Basics of Grain Sorghum Weed Control and Fertilizer Requirements | Cultures

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(Courtesy photo.)


Long-time grain sorghum growers realize that having a good pre-emergence herbicide is imperative. Since post-emergence herbicide options are limited, it is best to start with some clean fields and conditions are as good as they can get for effective pre-emergence weed control.

Herbicide choices for pre-emergence weed control are primarily limited to six active ingredients, with many producers opting for a combination of atrazine and one of the Group 15 herbicides which include S-metolachlor, acetochlor and dimethenamid. The most commonly used premixes are:

Bicep II Magnum, Bicep Lite II Magnum, Cinch ATZ, other generics contain atrazine and S-metolachlor, and some generics use metolachlor; Degree Xtra, Fultime NXT which contain atrazine and acetochlor; and Lumax EZ, Lexar EZ which contain a three-way mixture of atrazine, S-metolachlor and mesotrione.

The three-way blend in Lumax EZ or Lexar EZ contains not only atrazine and S-metolachlor, but a third active ingredient, mesotrione, which improves broadleaf weed control; however, mesotrione can only be used in heavier textured soils where crop damage is likely to occur.

In soils where atrazine and mesotrione cannot be used, growers use S-metolachlor sold as Dual, acetochlor sold as Warrant, or dimethenamid sold as Outlook, alone. A final option for these producers is to use Additional Outlook Rich Verdict. Verdict contains saflufenacil and dimethenamid. The most common rate is 10 ounces of Verdict plus an additional 10 ounces of Outlook.

Grain sorghum is similar to corn in terms of the number of nutrients needed per bushel of grain harvested. To achieve a specific yield goal, an adequate supply of nutrients, including nitrogen, phosphorus, potassium and sulfur, is necessary. It may be necessary to adjust the soil pH above 5.8 to ensure adequate nutrient availability in some soils.

The rule of thumb for sorghum nitrogen requirements is 1.12 pounds of nitrogen per bushel of grain. A yield of 100 bushels will require 112 pounds of nitrogen. The amount of nitrogen applied should be adjusted for residual soil nitrogen in addition to considering soil organic matter. The most efficient way to deliver nitrogen is to apply no more than 50% of the nitrogen needed in pre-planting, apply the remaining nitrogen in a starter mix with phosphorus and perhaps sulfur with a final application made during the growing season. Seasonal application should be made 30-45 days after emergence. By dividing the nitrogen fertilizer into multiple applications, the total amount of nitrogen applied can be adjusted to match a changing yield goal based on current and expected environmental conditions, especially precipitation.

Phosphorus and potassium fertilizer requirements should be based on soil test analysis. If phosphorus is needed, apply in a narrow band a few inches from the side of the sorghum rows to improve phosphorus utilization by the plant. Many soils where sorghum is grown do not require potassium and, if necessary, usually in small amounts.

Sulfur is often a overlooked nutrient in sorghum, but can be an important addition in high yielding environments. Growers should strive to keep the nitrogen to sulfur ratio at around 15 to 1.

Editor’s Note: Brent Bean, Ph.D., Director of Agronomy Sorghum Checkoff, Lubbock, Texas. For more information, visit www.sorghumcheckoff.com.


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