Louisiana Soybean Progress and Condition Update – June 2022

David Moseley, LSU AgCenter Soybean Specialist

Article Highlights:

• The Louisiana planting season was ahead of the five-year average and much faster than in 2021.
• The condition of the soybean crop is good, but dry conditions have caused some stress on the plants.
• Symptoms of manganese deficiency have been seen in several fields this year.
• Redbanded stinkbugs have reached threshold in some areas.

Louisiana Soybean Progress

According to the USDA, approximately 100% of the expected 1.2 million acres of soybean in Louisiana were planted by June 5, 2022. The planting progress was ahead of the five-year average (93% by June 5th) and much faster than last year (85% by June 5th). By June 12th, approximately 59% of the Louisiana soybean crop was blooming and 18% was setting pods (R3 growth stage). I have been encouraged to see many acres of soybean getting close to closing the canopy.

Louisiana Soybean Condition

In the June 12th report, the USDA estimated 87% of the Louisiana soybean crop is rated good to excellent. The condition has improved since the May 29th report where 71% was rated as good to excellent. Although the soybean crop looks good overall, we have had hot and dry conditions across the state. At one farm we compared the same variety planted on the same date in an irrigated field and a non-irrigated field. The plants in the non-irrigated field seemed to be one week behind in growth compared to the irrigated field. On June 15th, I was at the core-block in Franklin Parish. The farmer was irrigating the plot (Figure 1) for the third week in a row. Last year I wrote some farmers did not begin irrigating until late in the growing season due to consistent rainfall.

Figure 1. Irrigating the soybean core-block trial in Franklin Parish on June 15, 2022.

I have had a few conversations about manganese deficiency symptoms in soybean over the past few weeks. Dry conditions can cause manganese deficiency symptoms. A few weeks ago, we wrote an article on Identifying and correcting Manganese deficiency in Soybean. In the article we mention it is possible to correct manganese deficiency by apply manganese, but the deficiency may also be corrected after a rain or increased root growth.

Last week, Dr. James Villegas (LSU AgCenter entomologist) sent a text about redbanded stink bugs (RBSB) through the Remind texting system. Dr. Villegas was finding 12-16 RBSB per 100 sweeps on R3 to R4 soybeans at the Dean Lee Research Station. The threshold for RBSB is 16 adults and nymphs per 100 sweeps. Continue to scout the fields, especially as the plants begin to set pods and develop seed.

Cotton Mid-Season Management

Matt Foster, LSU AgCenter Cotton Specialist

Article Highlights:

• Dry conditions have been prevalent in some cotton producing areas of Louisiana.
• Cotton can lose at least 60-100 pounds of lint if drought stress occurs at the wrong time.
• Overall, the cotton crop looks good.

May and June have been unusually dry in most areas of Louisiana, which increases supplemental irrigation needs. Precisely timing irrigation when soil moisture is less than optimal can help preserve cotton yield potential. Research has shown that a 1-inch water deficit at the wrong time can result in the loss of at least 60-100 pounds of lint. The goal in Louisiana is to irrigate before plant stress occurs with a water amount that won’t waterlog the soil if subsequent rainfall is received.

Approximately 60% of cotton acres in Louisiana are irrigated, with furrow-irrigation being the main method. Irrigation timing varies due to weather, cultural practices, soil type, and the status of the crop. One method to aid in timing the first irrigation is to install soil moisture sensors that can determine soil moisture at 6 or 12-inch intervals in and below the root zone. In general, the first irrigation should begin when 50% of the available moisture has been depleted from the root zone. This ensures good root development and reduces the risk of soil saturation early in the growing season. At bloom, irrigation shouldn’t promote rank growth or hinder root development. In general, irrigation is terminated just prior to the first open boll. Caution should be used as excess soil moisture at this time can delay maturity and make defoliation more difficult.

Despite dry conditions, the cotton crop throughout the state looks good. Insect pressure from thrips was heavy enough in some areas to justify foliar insecticide applications. As of June 13, approximately 40% of the crop is squaring. Since squaring began, populations of aphids, fleahoppers, and plant bugs have reached treatable levels in some areas of the state. As more of the crop begins to square, growers will need to focus on square retention and managing plant height with plant growth regulator (PGR). Once cotton reaches match head square stage, plant growth, environmental conditions, and square load should be monitored. A few factors to take into consideration when planning for pre-bloom applications of PGR include variety growth habits, soil type, and total nitrogen available to the crop. PGR applications should be based on current plant growth characteristics and the anticipated growth rate based on expected growing conditions for the next 7 to 10 days.

