Released: March 08, 2002

Studies: Soy-Sorghum Rotation Boosts Sorghum Yields

MANHATTAN, Kan. – The benefits of crop rotations have been touted for years, but two studies by Kansas State University have documented specific yield benefits gained by planting soybeans as part of a sorghum-soybean rotation rather than keeping a field in continuous sorghum.

A five-year study at the K-State Research and Extension Cornbelt Experiment Field in northeast Kansas showed that there was a definite advantage to having soybeans in the rotation, said Mark Claassen, Agronomist-in-Charge of the field near Hesston.

In this study, five nitrogen (N) fertilizer rates were used including: zero pounds per acre; 40 pounds; 80 pounds;120 pounds, and 200 pounds. The five different rates were used on the continuous sorghum as well as on the sorghum-soybean rotation.

"Averaged over all of the fertilizer treatments there was a 17-bushel-per-acre advantage for sorghum after soybeans compared to continuous sorghum," Claassen said. "When we used no nitrogen, there was a 38-bushel-per-acre advantage for the sorghum in the sorghum-soybean rotation compared to the continuous sorghum. Sorghum yield in the rotation was near optimum in most years with 80 pounds of N per acre.

"On the other hand, with continuous sorghum, at least the 120-pound N-per-acre rate generally was required to approach the yields of sorghum in the soybean rotation with 40 or 80 pounds of N per acre," he said.

Research that spanned 18 years at K-State’s North Central Experiment Station revealed similar findings. In that study, which used four N fertilizer rates – zero, 30, 60 and 90 pounds per acre – continuous sorghum was also compared with a sorghum-soybean rotation, according to Barney Gordon, Agronomist-in-Charge.

There was an average 23-bushel-per-acre yield advantage for sorghum in the sorghum-soybean rotation over the continuous sorghum, he said.

"On fields where no nitrogen was used, the rotated sorghum yielded 32-bushels-per-acre more than the continuous sorghum," Gordon said. The actual average yields were 75 bushels per acre of sorghum on the sorghum-soybean fields versus 43 bushels on the continuous sorghum acreage.

"However, rotated sorghum increased yields only up to the 60-pounds-of-nitrogen-per-acre rate," he added. "At the 90-pound rate, rotated sorghum had a 12-bushel-per-acre yield advantage over the continuous sorghum."

Because continuous sorghum yields increased up to the 90-pound-per-acre rate, four additional N rates were added to the study six years ago, including 120 pounds per acre; 150; 180; and 210 pounds.

Averaged over all N rates, rotated sorghum yielded 16 bushels per acre more than the continuous sorghum, Gordon said. And just like the previous 18-year average, the six-year average indicated the rotated sorghum peaked out at 60 pounds of nitrogen per acre, and the continuous sorghum peaked out at 90 pounds of N per acre.

"One point of interest is that the rotated sorghum reached mid-bloom about seven days before the continuous sorghum," the agronomist said.

If one subscribes to the old rule of thumb that there’s one pound of nitrogen credit for every bushel of soybeans produced, Gordon said, we’re left with the question: Why didn’t the yields of continuous sorghum at the higher nitrogen rates ever catch up to the yields of rotated sorghum?

"There’s obviously more to the ‘rotation effect’ than just a nitrogen credit," he said. "There are several possible factors that may include breaking disease, weed and insect pressures and soil moisture differences."

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Mary Lou Peter, Communications Specialist
mlpeter@oznet.ksu.edu
K-State Research& Extension News