Louisiana Soybean May Update: Heavy Rain Raises Concerns About Final Plant Stand

David Moseley, LSU AgCenter Soybean Specialist

The 2025 soybean planting season in Louisiana began favorably, with 80% of the crop planted by May 4th, significantly ahead of the 5-year average of 54%. However, recent heavy rainfall has introduced concerns. Late April storms caused flooding in some fields, necessitating replanting in certain areas. During a field visit, I observed plants where floodwaters had deposited soil on the leaves. Fortunately, the floodwater receded quickly and subsequent rainfall washed away most of the soil. In this case the plants appeared healthy after the flooding. More recently, on May 6th, another intense storm brought several inches of rain to many parts of the state in a short period, potentially leading to further flooding.

Evaluating Plant Stand

Heavy rainfall and saturated soil conditions can negatively impact the final plant stand. Therefore, it is important to evaluate the existing plant stand (as outlined in Table 1) before making a decision about replanting. Research from the LSU AgCenter indicates that a plant population of at least 70,000 to 75,000 healthy plants per acre can still achieve approximately 95% of the potential yield. A national publication titled Soybean Plant Stands: Is Replanting Necessary?, from the Science For Success team (a group of soybean agronomists), points out that replanting can lead to a late planting date, which may reduce overall yield potential. If the final count of healthy plants is too low, repair-planting might be a more advantageous strategy than a complete replant.

Table 1: Row length for plant count by row spacing

(Multiply the number of healthy plants counted in the specified row length by 1,000 to estimate plants per acre. Repeat this evaluation in several representative locations within the field and calculate the average number of plants per acre.)

Tarnished Plant Bug Management in Pre-bloom Cotton

Dawson Kerns, LSU AgCenter Field Crop Entomologist

Tarnished plant bugs are among the most important pests of cotton in Louisiana. Injury inflicted by plant bugs results in square loss which has a direct impact on yield. Silking corn, blooming soybeans, and other flowering hosts all contribute to a growing plant bug population over the course of the growing season. Once cotton begins to square, adults will migrate into cotton and begin feeding. Before bloom, the goal is to maintain at least 80% square retention. Therefore, monitoring square retention is the best way to determine if early-season management of plant bugs is adequate. If square retention is at risk of falling below 80%, then treatment may be warranted. Additionally, a sweep net can be used to monitor adult plant bugs migrating into the field. However, it is important to consider that plant bugs will move in and out of cotton quickly during the pre-bloom window, so it is not uncommon to find poor square retention with little or no plant bugs present in the field.

Figure 1. Tarnished plant bug

ThryvOn cotton should be monitored just as non-ThryvOn cotton. The ThryvOn trait provides some suppression of adult plant bugs, but it is still prone to heavy square loss, especially when large infestations of plant bugs occur. The biggest benefit provided by ThryvOn is in the reduction of immature plant bugs during bloom.

Neonicotinoids such as Centric at 2.5 oz/a or imidacloprid at the highest labelled rate are early season options for tarnished plant bugs, but follow-up applications may be needed to maintain adequate control. Other options for plant bug control include: acephate which carries the risk of flaring spider mites or aphids, Vydate which is effective but has a limited residual of only three days, Bidrin which is not labelled for plant bugs before bloom, and Transform which is very effective, but may best be saved for the blooming window. The third week of squaring to first bloom window is the best time to begin applications of Diamond. Applications of Diamond targets immatures and will help prevent establishment of plant populations that require repetitive insecticide applications.

Boosting Soil Biology: How Organic Matter Makes a Difference

Leandro O. Vieira, LSU AgCenter Soil Fertility Specialist

Organic matter is essential for the improvement of soil biological properties, which is crucial to maintain nutrient balance and support healthy crop growth. The metabolization of organic matter in the soil, which will result in the release of nutrients through mineralization and the emission of carbon dioxide are all dependent on microbial activity. Additionally, microbial metabolism generates stable organic compounds that can remain in the soil for decades, and in some cases, centuries, before they are oxidized and released as carbon dioxide. This resistance to decomposition enables the accumulation of organic matter in the soil, which in turn contributes to the long-term health of the soil.

The increase in microbial diversity is another significant effect that is likely to have a greater impact than the increase in microbial activity alone. The proliferation of a variety of microorganisms is facilitated by the abundance of fresh organic matter, which serves as their primary food source. This diversity improves the availability of nutrients for plants by enabling various microorganisms to fix atmospheric nitrogen, break down minerals and increase the availability of phosphorus.

Figure 1. The adoption of conservation practices can lead to an increase in the population of macro- and mesofauna, such as earthworms. Earthworms play an important role in enhancing the persistence of soil organic matter and improving drainage.

Furthermore, organic matter increases the population of macro- and mesofauna, including earthworms and other soil organisms (Figure 1). There are numerous chemical and physical properties of soil that are enhanced by an increased population of these organisms, such as aeration, water infiltration, and retention. This improved soil structure promotes the growth and resilience of healthier plants.

