Soybean production guidelines prepared by LSU AgCenter cooperating personnel from Louisiana Agricultural Experiment Station researchers and by Louisiana Cooperative Extension Service specialists.

Grain yield is probably the single most important trait to consider!! Other variety characteristics indirectly affect yields as well. The data in Table 1 give current year and two-year averages when available. Performance of superior varieties is indicated by bold type and footnotes in Table 1. Varieties are listed alphabetically.

Soybean varieties in Maturity Group IV-V are suggested for Louisiana because they consistently outperform other maturity groups in quality and yield per acre. The early indeterminate varieties have been grown successfully in certain parts of Louisiana. But, poor seed quality and excess shattering can be a problem with them if weather conditions before harvest are warm and wet.

For best use of this guide, refer to Tables 1A-1C for variety yield results and pick the location fitting your situation. If your farm does not fit any location, choose varieties that perform well at multiple locations.

Other Varietal Characteristics

Yield per acre is an important trait in selecting a variety, but other varietal characteristics should be considered. One or more of the following may be important, depending on the individual farm situation.

Herbicide Tolerance and Weed Management Programs

Liberty Link^® soybean is tolerant of the herbicide glufosinate (Liberty), which is a non-selective, broad spectrum herbicide. There is information from the LSU AgCenter on Liberty Link^® variety performance. Do not apply Liberty^® herbicide to any variety not labeled as Liberty Link^®, which includes, but not limited to, Roundup Ready^® and/or STS^® varieties. Applications of Liberty (glufosinate) to non-Liberty Link^® soybean will cause significant injury to and/or death of non-Liberty Link^® soybean. Regardless of the herbicide tolerant technology utilized, application of a residual herbicide prior to soybean emergence followed by a postemergence application of a residual herbicide that is tank-mixed with a non-selective herbicide is the best strategy to manage herbicide-resistant weeds. Furthermore, research has shown that maintaining soybean weed-free for the first five weeks after emergence can maximize soybean yield.

Disease Resistance

Varieties of soybeans differ in susceptibility to diseases and nematodes. Aerial blight is an important disease south of Alexandria, but can spread into other parishes during wet seasons. The disease Cercospora (spp.) has become major disease problem throughout the state. These late-season diseases cause yield loss and harvest delays. For other diseases, Phytophthora root rot is more prevalent in clay or poorly drained soils, and root knot nematodes are more prevalent in sandy soils. Cyst nematodes may occur on all soils. When these diseases occur, results can be devastating and variety selection is very important.

Salt-Chloride Tolerance

Soybeans under continuous irrigation may be subjected to high levels of salts or chlorides from well or surface water. Observations from several years at the Macon Ridge Branch Station at Winnsboro have made it possible to pinpoint varieties that have resistance or sensitivity to the problem. The problem shows up as leaf scorching and usually occurs shortly after irrigation water is applied.

Maturity

There is a certain amount of overlap in maturity between groups within the state. Environmental conditions, especially drought, can cause variation in maturity. In the southern region of Louisiana, varieties may mature five to 10 days later than in the northern region. Most varieties within a group mature in the following range when planted at recommended times:

• Very early maturity — Aug. 10 - Aug. 19

• Early maturity — Aug. 20 - Sept. 10

• Early medium maturity — Sept. 11 - Oct. 1

Table 2 indicates the approximate date of maturity of varieties if planted at the optimum time. Where large acreages are involved, varieties of differing maturity should be selected to stagger the harvest and avoid loss from shattering and poor quality.

Lower Pod Height

Pod height is especially important in rough, poorly drained or new ground, but is important for all varieties to set pods a reasonable distance above the soil surface to aid in harvest.

Poor Drainage

Most soybeans in Louisiana are planted on heavy clay soils with poor internal drainage. Research has determined that certain varieties are superior to others under these conditions. Consult results from the St. Joseph Sharkey clay test to select varieties for tolerance to poor drainage. Planting on raised beds is desirable where drainage is less than optimal.

