in most soils with medium to high CEC (>10; Please read the article “Split application of nitrogen can reduce nitrogen fertilizer inputs in Louisiana corn” for more information regarding the benefit of split application), a 3-way split of N with a 3^rd application (~45 to 60 lbs N/acre) at or before tasseling stage (V11-V13; about 2 weeks prior to tassel) is recommended especially for coarse-textured low CEC (<10) soils as well as for poorly drained soils that are very prone to water-logged conditions. This helps protect corn yield losses in years with excessive rainfall during the early corn growing season, which increases N losses. Researchers from many land-grant universities including LSU AgCenter found that pre-tassel N application can increase corn yield when part of the pre-plant and sidedress N are lost due to excessive rainfall during the early growing season. However, for fields that already received all the N fertilizer in 2-splits, the need for a 3^rd N application at or before tasseling should be based on soil type, crop growth, rainfall amount and soil conditions during the growing season, yield potential, environmental forecasts, reference strips (NDVI), and/or leaf N concentration. The NDVI reading from N reference strips and leaf N concentration from V10 to tasseling stage can be used to determine the need for a pre-tassel (V12 to 14) N application. A detailed article regarding corn pre-tassel nitrogen application will be posted in May newsletter issues. Overall, an ideal N management program for over 200-bushel corn yield/acre should include at least 30 to 45 lbs N at planting and the remaining amount at V6 to V8 stage with or without 45 to 60 lbs N before tasseling based on NDVI reading from reference strips and/or leaf N concentration.

Figure 1. Corn seasonal nitrogen uptake (Source: Bender et al. 2013. Modern corn hybrids’ nutrient uptake patterns)

Figure 2. Corn yield component development across growth stage(Source: University of Nebraska-Lincoln Extension).

Split application of nitrogen can reduce nitrogen fertilizer inputs in Louisiana corn

Rasel Parvej, Syam Dodla, Matthew Foster, and Rezaul Karim, LSU AgCenter Scientist

For corn production in Louisiana, it is recommended to apply nitrogen (N) in at least 2 splits during the growing season to reduce losses and increase N use efficiency. However, many producers apply the total amount anywhere from corn emergence to V6 stage (6 visible collar leaves and plant is about 12-18 inches tall). In an ideal year with moderate temperature and adequate and evenly distributed rainfall, a single application of the total N can be good enough to reach the yield goal especially in moderately fine to fine textured soils such as silty loam to clay with cation exchange capacity (CEC) >10. However, corn in coarse textured soils such as loamy sand to sandy loam with CEC < 10 should receive N in 2-3 splits to minimize losses and ensure adequate supply during the growing season.

Corn produces different yield components at different stages of plant growth with the most important yield component, kernel rows per ear being set from V1 to V7 stages, potential kernels per row from V7 to tasseling, and harvestable kernels per row from tasseling to R3 stages. Adequate supply of N before the onset of each yield component is very important. If a corn plant senses insufficient N in soils, it can adversely affect the development of each yield component. In other words, production of a particular yield component at a specific growth stage is greatly reduced if N deficiency occurs at that growth stage.

Considering season-long N uptake curve and yield component development, corn requires a small amount of N from planting to V7 stage to maximize kernel rows per ear, a high amount of N from V7 to tasseling to maximize potential kernels per row, and another small amount of N from tasseling to R3 to fill maximum number of kernels per row. If all N is applied during V6-7 stage, as many producers do, the target to maximize kernel rows per ear will be missed. Providing all the N at or right after planting can target all the yield components but practically more N will undergo loss mechanisms than crop uptake due to a small root system with small N demand early in the growing season. To ensure maximum development of all the yield components, an ideal 2-way split application should include a small amount of N (~15% of the total) during planting and the rest (~85% of the total) during V6-7 stage. If a producer had to apply all the N a few days after corn emergence due to time limitation and equipment or management issues, they should apply ¾ of the total N at this time and the rest ¼ during pre-tassel (V10-12); but this practice is not recommended.

Considering producer’s limitation and issues, we established two corn trials in 2021 at the Macon Ridge Research Station (MRRS) in Winnsboro and Northeast Research Station (NERS) in St. Joseph. These trials were conducted in silt loam soils with 38- or 40-inch-wide beds at both locations. Our results showed that a split application of 90 lb. N per acre a few days after planting (AP) plus 60 lb. N per acre before tasseling (V10-12; PT) yielded statistically similar compared to a full rate of N (200 lb. per acre) few days after planting (AP) plus 60 lb. per acre before tasseling (PT) at both locations (Fig. 1). According to LSU AgCenter current recommendations (1 lb. of N per bushel of corn harvested for silt loam soils), around 200 to 220 lb. N/acre was recommended to produce the maximum yield at both locations, but we were able to reach our targeted yield with application of only 150 lb. N per acre in 2-splits. Although this type of splitting is not recommended (mentioned above), our 1-yr (2-sites) results highlight the importance of splitting N application in Louisiana corn production. Adequate N amount with proper timing would reduce our N fertilizer needs and maximize yield and profit especially when fertilizer price is at a record high.

