Louisiana Sorghum: Planting and Nutrient Management Tips

    Flowering sorghum. Image from Texas AgriLife Extension

    Louisiana growers are expected to plant more acres to grain sorghum this year compared to previous years. 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 some key tips for growing a successful sorghum crop.

    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. Soil temperature is the main factor influencing germination rate.

    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.

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    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, growers should be aware that populations can vary greatly (Table 1). Seeding rates should be based on seed per acre and not pounds per acre.

    Soil pH

    Grain sorghum is sensitive to low soil pH. The optimal soil pH for grain sorghum ranges from 5.8 to 6.5. Continued cultivation and the use of chemical fertilizers, especially those containing ammonium and sulfur, tend to decrease soil pH over time.

    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.

    Yield becomes less responsive to nitrogen as yield approaches 150 bushels per acre (Figure 1). 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.

    Sulfur

    A 125 bu/A grain sorghum crop takes up about 23 pounds per acre sulfur with about 8 pounds per acre removed in the grain at harvest. When a soil test is utilized to determine if sulfur is needed, values of less than 12 ppm (Mehlich 3 extraction) suggests that additional sulfur may be needed. In this case, the typical recommended rate of sulfur is 20 pounds per acre in the sulfate form.

    Table 1. Effect of seed size on planting rate and plant population when planting is based on pounds per acre.

    Hybrid A B
    Seed weight grams/1,000 seed 38 25
    Number seed per pound 12,000 18,000
    Number of seeds @ 6 lbs/acre 72,000 108,000
    Grain sorghum yield response to N_Matt Fosterpng

    Figure 1. Grain sorghum yield response to nitrogen rate (Source: Mengel, Kansas State University). Click Image to Enlarge




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