Should you apply pre-tassel nitrogen in corn?

Rasel Parvej, Dan Fromme, Josh Copes, and Syam Dodla

Nitrogen is the most yield limiting nutrient for corn production. Corn requires nitrogen for amino acids, protein, and chlorophyll production. Chlorophyll is the key component for photosynthesis. Insufficient chlorophyll content results in reduced yield potential. A 200-bushel corn crop 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 are prone to loss to the environment through volatilization, denitrification, and/or leaching. Volatilization loss is high in hot and humid climates such as Louisiana and alkaline soils if the fertilizer (especially urea) is not incorporated within a few days. Leaching loss is high in high rainfall areas especially in sandy to sandy loam soils with low cation exchange capacity (CEC). Denitrification loss is the main concern in poorly drained soils but can occur in any soils with excessive rainfall that creates water-logged anaerobic conditions. Like this year, excessive rainfall often occurs in lower Mississippi Delta during the early corn growing season resulting in saturated soils for several days and accelerates nitrogen loss via denitrification. Therefore, nitrogen management in corn is one of the biggest concerns for producers every year. It is recommended to apply nitrogen in at least two 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 near 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 research shows that two applications are good enough to maximize corn yield under ideal conditions for most soils with medium to high CEC (>10), nitrogen application in three splits with 1/4 at planting, 2/4 around V5-V6 stage, and 1/4 before tasseling could be beneficial for coarse-textured low CEC (<10) soils as well as for poorly-drained soils that are very prone to water-logged conditions. This also helps in years with excessive rainfall during the early corn growing season, which increases nitrogen losses. Including a pre-tassel application in nitrogen fertilization program can help reduce nitrogen losses and ensure adequate nitrogen supply during the maximum nitrogen uptake period from V10 to grain-filling stage. It also helps adjust nitrogen rate based on crop growth, yield potential, environmental forecasts, crop sensing (NDVI, SPAD, etc.), and tissue testing. Many land-grant universities including LSU AgCenter trials showed that pre-tassel nitrogen application can increase corn yield when part of the 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 uppermost fully developed entire leaves below the whorl should be collected around V12-V13 stage and sent immediately to the lab for nitrogen concentration. This would allow enough time for the producer to get the results back and make a decision. The critical (normal) corn leaf nitrogen concentration around pre-tassel stage ranges from 2.75 to 3.5%. Leaf nitrogen concentration below 2.75% would be considered low and above 3.5% would be high. One caveat about tissue testing is, it may not always accurately diagnose nitrogen deficiency and indicate pre-tassel nitrogen need because nitrogen concentration in corn leaf is highly influenced by crop growth and dilution factor. For example, leaf nitrogen concentration can be high due to insufficient plant growth (low dilution) associated with drought, diseases, and pest infestation. Therefore, care should be taken interpreting leaf nitrogen concentration. Overall, a producer should consider rainfall amount following sidedress nitrogen application, field conditions, crop growth, yield potential, and/or tissue-testing when deciding to apply pre-tassel nitrogen.

Applying high rates of urea ammonium nitrate (UAN) as a foliar application is not recommended due to the potential for severe foliage burn (Figure 2). The pre-tassel nitrogen rate should be 15 to 25% of the total nitrogen applied i.e. roughly 40 to 60 lb nitrogen per acre. Producer can choose dry (urea) or liquid (UAN) nitrogen source. Both dry and liquid nitrogen fertilizers 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, minimize nitrogen losses, and avoid foliage damage. Application before an expected rain (about 0.25-inch) or pivot irrigation is recommended to incorporate applied nitrogen that will minimize foliage burn as well as volatilization loss. Further, multiple studies conducted at LSU AgCenter showed that use of N-stabilizers improves the efficacy of applied nitrogen fertilizer up to 20%.

Figure 1. Nitrogen deficient corn in saturated soils due to
excessive rainfall. (Source:Pioneer - Nitrogen Application Timing)

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.

Nitrogen fixation in soybeans

By LSU AgCenter soybean specialist David Moseley and agronomist Rasel Parvej

There have been recent concerns about possible conditions leading to deficient nitrogen fixation in soybean plants. A planting date trial at the Dean Lee Research Station was established on March 30, 2020. It has been between five to six weeks since planting; therefore, this is a good time to evaluate for active nitrogen fixation.