Fig. 1. Corn grain yield response to different nitrogen application rates and timings for research trials conducted at the Macon Ridge Research Station (MRRS) in Winnsboro and Northeast Research Station (NERS) in St. Joseph, LA in 2021.

Consider Nitrogen Fixation When Planting Soybean

David Moseley, Rasel Parvej, Syam Dodla, and JimWang, LSU AgCenter Scientist

Soybean acreage has been projected to increase in 2022 in Louisiana as well as in several other states due, in part, to the high price of fertilizer. Since both corn and cotton require more fertility compared to soybean (does not require N) to maximize yield, many producers will rotate their continuous corn and cotton fields with soybean or plant soybean after soybean. It is also possible producers will plant into marginal land that has not been profitable in recent years or pasture acres may be converted to row crops. If the field has not been planted to soybean over the previous three to five years, it is recommended to inoculate the seed with Bradyrhizobium japonicum bacteria. The bacteria and the soybean plants will form a symbiotic relationship to convert atmospheric nitrogen gas (N₂) into ammonium (NH₄+). This nitrogen fixation process, that is common among different legumes, converts a form of nitrogen that is unavailable to plants (N₂) into plant available nitrogen (NH₄+). Further, many commercially available Rhizobia inoculants may have higher nitrogen fixation potential. Multiple studies conducted at the Red River Research station showed that use of inoculant even in the fields that had soybean grown in the last three years had a 3 - 7% increase in yield. When inoculating the seed of a legume plant, it is important to distinguish between the different types of Rhizobia bacteria that is required by the specific legume. For example, Bradyrhizobium japonicum is appropriate for soybean, but Rhizobium leguminosarum biovar trifolii and Sinorhizobium meliloti is appropriate for a clover and alfalfa mix. Therefore, although alfalfa and clover are legumes, the appropriate bacteria may not be present in the soil to form the symbiotic relationship with soybean plants if the clover and alfalfa field has not been planted to soybean in the previous three to five years. More information on types of inoculants can be found at this Penn State Extension publication.

If the soil has low pH (acidic soils), the nitrogen fixation process can be limited due to a low availability of molybdenum (Mo). A Mo trial was conducted in a Louisiana acidic soil (pH 5.6 to 6.2) in 2018 and 2019. The results of the LSU AgCenter trial indicated applying Mo in acidic soils can significantly increase soybean yield from 6.9 to 17.4%. If a Mo treatment is required, it is important to remember that Mo should not be combined with Bradyrhizobium japonicum inoculum, unless they are combined immediately prior to planting. The Mo treatment can desiccate and decrease number of viable bacteria. For more information, read "Nitrogen Fixation in Soybeans" in the Louisiana Crops Newsletter Volume 10, Issue 4 – May 2020.

Downloading free rainfall data from delimited boundaries using CHIRPS

Armando Lopes de Brito Filho, PhD student, Franciele Morlin Carneiro, Post-Doctoral Researcher, and Luciano Shiratsuchi, LSU AgCenter Associate Professor in Precision Ag.

Rainfall data is essential information for farmers to help manage their fields. However, one of the main challenges is to have good coverage in areas where there are only a few weather stations close by. In addition to that, the cost for farmers to acquire weather stations is still significant. Therefore, is there any other way to acquire public weather data in areas with few weather stations?

Currently, one of the leading technologies that have been supporting farmers is through remote sensing (RS), that allows you to obtain data from different platform levels, such as terrestrial, aerial, or orbital. Satellite images are used to supervise crop development, area delimitation, and climate assessment.

Thus, answering the question of the first paragraph, it is possible through remote sensing to measure rainfall data. Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) is a near-global precipitation dataset of over 40 years. It uses satellite imagery at 0.05° spatial resolution (or about 5x5 km pixel) and intelligent interpolation techniques to build rainfall estimation models. The product is proving to be an efficient alternative for farm management support, especially in regions where weather stations are scarce.

Next, we will present how to download the CHIRPS data in a daily basis, using a delimited area or boundary to download rainfall data. A sequence of this operation is presented in Figure 1.

Figure 1. Steps we need to follow to obtain rainfall precipitation data from CHIRPS.