Lodging Resistance

Soybean varieties are more likely to lodge if a population of more than six plants per foot of row is present and if grown on highly fertile soil. Tall varieties tend to lodge more severely than short ones. When lodging occurs, seed quality and yield are affected. A lodged field is more susceptible to disease and reduces harvest efficiency.

Plant Height

Plant height varies according to growing conditions, planting date, soil type and variety. Table 2 indicates relative plant height among varieties tested in 2015. If canopy closure has been a problem, a taller variety should be selected or closer row spacing adopted. On highly fertile soil, too much growth is sometimes a problem, and a shorter variety is the better choice.

Seed Quality

Poor seed quality is more often found in early maturing varieties. This is especially true for indeterminate varieties which do not mature uniformly. However, in wet harvest seasons when temperatures and humidity remain high, most varieties will have the problem. Poor seed quality occurs when fields are not harvested when ready or under heavy pressure of some diseases. When poor conditions occur between physiological maturity (maximum dry matter accumulation) and harvest, chances increase for a decline in seed quality.

Cultural Practices

Lime and Molybdenum

Availability of most plant nutrients is usually best in soils with a pH of 5.8-7. When the soil pH drops below 5.2 on sandy loam and silt loam soils, and below 5 on clay soils, manganese toxicity may occur. When the soil pH drops below 5, aluminum toxicity may also occur.

In extreme cases, manganese toxicity is expressed as a stunted plant with crinkled leaves. In milder cases, manganese toxicity may not show, but yield decreases will occur. Aluminum toxicity affects the roots. Roots on plants with aluminum toxicity are shorter and thicker than normal, resulting in a condition known as club root. Manganese and aluminum toxicities can be controlled by keeping the soil pH above the critical levels.

Molybdenum is a nutrient needed by soybeans in small quantities. There is enough molybdenum in our soils for optimum growth, but molybdenum is less available to plants as the soil becomes more acidic. At a pH higher than 6.2, additional molybdenum is not needed as seed treatments or fertilizer. When the soil pH is below 5.5, both lime and molybdenum are needed. The lime (enough to raise the soil pH to 5.5 or higher) is needed to eliminate the possibility of manganese and aluminum toxicities. When the soil Ph is between 5.5 and 6.2, molybdenum should be used.

Nitrogen

Nitrogen is needed in large quantities by soybeans. Soybeans remove about 4 pounds of nitrogen in each harvested bushel. Fortunately, soybeans are legumes and can obtain most of their nitrogen from the atmosphere. They accomplish this with the aid of the bacteria Rhizobium japonicum. These bacteria use soybean roots as a livable environment. They form nodules on soybean roots that capture nitrogen from the atmosphere and fix it in a usable form. Seed should be inoculated with Rhizobium japonicum bacteria in soils with no recent history of soybeans or when conditions have reduced Rhizobium japonicum bacteria survival.

Phosphorous

Phosphorous is critical in the early stages of soybean growth. It stimulates root growth, is essential in the storage and transfer of energy, and is an important component of several biochemicals that control plant growth and development. Phosphorus is concentrated in the seed and strongly affects seed formation. Soybeans remove about 0.8 pounds of phosphate (P₂O₅) per bushel in the harvested portion of the crop.

Phosphorus deficiencies are not easily observed. Usually no striking visual symptoms indicate phosphorus deficiency in soybeans. The most common characteristics of phosphorus-deficient soybean plants are stunted growth and lower yields.

Phosphorus fertilization rates should be based on soil test results. Remember soil pH affects the availability of phosphorus; it is most available to soybeans at when the soil pH is between 6 and 7.

Potassium

Potassium is essential in the growth and development of soybeans. Potassium is indirectly related to many plant cell functions. Some 60 enzymes require the presence of potassium. Plants with adequate amounts of potassium are better able to fight diseases than potassium-deficient plants. About four times as much potash (K₂O) is required by soybeans as phosphate (P₂O₅). About twice as much potash (K₂O) is removed in the seed as phosphate (P₂O₅). Soybeans remove about 1.4 pounds of potash (K₂O) in the harvested portion of the plant.