Soybean plants are able to fix up to 70% to 75% of their nitrogen requirement by converting atmospheric nitrogen gas (N₂) into ammonium (NH₄⁺). This nitrogen fixation process is from a symbiotic relationship between the soybean plant and Bradyrhizobium japonicum bacteria colonies living on the soybean root. The colonies appear in the form of nodules attached to the root. The remaining nitrogen requirement of the soybean plant is usually available from the soils, with no supplemental nitrogen required. Furthermore, applying supplemental nitrogen to soybean plants can decrease the amount of nitrogen fixed through the symbiotic relationship. The seed should be inoculated with Bradyrhizobium japonicum bacteria when planting into fields that were not planted to soybean for the previous three to five years and in fields that were previously under sustained flooded conditions.

Unfortunately, the symbiotic nitrogen fixation process can fail under certain environmental conditions. Soils that are coarse-textured, compacted, have low or high pH, contain high residual soil nitrogen levels or are flooded for three or more days may negatively affect the nitrogen fixation process. In low-pH soils, nitrogen fixation is negatively affected due to the low availability of molybdenum (Mo), a key component of the nitrogenase enzyme that drives the biological nitrogen fixation process. Poor nodulation may also result from unhealthy soybean plants that cannot supply an adequate amount of carbohydrates to the bacteria.

Soybeans should be planted in fields with a soil pH around 6.5. If soybeans are planted in fields below 6.0 pH, molybdenum should be applied as a seed treatment. However, molybdenum should not be used as a seed treatment with Rhizobium inoculum unless planting immediately after treating the seed. Fields with soybean plants showing nitrogen deficiency (short and pale green to yellow plants with non-prominent veins) should be evaluated for poor nodulation by digging up and washing the roots. At the V3 to V5 growth stages, at least seven nodules (2 millimeters or greater in size) with a pink or red colored cross-section should be found in each plant. If the plants show nitrogen deficiency, applying supplemental nitrogen may be economical. If applying nitrogen, use granular/dry nitrogen fertilizers that would reduce foliage burning. The rate of supplemental nitrogen required depends on soybean growth stage, average yield history and available nitrogen from the soil. Usually, soybeans remove about 4 pounds of nitrogen per bushel of grain harvested.

Figure 1. Soybean plants with nitrogen deficiency symptoms. The
limited quantity of nitrogen in the plant is translocated to newer
leaves, leaving older growth with a pale-green to yellow color.

Figure 2. Bradyrhizobium japonicum bacteria nodules attached to
the roots of a soybean plant at the V3 growth stage. A penny gives
perspective to the size of the nodules.

Figure 3. A cross-section view of a Bradyrhizobium japonicum
bacteria nodule from a soybean root. The cross-section view of the
nodule has a pink or red color, suggesting the nodule is healthy and
actively fixing nitrogen. A penny gives perspective to the size of the
nodules.

Insect situation for May 2020

By LSU AgCenter entomologists Sebe Brown and Tyler Towles

Cotton

Thrips pressure around much of the state has been light thus far in the growing season. Based on our thrips surveys in central Louisiana, tobacco thrips are the predominant species. These findings coincide with what consultants are seeing in north Louisiana as well. Much of the April-planted cotton has surpassed two to three true leaves, and insecticide seed treatments appear to be performing well in most scenarios. However, several central Louisiana cotton fields 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 to 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. 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.

Soybeans

With the lack of winter two years in a row, redbanded stink bug (RBSB) numbers have been building in alternative hosts around the state. RBSBs were found the first week of January at the Dean Lee Research Station and have continued to increase as spring approached. RBSBs were found in February in northeast Louisiana. This is not unexpected and should not alarm soybean producers. These numbers serve as an indicator that RBSBs are in the environment and populations may be high. RBSBs are typically attracted to beginning seed (R5) soybeans; however, when alternative legume hosts begin to senesce, RBSBs will often move into vegetative and early reproductive stage soybeans. Although young soybeans do not have reproductive structures for RBSBs to directly feed on, these insects will feed on xylem. With no soybean seeds present to injure, stink bug feeding on young beans may contribute to green bean syndrome. With this in mind, are insecticide applications targeting stink bugs on vegetative and early reproductive soybeans justified? With low soybean prices and rising insecticide costs, protecting yield and seed quality is fundamental, but receiving the largest economic return for your insecticide application is important to consider also. Saving insecticide applications for instances that produce direct yield and quality savings may be the best option with an uncertain soybean market.