Development of each step of the rainfall data collection process

The first step is to know the exact location of your field of interest. You can build a delimited area or boundary in any GIS software such as Qgis or Arcgis (Figure 1.1). This step is essential to create a polygon of the field of interest in a shapefile format and then compress or zip the file necessary in the second phase.

After creating the zipped file, we need to create a Google Earth Engine account (https://earthengine.google.com). A sharing link will be generated on this site, which will be used in the third step (Figure 2).

Figure 2. Steps on Google Earth Engine to upload the zip file and add an asset.

With the link generated in the previous step (Figure 2.2), you will open a new web page, Climate Engine (https://climateengine.com). On this site, you will see loaded the area of interest delimited in the first phase (Figure 3). And according to the selected input mechanisms, we will have the precipitation graph plotted for visualization (Figure 1.4) and the availability to download the raw data in CSV format. If you have problems loading a boundary, the other option is check in the box Region Point and then move the point to a certain location in climate engine map, but in this case, you will download the data for that specific location and not for an area of interest such as a basin or farm.

Figure 3. Steps in Climate Engine.

We emphasize that it will be necessary to create an account to use the site in both the second and third phases. However, it is easy to make. You will need just your existing Gmail account to log in and explain the reasons for using the sites. In both cases, these steps are self-explanatory.

CHIRPS data can be used as an input in a decision support tool. With such data, it is possible to have a historical series of the rainfall pattern in a region or in each production field. Thus, farmers can more effectively manage their mechanized operations. It can be used to select tractor power and number of farm machinery needed to cover dry days to plant, spray and harvest. Furthermore, they will have more support in choosing the ideal planting window to match all operations with rainfall events.

Thrips Control Methods Important in 2022

Tyler Towles ( LSU AgCenter Entomologist) and Sebe Brown (University of Tennessee Entomologist)

Early-season thrips control in cotton is an extremely important consideration since thrips infest 100% of cotton acreage in Louisiana. Thrips are the most significant pest of seedling cotton with large infestations causing terminal damage, delayed maturity, and potential yield loss. When scouting for thrips, finding adults is less important than finding immatures. Since adults have wings, they can be found everywhere. However, since the immatures are wingless, finding them on cotton seedlings indicates that feeding and damage are occurring. Another thing to remember is that if immature thrips are present, it could indicate that the incorporated seed treatment may be breaking down and reproduction is occurring. Thrips are most injurious to cotton from emergence to the 4^th – 5^th leaf stage, meaning that thrips applications are not warranted after this stage.

Historically, cotton producers have relied on neonicotinoid seed treatments for thrips control. Since thiamethoxam resistance has been well documented in thrips populations across the cotton belt, this active ingredient shouldn’t be utilized as a stand-alone seed treatment for thrips control. This leaves one neonicotinoid, imidacloprid, which continues to provide adequate control on thrips. That control can be enhanced when the seed is also overtreated with acephate at a rate of 6.4 oz/cwt. It should be noted that if seed is overtreated with acephate, that seed cannot be returned to a supplier.

Aldicarb is available for thrips and nematode control in cotton under the trade name AgLogic. Aldicarb is more expensive than other available preventative thrips treatments, however, it provides very good control at the 4 lb. per acre rate. Additionally, Temik boxes are required to apply AgLogic in-furrow. Other options to enhance thrips control include in-furrow sprays of imidacloprid (9.2 oz/acre) or acephate (1 lb./acre) during planting.

There are several foliar rescue treatments commercially available that provide thrips control. However, there are some considerations when making these selections. Keep in mind that price, product availability, and flaring non-target insects should all play a role in making a decision.

Considerations when picking foliar insecticides for thrips control

Dimethoate

• Pro: Relatively inexpensive, decent efficacy at high rates, less likely than acephate to flare spider mites and aphids.
• Con: Less effective on western flower thrips, less effective than acephate or bidrin when applied at lower rates.

Acephate

• Pro: Relatively inexpensive, effective towards western flower and tobacco thrips.
• Con: May flare spider mites and aphids if present.

Bidrin

• Pro: Effective, less likely than acephate to flare spider mites and aphids.
• Con: More expensive, less flexibility with applications early season.

Radiant

• Pro: Effective, least likely to flare spider mites and aphids.
• Con: More expensive, requires adjuvant, more effective on westerns than tobacco thrips.

Intrepid Edge

• Pro: Effective, unlikely to flare spider mites and aphids. Intrepid Edge is a mix of Radiant and Intrepid. Activity is similar to Radiant.
• Con: Requires the application of two modes of action, but only gets the benefit of one.