Potassium deficiency symptoms are fairly easy to diagnose when they are severe enough to be seen visually. Potassium deficiency symptoms usually occur on the lower leaves. The deficiency symptom will usually occur during bloom or pod fill. The margins (edges) of the leaves are necrotic (dead and brown). Severe potassium deficiencies can greatly reduce yields.

Potassium fertilizer rates should be based on soil test results.

Early Planting

Soybeans should not be planted until soil temperature reaches 60 F. Because emergence may also be affected by cool soil temperatures after planting, early planting decisions should also consider the forecast. Adequate soil temperatures occur in April but it can vary by location and year. Maturity Group IV and indeterminate Group V varieties do best in April plantings. Research in North Louisiana has instances of high yields for Group IV’s and V’s planted in mid to late March. In these cases, daily average soil temperatures were generally at or above 60 F at planting. A few (especially determinate types) may be sensitive to planting before early-May. Narrow row spacing may be beneficial when planting early due to the potential of reduced plant height. Always use fungicide seed treatment when planting early and conditions are less than favorable.

Late Planting

When planting is delayed until June 15 or later, the amount of vegetative growth that the plant produces becomes more critical. It is important to choose varieties that grow rapidly in a short time. Once blooming starts, most vegetative growth ceases in determinate varieties. Maturity Group VI soybeans should be used when planting after June 1. When planting late, seeding rates should be increased to compensate for reduced vegetative growth.

Seeding Rate

Too dense a plant population reduces yields, encourages diseases and lodging and increases seed cost. Use seed per foot as a guide rather than pounds of seed per acre when calibrating planters. In the following table, the estimated pounds per acre should be used only to calculate how much seed to buy. Because of varietal difference in seed size, as well as seasonal variation within lots of the same variety, planting rates can be misleading if expressed in pounds per acre.

Dates of Seeding

Because weather conditions are different from year to year, seeding dates can be affected by environmental conditions. Early or late planting can cause reduction in plant height in many varieties. Generally late plantings have less chance of success unless irrigation is available or optimal weather and timely rains occur throughout the growing season. A general rule is that 1/2 bushel per day is lost for every day that planting is delayed past the first week of June.

Optimal seeding dates for each maturity group planted in Louisiana are:

• Group III – April 15 – May 10

• Group IV – April 15 – May 10

• Group V – March 25 – May 5

• Group VI – March 25 – April 30

Row Spacing

Varieties respond differently to row spacing. The most important consideration is that the canopy be closed as quickly as possible to avoid late-season weed problems and low yields. Research has shown that narrow row spacing (30 inches or less) has out-yielded wide row spacing.

Depth of Seeding

Plant just deep enough to get the seed in moist soil. On sandy or silt loam soils, plant just 1 inch deep if moisture is available. On clay soils, plant 1 to 2 inches deep, depending on moisture conditions. Rolling the soil, especially clays, after planting will help obtain a stand by conserving moisture.

This material was prepared by the following personnel of the LSU AgCenter:

Ronald Levy, associate professor, Dean Lee Research Station

Boyd Padgett, Central Region director, Dean Lee Research Station

Trey Price, assistant professor, Macon Ridge Research Station

Dustin Harrell, associate professor, Rice Research Station

Daniel Stephenson, assistant professor, Dean Lee Research Station

Eric Webster, professor and coordinator, Iberia Research Station

Pat Bollich, resident director, Central Region Station

Clayton Hollier, professor, Department of Plant Pathology and Crop Physiology

Jeff Davis, assistant professor, Department of Entomology

Blair Buckley, professor, Red River Research Station

Caitlin Woodard, research associate, Dean Lee Research Station

Keith Normand, extension associate, (Soybean) Central Region