Finally, an increase in organic matter in the soil can foster the suppression of plant pathogens by improving the diversity and activity of microorganisms. The proliferation of antagonistic microorganisms can effectively suppress detrimental plant pathogens, resulting in healthier crops and a decrease in the incidence of certain diseases.

There are several conservational practices that can promote organic matter accumulation in the soil, such as adopting cover crops, implementing no-till or reduced tillage, and applying organic amendments. Increasing the organic matter content in the soil is expected to positively impact not only soil biological properties but also its chemical and physical properties.

Nickel-fertilization in soybean can improve grain yield and nitrogen metabolism

Saulo Augusto Quassi de Castro, LSU AgCenter, Sugarcane & Soybean Agronomist

In the LA Newsletter edition of February 2025, the functions of the micronutrient Nickel (Ni) and its importance for soybean were presented. It was also pointed out that research is scarce on Ni. However, hidden Ni deficiency was already reported in soybean in field conditions worldwide. Therefore, in this edition, some results of Ni-fertilization on soybean will be presented on grain yield, nitrogen concentration, urease activity, and biological nitrogen fixation.

The first studies on Ni-fertilization in soybeans were done in controlled conditions to investigate the potential Ni toxicity on seed treatment. A dose-curve response trial was conducted with Ni rates ranging from 20.4 to 245 mg Ni lbs^-1 of seeds (45 to 540 mg Ni kg^-1 of seeds) as nickel sulfate (Lavres et al., 2016). The rate of 81.6 mg Ni lbs^-1 of seeds (180 mg Ni kg^-1 of seeds) promoted the greatest aboveground biomass, on the other hand, rates higher than this promoted Ni toxicity, reducing grain yield and aboveground biomass. Regarding mass of nodules and biological nitrogen fixation, the best rate was 20.4 mg Ni lbs^-1 of seeds, in which 99% of the nitrogen accumulated in the grains was provided by biological nitrogen fixation (Figure 1A). However, as mentioned in the previous edition, the effect of Ni-fertilization relies on genotype and edaphoclimatic conditions.

Figure 1. Soybean at reproductive stage (R1) without (left) and with 20.4 mg Ni lbs-1 of seeds, equivalent to 45 mg Ni kg-1 of seeds (right). The cups contain the fresh nodules obtained from each respective plant, showing the increase in nodule mass. Source: Lavres et al. (2016).

To evaluate the genotypes’ influence, 15 soybean genotypes were selected, and an investigation was done in controlled and in field conditions (Siqueira-Freitas et al., 2018). These authors were the first to report hidden Ni-deficiency in soybean cropped in field. The rate of 0.89 lbs Ni Acre^-1 (1 kg Ni ha^-1) as nickel sulfate increased 1.1 times the urease activity of 10 genotypes, 1.4 times the ammonia concentration in 13 genotypes, and 1.8 times ureide concentration (that is, allantoin and allantoic acid, the main form of nitrogen exported from the nodules) in 4 genotypes. These results show that nitrogen metabolism (nitrogen fixation and assimilation) was improved on soybean plants with Ni-fertilization, resulting in an increase in grain yield of 4 genotypes (up to 1,340 lbs Acre^-1). So far, Ni-fertilization response relies on soybean genotypes, and the improvements in nitrogen metabolism do not always reflect on benefits in grain yield.

To bypass any potential plant toxicity due to seed treatment or soil fertilization with Ni, an alternative is to adopt foliar Ni-fertilization. To proceed with foliar fertilization of any essential element to the plants, first of all, it is important to know if the nutrient can be absorbed through the leaves or remains on the leaf surface. Nickel applied as nickel sulfate was already demonstrated to be absorbed by the leaf, being the leaf trichomes an important pathway (Figure 2; Oliveira et al., 2024).

Figure 2. Soybean leaf 3 hours after being exposed to a 5-µL droplet of 50 mg Ni L-1 as nickel sulfate. Image is an elemental map (µg g-1) obtained by synchrotron µXRF showing nickel distribution in the leaf blade and mainly in the trichomes. Source: Oliveira et al. (2024).

Knowing that Ni-fertilization has been shown to increase nitrogen metabolism and grain yield, and that nickel sulfate can be taken up by the soybean leaves, application methods were recently investigated (Rodak et al., 2024). Trials were carried out in the field and under controlled conditions. Application of nickel sulfate through foliar (0.29 oz Ni Acre^-1, equivalent to 20 g Ni ha^-1), soil (0.89 lbs Ni Acre^-1, equivalent to 1 kg Ni ha^-1), or seed treatment (0.04 oz Ni Acre^-1, equivalent to 2.5 g Ni ha^-1) as well as its interaction were investigated in both field and controlled conditions. Regarding the field trials, the combination of soil + foliar application and seed treatment + foliar application promoted the highest grain yield, an increase of 583 lbs Acre^-1 on sandy clay soil and 1036 lbs Acre^-1 in a sandy loam soil. These application methods also promoted the greatest biological nitrogen fixation efficiency, while photosynthesis and leaf urease activity increased regardless of Ni application method (Figure 3). A concern of Ni-fertilization is a potential increase in nickel content in the grains. So far, in this work, the nickel concentration in the grains didn’t differ from the control treatment without Ni-fertilization, showing that Ni-fertilization will not affect grain food safety.