New entomologist

Tyler Towles has recently accepted the role as a field crop entomologist stationed at the Tom H. Scott Research and Extension Center near Winnsboro. His official start date was May 1. His position’s responsibilities are split as 80% research and 20% extension. Towles graduated with his doctorate from Mississippi State University in May 2020, and his dissertation focused on cotton bollworm emergence from various corn refuging strategies. He can be reached by cell phone at 662-820-4217 or by email at TTowles@agcenter.lsu.edu.

Economics of replanting soybeans

By LSU AgCenter economist Michael Deliberto, weed scientist Josh Copes and soybean specialist David Moseley

Although there have been dry conditions in the south and wet conditions in the north, the 2020 planting season in Louisiana is off to a good start. The USDA National Agricultural Statistics Service reports 68% of the soybean crop in Louisiana has been planted, with 46% of the crop emerged as of May 10. The percent planted and emerged is ahead of last year and even to slightly ahead of the five-year average. The statewide soybean crop is rated as 96% fair to excellent. Hopefully, the forecasted heavy rains will not slow down the planting progress or cause damage to the planted acres.

Severe weather, including heavy rains and tornadoes, hit parts of Louisiana in late April. There have been observations of poor soybean stands, including some acres requiring a replant. There have also been a few concerns of soybean stands resulting from poor seed-to-soil contact in cover crop and no-till systems.

Economics is the most important aspect to consider when deciding whether to replant soybean. Factors that impact the economics of replanting are final plant stand, date of replanting, and the additional costs of seed and planting. Typically, mid-April- to mid-May-planted soybeans will produce the highest yields; planting later can result in yield loss. A thorough assessment of the final soybean stand should be conducted prior to replanting. Data from the LSU AgCenter suggest final populations of less than 70,000 to 75,000 may result in significant reduction in yield. In addition, inspect the fields for wide gaps where the soybean canopy will not close, as this can result in increased weed pressure, decreased soil moisture and potentially a decrease in yield.

The economic impact on direct farm-level production costs from replanting soybeans can result in an increase in the number of bushels that will be required to offset the incurred production expenses associated with replanting field operations. The severity of this will depend on the type of soybean technology employed, as differences in the prices for seed, seed treatments and seeding rate can influence the replanting costs and, hence, the number of additional bushels required at harvest to offset those costs.

The following economic analysis employs a general farm management approach to calculate the breakeven yield required to cover the increase in replanting costs across alternative soybean price levels. Under the assumption that the farm’s yield is expected to be 55 bushels per acre, estimated total variable production costs for Roundup Ready soybeans under irrigation are $397.30 per acre. The breakeven yield under these imposed conditions is calculated to be 44.14 bushels per acre, assuming a $9.00 per bushel price. The breakeven price is calculated to be $7.22 per bushel. This can be interpreted to the extent that a producer would need to receive in excess of $7.22 per bushel to cover the total variable production costs per acre.

Planting costs (machinery, fuel and labor) are comprised of a planter, tractor and seed. Planter costs are estimated to be $4.08 per acre in addition to the $65.00 seed cost to total $69.08 per acre. If a producer determines that a field must be replanted, the total variable costs for the growing season will increase from $397.30 to $466.38 per acre, reflective of the aforementioned replanting costs. Normal production conditions are assumed. The breakeven yield under these imposed conditions is calculated to be 51.82 bushels per acre, assuming a $9.00 per bushel price. This in an increase of approximately 8 bushels (7.68 bushels) per acre to offset the increase in production costs while the $9.00 price is held constant. The breakeven price is calculated to be $8.48 per bushel, an increase of $1.26 per bushel.