2022 is going to be a challenging production year for several reasons and creating additional problems will only exacerbate our current situation. Two imperative things need to happen to ensure cotton gets a quick start in 2022: (1) Controlling thrips in seedling cotton to prevent delaying maturity. Delayed cotton would result in additional tarnished plant bug and/or cotton bollworm sprays. (2) Consider any off-target insect pests when spraying for thrips. Flaring aphids or spider mites would require additional insecticidal applications.

Tips for a Healthy Grain Sorghum Stand

Matt Foster, LSU AgCenter Grain Sorghum Specialist

Grain sorghum is a good option for dryland fields where productivity is marginal. It has a longer planting window and provides many benefits when used in a crop rotation with cotton and/or soybean. Although grain sorghum production has many benefits, it has some drawbacks such as sensitivity to off-target movement of glyphosate, limited herbicide options for weed control, and insect pest issues (sugarcane aphid and others). This article will outline a few key tips for getting your 2022 crop off to a good start.

Planting Date and Soil Temperature

Grain sorghum typically responds well to early planting but has less seedling vigor when compared to corn. The recommended planting window ranges from April 1 to May 1 in south Louisiana and April 15 to May 15 in north Louisiana. However, planting decisions should be based on soil temperature and not the calendar. When considering the ideal time to plant sorghum, the five-day average soil temperature should be at least 60 degrees Fahrenheit at the 2-inch depth and the seven-day forecast is for warm weather. Optimal temperature for quick germination and establishment of grain sorghum is near 65 degrees Fahrenheit.

Seeding Rate and Depth

Grain sorghum should be planted at a rate of approximately 75,000 seeds per acre. This is equivalent to five to six seeds per row foot on 40-inch centers, four to five on 30 to 36-inch centers, and three to four on 20-inch centers. If rows are 10 inches or less spaced, three seeds per row foot should be adequate. Sorghum can be grown in a variety of row widths, but research has shown that yield responds well to row spacing of 30 inches or less. Seed should be planted in adequate moisture no deeper than 2 inches. Optimum depth ranges from 0.75 to 1.5 inches deep. Sorghum seed varies in size from 12,000 (38 grams per 1,000 seed) to 18,000 (25 grams per 1,000 seed) seeds per pound. If using pounds per acre to plant, you should be aware that populations can vary greatly. Seeding rates should be based on seed per acre and not pounds per acre.

Nitrogen, Phosphorus, and Potassium

Nitrogen should be applied between 100 to 125 pounds per acre on upland soils and 125 to 150 pounds per acre on alluvial soils. Clay soils typically require a higher nitrogen rate compared to sandy/silty soils. A rough rule of thumb is to apply 1.12 pounds of actual nitrogen for each bushel of grain sorghum produced. The amount of applied nitrogen should be based on crop yield goal and the amount of residual nitrogen in the soil. All the required nitrogen can be applied before or at planting, but this increases the risk of fertilizer burn on seedlings and nitrogen losses through volatilization, leaching, or denitrification. Therefore, nitrogen is recommended to be applied in a split application with 50 to 75% before or at planting and the remainder no later than the 6- to 8-leaf stage.

Grain sorghum utilizes phosphorus and potassium during the early part of the growing season, so these nutrients should be applied pre-plant or at planting. Soil testing is recommended to determine phosphorus and potassium needs for each field. The soil-test-based fertilizer recommendation for grain sorghum can be found at the LSU AgCenter Website.

2022 March Wheat Update

Boyd Padgett, Trey Price, and Steve Harrison, LSU AgCenter Scientist.

At the time of this article, May wheat prices were $10.42/bu. Therefore, producers should consider managing wheat for maximum profits. While most, if not all nitrogen has been applied to our crop, producers should have applied (preferably a split application) up to 110 units. Continue to monitor fields for yellowing and consider applying additional nitrogen at or just after jointing. Keep in mind that additional N applied following flag leaf typically contributes very little to crop yield.

Rust susceptible varieties should receive an application of a fungicide (triazole or triazole/strobilurin) at flag leaf. There are several low-cost fungicides that are very effective for rust management. Since most varieties have some degree of susceptibility to Fusarium head blight (scab), a fungicide recommended for this disease should be applied at flowering. The average response to two applications of fungicide across north Louisiana in 2021 was 8.8 bushels per acre with a test weight increase of 0.5 pounds per bushel. See Table 9. Yield response ranged from 0 to 21.4 bushels per acre across all varieties.

A list of fungicides and rates for wheat diseases can be found at this LSU AgCenter publication.