Plant Bugs: Scout, Identify, and Control

James Villegas and Tyler Towles, LSU AgCenter Entomologists

Article Highlights

• Scout frequently and rotate insecticides
• Pre-bloom thresholds – 8 plant bugs per 100 sweeps and 80% square retention
• Post-bloom threshold – 2-3 plant bugs per drop cloth sample
• Multiply clouded plant bugs by 1.5 when determining treatment decisions

Plant bugs are among the most economically important insect pests of cotton. In Louisiana, the term “plant bugs” refers to a group of closely related pests that include the tarnished plant bug, the cotton fleahopper, and the clouded plant bug (see pictures below). Both adults and nymphs (immature stages) have piercing, sucking mouthparts and feed on squares, flowers, and bolls. Feeding injury by plant bugs on small squares and bolls results in reductions in size, quality, and yields, and delays maturity. Larger bolls are relatively unaffected by plant bug injury; however, clouded plant bugs are known to injure larger bolls. To reduce economic losses from plant bugs, scouting and treatment should be conducted in both pre-and post-bloom cotton.

*Tarnished plant bug and cotton fleahopper – photos courtesy of LSU AgCenter. Clouded plant bug – photos courtesy of the University of Tennessee.

Prior to bloom, the goal is to maintain between 70–85% first position square retention. If squares are shedding at pre-bloom, there’s a big chance that plant bugs are the culprit. Use a sweepnet to check for plant bug presence and monitor square retention before the treatment decision. The threshold to treat at pre-bloom is 8 or more plant bugs per 100 sweeps and 80% square retention. Neonicotinoids generally perform better at pre-bloom. Avoiding the overuse of pyrethroids, organophosphates, and carbamates in pre-bloom cotton preserves beneficial insects, prevents flaring secondary pests, and aids in resistance management.

Sampling and plant bug thresholds changes when cotton plants bloom. The threshold to treat is 2–3 plant bugs per drop cloth sample (0.6 per row foot). At early bloom, both sweepnet and drop cloth can be used to monitor plant bugs. Multiply clouded plant bugs by 1.5 when determining densities for treatment decisions. Several insecticides are effective against plant bugs at post-bloom but choose carefully depending on individual field scenarios. For instance, growth regulators (e.g., Diamond) control only immature plant bugs and thus should be tank-mixed with an adulticide if substantial adults are present. Frequent scouting and insecticide rotation are keys to long-term success in managing plant bugs. Table 1 below shows recommended insecticides from LSU AgCenter 2022 Insect Pest Management Guide.

Table 1. Insecticide recommendations for plant bugs (cotton fleahoppers, tarnished plant bugs, and clouded plant bugs) in pre-and post-bloom cotton.

Cotton In-Season Nitrogen Deficiency Diagnosis

Rasel Parvej and Jamil Uddin, LSU AgCenter Soil Scientists

Article Highlights:

• Petiole sampling to monitor in-season N status should be started at one week prior to first bloom and continue for 4 to 5 weeks after first bloom.
• The sufficient petiole nitrate-N concentration ranges from 10,000 to 35,000 ppm (1 to 3.5%) at first bloom and the sufficient leaf blade N concentration ranges from 3 to 4.5% N at first bloom.
• Petiole nitrate-N concentration should be interpreted, and in-season N fertilization decision should be made based on physiological, environmental, and nutritional conditions to make sure that the observed petiole nitrate-N concentrations have not been influenced by any stress during the growing season.

Monitoring petiole nitrate-N concentration is the best tool of tracking in-season nitrogen (N) status in cotton. A successful petiole nitrate-N monitoring program consists of several weekly petiole samplings starting at one week prior to first bloom and continue for 4 to 5 weeks after first bloom. At each sampling time, at least 20 uppermost recently mature leaves with petioles on the vegetative stem should be collected. These leaves are usually the 3^rd to 5^th leaf from the terminal. The petiole of each sampled leaf should be separated from the leaf blade, placed in a labeled paper bag, and sent immediately to the plant diagnostic lab for nitrate-N, total phosphorus (P), and total potassium (K) concentrations. The total N concentration in leaf blade at first bloom is also important in interpreting petiole nitrate-N concentration. Therefore, the leaf blade (without petiole) of the collected sample at first bloom should also be analyzed separately for total N concentration. Leaf blade testing is not required at other sampling times. The K concentration in the petiole is also valuable in monitoring in-season K nutritional stress since K is one of the key nutrients in cotton production.