When the market price is varied from $12.00 to $8.00 per bushel (yield of 55 bushels per acre held constant), it is observed from Figure 1 that as price decline, more production is required to cover the increase cost of replanting.

Figure 1 that as price decline, more production is
required to cover the increase cost of replanting.

Figure 2. The required increase in breakeven yields from replanting
a 55-bushel-per-acre potential field across multiple price levels. As
the soybean market price declines the required increase in
breakeven yield also increases from 5.76 ($12.00 price) to 8.64
bushels per acre ($8.00 price). Alternatively, if a producer were to
receive a discount on soybean seed, the required level of additional
yield would decrease. The economic logic behind this is that a
smaller increase in seed costs coupled with planting expenses
($4.08 per-acre) would require a smaller number of additional
bushels being required to offset the increased costs. For example,
is the seed is discounted 50%, the seed costs would be $32.50 per
acre. When coupled with the replanting expenses, the discount
would equate to a $36.58 per-acre cost compared to the $69.08 per
acre expense under the first scenario.

Figure 3. The required increase in breakeven yields from replanting
a 55 bushel per acre potential field across multiple price levels
under the discounted seed scenario. As the soybean market price
declines, the required increase in breakeven yield also increases
from 3.05 ($12.00 price) to 4.57 bushels per acre ($8.00 price).

Corn disease update

By LSU AgCenter plant pathologists Trey Price and Boyd Padgett

Holcus spot/paraquat drift

A few reports of paraquat drift or Holcus spot have surfaced statewide. The two maladies display round to oval, light tan to white spots with or without yellow halos. They are difficult to distinguish from each other based on symptoms alone. Generally, if a drift pattern (gradient) is observed, if affected areas are large and more jagged than round, or if secondary fungi are within lesions, it is likely paraquat drift (Figure 1). If the distribution is random, the spots appear within 48 hours of a thunderstorm and water soaking is observed, it is likely Holcus spot (Figure 2). Microscopic observation of Holcus spot may reveal bacterial streaming, as the disease is caused by Pseudomonas syringae pv. syringae. Both issues are usually of minor concern, with the exception of paraquat drift heavy enough to affect corn stand and yield.

Figure 1. Paraquat drift on corn.

Figure 2. Holcus spot of corn.

Common rust

Common rust may be found this time of year in most corn fields. Pustules of common rust are brick red to dark orange in appearance, somewhat elongated and will appear on both leaf surfaces (Figure 3). Common rust will progress during relatively cool, rainy and cloudy weather; however, very rarely are fungicide applications warranted for common rust. Warmer temperatures will greatly slow common rust development. We do not have any confirmed southern rust observations in Louisiana to date. Southern rust pustules are more orange than brick red, usually not as elongated and almost always appear on the upper surface of leaves (Figure 4).

Figure 3. Common rust.

Figure 4. Southern rust.

Northern corn leaf blight

We are starting to find northern corn leaf blight (NCLB) in central and northeast Louisiana (Figure 5). This disease will first appear in susceptible hybrids in fields following corn with reduced tillage. The disease will progress slowly during dry weather and more quickly during regular rainy periods. Most of the time, fungicide applications are not needed for NCLB. Most hybrids have some degree of resistance, and the corn crop usually fills out before the disease is severe enough to impact yield. However, severe disease may occur in susceptible hybrids that are following corn in reduced tillage situations. These are the fields that need to be watched closely.

Figure 5. Northern corn leaf blight.

Fungicide considerations

Fungicide application decisions should be carefully considered field by field based on disease severity (Figure 6), crop stage (Table 1), hybrid susceptibility (link), fungicide efficacy (Table 2), tillage regime, prevailing environmental conditions, previous experience, commodity price and the probability of a return on the investment. If applications are warranted, apply at labeled rates using maximum (5 GPA by air minimum) water volume is recommended.

Figure 6. NCLB disease severity scale.

Table 1. Percent yield loss as a result of defoliation by crop stage. For example, 30% defoliation at dent stage results in a 2% yield loss.

% Defoliation

CDWG Fungicide efficacy table-2020-LApdf

LSU AgCenter specialists