Figure 3. Comparison of soybean plants and their respective leaves at R2 stage (full flowering stage) grown without Ni-fertilization (Control treatment) and when nickel sulfate was applied through soil application + foliar fertilization (Soil + Leaf) and through seed treatment + foliar fertilization (Seed + Leaf). Source: adapted from Rodak et al. (2024).

It is important to highlight that Ni response relies not only on Ni rate but also on soybean genotypes and edaphoclimatic conditions, therefore, the rates that were presented in the text are not Ni-fertilization recommendations and must be tested in each condition prior to broad use. Trials have been carried out at Dean Lee Research and Extension Center and at Iberia Research Station to investigate Ni rates using nickel sulfate applied in-furrow or/and by foliar application.

References

1. Lavres, J., Castro Franco, G., de Sousa Câmara, G.M. (2016) Soybean Seed Treatment with Nickel Improves Biological Nitrogen Fixation and Urease Activity. Frontiers in Environmental Science 4: 184203

2. Oliveira, J.B., Lavres, J., Kopittke, P.M., Chaney, R.L., Harris, H.H., Erskine, P.D., Howard, D.L., dos Reis, A.R., van der Ent, A. (2024) Unravelling the fate of foliar-applied nickel in soybean: a comprehensive investigation. Plant and Soil 502: 537–556

3. Rodak, B.W., Freitas, D.S., Rossi, M.L., Linhares, F.S., Moro, E., Campos, C.N.S., Reis, A.R., Guilherme, L.R.G., Lavres, J. (2024) A study on nickel application methods for optimizing soybean growth. Scientific Reports 14: 10556

4. Siqueira Freitas, D., Wurr Rodak, B., Rodrigues dos Reis, A., de Barros Reis, F., Soares de Carvalho, T., Schulze, J., Carbone Carneiro, M.A., Guimarães Guilherme, L.R. (2018) Hidden Nickel Deficiency? Nickel Fertilization via Soil Improves Nitrogen Metabolism and Grain Yield in Soybean Genotypes. Frontiers in Plant Science 9: 328382

Predictions for Irrigation in 2025

Stacia L. Davis Conger, LSU AgCenter State Irrigation Specialist

While spatial variability of rainfall remains high, Louisiana experienced “normal” spring weather conditions with some favorable windows for planting agronomic row crops between rain events (Fig. 1). If anything was abnormal, peak daytime temperatures remain mild for May. Progression reports for the end of April indicate that most corn, rice, and soybeans were planted with cotton catching up quickly.

As planting transitions into management, one of the many next steps should include decisions associated with setting up irrigation. Current climate predictions suggest above-normal temperatures and above-normal rainfall this summer. Since we’ve all been led astray by weather forecasting models in the past, predictions from the 2025 Old Farmer’s Almanac also describe the same conditions. For planning purposes, the timing of rainfall will become extremely important to irrigation strategies this year.

Above-normal rainfall can occur in three ways: 1) More frequent rainfall events during the crop season, 2) More intense rainfall when events happen, or 3) A combination of both options 1 & 2. While options 1 & 3 will introduce an issue with drainage more than irrigation, option 2 will manifest the need for critical and timely irrigation despite climbing rainfall totals.

High intensity rainfall will generate mostly surface runoff, especially in our heavy soils with low infiltration rates. For example, it’s unlikely that 4.5 inches falling on a single day will satisfy crop needs for longer than a week (more or less) because only a fraction of the water reaches the root zone where it becomes beneficial to the crop. Slow and steady rain will always have higher water use efficiency than intense rain that sparks flash flooding risks.

The availability of soil water becomes critical as crops transition into reproductive stages. This translates to V10 through R6.5 for soybean, V8 through ¾ milk line for corn, first flower through open bolls for cotton, and active tillering through flowering for rice. These stages reflect general averages across varieties and technologies; please consult LSU AgCenter crop specialists for more specific recommendations.

Since above-normal temperatures suggest increased evapotranspiration this summer, irrigation planning should reflect option 2 to reach yield goals, which means that we will likely need irrigation between rainfall events. This will be a great year to try out LSU AgCenter’s DIRT webtool for scheduling irrigation found here: www.lsuagcenter.com/DIRT.

Figure 1. Spring 2025 monthly rainfall totals for each region of Louisiana. Data was summarized from the Louisiana Agriclimatic Information System (www.lsuagcenter.com/weather).

LSU AgCenter Specialists