As spring approaches, temperatures can range from below freezing to over 80oF from now until harvest. During the week of 3/6/22, temperatures dipped into the mid-twenties in north Louisiana. Our wheat entered that period when sub-freezing temperatures could cause damage to the crop. This begs the question: At what point do freezing temperatures damage wheat? This is highly dependent on the temperature, duration of plant exposure, and crop growth stage. Other factors affecting injury are variety and plant moisture content at time of exposure. Plants with adequate moisture are more sensitive to freeze injury than plants in drought stress. Wheat that is in the jointing to milk growth stages is most sensitive to freezing temperatures. Wheat that was planted in a timely manner (not too early) is probably in early stages of jointing in north Louisiana and should withstand temperatures in the mid-20s. Specifics on growth stage and exposure for two hours at injurious temperatures are listed in Table 1.

Table 1. Temperatures that cause freeze injury to wheat at spring growth stages and symptoms and yield effect of spring freeze injury.

Source: Kansas State Cooperative Extension Service, Manhattan, KS

Stripe rust has been observed in variety tests located at the Dean Lee Research Station (Alexandria). Information on leaf and stripe rust disease development and symptoms are listed below.

Stripe rust (also called yellow rust) development is most aggressive when temperatures are 50 to 65°F in the presence of intermittent rain or dews (6 to 8 hours). However, development can occur when temperatures range from near freezing to 70°F. Initial infections on seedling wheat may not have the characteristic striping pattern that occurs on older plants. Seedling infections often occur in ‘thumb-sized’ clusters on the leaves, as opposed to a random distribution that occurs with leaf rust. Infections may appear as linear rows of small yellow to light orange pustules (stripes) on the lower leaves during late winter or early spring. Striped patterns are typical of infections in older pants. If conditions remain favorable for development, pustules may cover the entire upper leaf surface, as well as portions of the head. A lifecycle (infection to reproduction) can be completed within 7 to 10 days under optimum conditions.

Leaf rust is usually evident later in the season than stripe rust. This is because the leaf rust pathogen requires warmer temperatures for development. Initial symptoms of leaf rust begin as orange/light-orange/yellow spots, usually on the lower foliage. As the disease develops, small pin-point pustules form on the upper leaf surface. Pustules are brick or dark red and occur randomly on the leaf. Similar to stripe rust, pustules can cover the entire leaf surface if conditions remain favorable for development. The disease develops optimally when nighttime temperatures are 50 to 70°F and leaves remain wet for 6 to 8 hours.

Stripe Rust

Stripe Rust in the lower canopy on early season wheat

Stripe rust on tillering wheat

Hot spot of stripe rust

Leaf Rust

Leaf Rust

Managing scab begins with a knowledge of the conditions that favor infection and disease development. The fungus can infect corn; therefore, wheat grown in fields planted to corn the previous year are at higher risk to this disease. Infected corn debris (also wheat straw and other hosts) can serve as initial inoculum. Fungal spores produced on this debris are dispersed by rain splash or wind to nearby wheat plants. Later in the season, plant to plant spread is possible. Infection can occur from head emergence to harvest, but infection during flowering through soft dough is most damaging. Conditions that favor infection are temperatures from 75-85oF and 48-72 hours of free moisture.

Symptoms of the disease can appear 10 to 14 days after flowering as bleached heads, which will be evident from the turn row. This symptom is often mistaken with the appearance of maturing wheat. Upon closer inspection, affected wheat heads will usually have infected kernels showing the characteristic bleached appearance with pinkish/salmon/light orange coloration along the glumes. This coloration is millions of microscopic spores (reproductive structures) of the fungal pathogen. There are usually healthy kernels along with the diseased kernels on the same head. In extreme cases, however, the entire head may be infected. At harvest, affected seed will be shriveled, off color, much lighter than healthy kernels, and are referred to as “tombstones”.

A list of varieties with a reasonable level of resistance to scab and recommended fungicides for control can be found at the US Wheat and Barley Scab Initiative web site: (please note that the table titled ‘2021 MR Soft Red Winter Wheat Varieties’, the FHB Rating column reflects the scab index which is a calculation using scab incidence and severity, as well as Fusarium damaged kernel and DON (mycotoxin) Not a 1-9 scale ).

Bleached heads caused by Fusarium graminearum (Scab fungus)

Pinkish/salmon/light orange spores of the scab fungus

Healthy and infected kernels on the same head

Infected kernels on left ‘tombstones’ and healthy kernels on right

Authors greatly appreciate funding and support for research from the Soybean and Grain Research & Promotion Board and the U.S. Wheat & Barley Scab Initiative

LSU AgCenter Specialists