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Tweaking sorghum

Arkansas grain sorghum acreage has increased this year. With renewed interest in the crop, the Arkansas Extension Service believes it might be time to revise some of the cultural practices it recommends. One practice is fertility — particularly the use of nitrogen.

"Our fertilizer recommendations are based on information that’s 30 years old. With new varieties, we need to determine the most effective and economical rates of inputs," says Leo Espinoza, Arkansas Extension soil specialist.

To check recommendations, Espinoza and colleagues have a study on the Cotton Branch Experiment Station using three cultivars — Terral 1050, Pioneer 84G62 and Terral 981. On those varieties researchers are putting out several rates of nitrogen in irrigated conditions: 0, 50, 100, 150, 200 and 250 pounds per acre. They also have a test under dryland conditions with the rates at 0, 40, 80, 120, 160 and 200 units of nitrogen per acre.

"In a typical field, right now, we’d probably suggest 120 units of nitrogen. If there’s a possibility the yields will be higher than 6,000 pounds per acre, we’d suggest adding 30 units more to reach 150 units of nitrogen per acre," says Espinoza, who spoke at the Precision Agriculture Tour held in east Arkansas.

"We also think it’s a good idea to split fertilizer shots — one preplant or right after planting and one around B-5 or B-6."

There are several reasons for a split application. One, sorghum needs the nutrients early because the seed is small and doesn’t have the energy of larger seeds like soybeans or cotton. The second application comes at a time when the sorghum plant is determining how many grains per head it will produce. If the plant has plenty of food, it’ll produce a nice, large grain head, says Espinoza.

Also, fertility and water go hand-in-hand. Fertility without moisture means very little.

"Water stress at the B-5 point will also affect the number of grains a head will produce. It seems to me that we normally have good moisture at season’s beginning and then the sun comes out and moisture becomes a problem around the booting stage. Stress at that time is critical. It’s very important to keep the soil moist during that stage."

Sorghum is a tough plant, but it won’t yield to its potential without proper management. If the growing point is in the ground when stress occurs, the plant has the opportunity to recover nicely. But as soon as it emerges, stress is much harder to deal with.

"We planted the irrigated plots April 25. We tried to get six plants per foot on the irrigated test and four plants per foot in the dryland test.

"It may vary a bit, but as a rule of thumb, you need about 10 inches of water to produce a nice grain head. After that, an extra inch of water produces about 300 to 400 pounds of grain per acre. That’s significant. Since we planted we’ve had around 10 inches of water on the test plots. What I wonder is will we have enough moisture to fill the grains properly."

Espinoza also is measuring biomass — how much stubble the sorghum crop is producing. Around 50 percent of the nitrogen applied to the soil will be used for producing grain. Only 10 percent of the potassium is used by grain, though.

"If a plant uses 200 units of potassium, there’s the potential to recoup 180 units through the stubble. We’re checking how much potassium is recoverable."

The roots of sorghum are aggressive. Plants like to have their own space. In deep soils, roots can travel as much as 6 feet.

"We wondered if we arranged plants differently, could we better utilize water and land? Checking that, we planted sorghum on 38-inch and 22.5-inch rows — both in twin rows and three rows per bed. In Texas, there are consistently better yields from using different plant and row spacings and configurations — as much as 20 percent higher yields are seen from shifting patterns. We’re checking to see if that holds true in Arkansas as well."

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