Cotton petiole nitrate-N concentration across the blooming period can be interpreted using the nitrate-N concentration in Table 1. The sufficient petiole nitrate-N concentration ranges from 10,000 to 35,000 ppm (1 to 3.5%) at first bloom and 1,000 to 5,000 ppm (0.1 to 0.5%) 6-wk after first bloom and the sufficient leaf blade N concentration ranges from 3 to 4.5% N at first bloom stage. It is very important to note that these sufficiency ranges of petiole nitrate-N concentration across the blooming stages are not the critical levels, but desirable ranges, and nitrate-N concentrations below or above these desirable ranges do not directly indicate N deficiency or sufficiency because petiole nitrate-N concentration is greatly influenced by plant stress caused by several abiotic and biotic factors. Along with nitrate-N concentration, P concentration in petiole during the blooming period is very important in understanding environmental or physiological stress that can influence petiole nitrate-N concentration. For example, petiole P concentration at first bloom should be >800 ppm (>0.08%) and a decrease of >300 ppm (>0.03%) petiole P concentration from the previous week during blooming period is a good indicator of water stress and in-season N fertilization decision should be delayed until the next sampling results are received. For proper interpretation of petiole nitrate-N concentration, it is very important to make sure that the observed petiole nitrate-N concentrations have not been influenced by any stress during the growing season.

Table 1. Sufficiency ranges of nitrate-N (NO₃-N) concentration in cotton petiole during blooming period in Arkansas.

Source: Mitchell C.C., and W.H. Baker. 1997. Plant nutrient sufficiency levels and critical values for cotton in the southeastern U.S. Proceedings of the Beltwide Cotton Conference, National Cotton Council, Memphis, TN. 1:606-609.

Soybean In-Season Potassium Deficiency Management

Rasel Parvej, David Moseley, and Jamil Uddin, LSU AgCenter Scientists

Article Highlights:

• Hidden K deficiency often occurs in soybean fields that are low to medium in soil-test K level.
• Hidden K deficiency can be diagnosed by taking trifoliolate leaves sample at the R2 stage and analyzing for K concentration.
• The critical leaf K concentration at the R2 stage is 1.46% and K deficiency can be corrected by applying 60 pounds K₂O/acre (100 lb Potash/acre).

Potassium deficiency symptoms in soybean first appear as irregular yellowing on the edges of K deficient leaves and can occur as early as at the V3 vegetative stage (three trifoliolate leaves) mainly on the lower older leaves (Figure 1). But symptoms often occur on the upper younger leaves during the reproductive stages especially in severe K deficient soils (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 growth 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 (80 to 120 ppm or 160 to 240 lb Mehlich-3 K for 0- to 6-inch soil depth), have not received K fertilization, have coarse-textured soils with high leaching potentials due to low cation exchange capacity (CEC) and excessive rainfall, or undergo severe drought conditions.

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. For proper tissue sampling, 15 to 20 recently mature trifoliolate leaves excluding petioles from the 3^rd node from the top of the soybean plant should be collected, the date and soybean growth stage should be recorded, and the sample should be sent immediately to the plant diagnostic lab for K concentration. The critical K concentration at the R2 stage ranges from 1.46 to 1.90% and any K concentration below the critical level would be deficient and above the critical level would be sufficient (Figure 3). 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 for properly interpretation of tissue K concentration.

Soybean K deficiency can easily be corrected by top-dressing or flying 60 pounds K₂O per acre (100 lb Muriate of Potash per acre; 0-0-60) until the R5 stage or about 5-weeks past the R2 stage. Foliar application of liquid K would not be effective and economic to correct severe K deficiency since foliar product contains very small amount of K. Also, foliar product requires several applications since K has a high salt index that can burn soybean foliage if applied in high concentrations.

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 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)

Water Quality and Irrigation Workshop

Stacia Conger, LSU AgCenter Irrigation Specialist

There will be a water quality and irrigation workshop at the Red River Research Station on July 14, 2022 from 3:30 – 6:30 PM. Several topics on irrigation, soil health, fertility, precision ag, row rice, crop updates, and pest management will be included. RSVP by clicking this link, scanning the QR code below, or by contacting Donna at DHaynes@agcenter.lsu.edu (318-741-7430).

Figure 1. QR code to register for the Water Quality and Irrigation Workshop.

LSU AgCenter Specialists