Crop rotation is commonplace in today's agriculture, but wheat planted no-till after sorghum often yields less than does wheat after soybeans or corn. A Kansas State University researcher has examined the issue and found a major reason why.

A three-year study at the Harvey County Experiment Field near Hesston addressed the question, said Mark Claassen, agronomist-in-charge of the Kansas State University Research and Extension field.

"We found that wheat no-till planted after grain sorghum harvest can be successful if enough nitrogen (N) is applied to the wheat crop," Claassen said. "Wheat yields increased with each increase of 40 pounds of nitrogen per acre up to 120 pounds of N per acre. Grain sorghum yields also increased with higher nitrogen rates (60 versus 120 pounds per acre), but a farmer wouldn't apply extra nitrogen to sorghum to help with the subsequent wheat crop."

The research site, Claassen explained, was a Geary silt loam soil. Wheat was no-till planted (Oct. 16, Oct. 2, and Oct. 18) each year after sorghum harvest. The grain sorghum had been fertilized with either 60 or 120 pounds N per acre.

Within the 60 and 120 pounds N-per-acre blocks, four nitrogen rates were applied to the wheat (0, 40, 80, and 120 pounds N per acre). Three seeding rates (60, 90, 120 pounds per acre) were used within each nitrogen rate.

The average grain sorghum yields for the 60- and 120-pounds nitrogen rates were 93 and 110 bushels per acre, respectively. There was little difference in residual soil nitrogen between the two nitrogen- treated sites.

The three-year average yields for wheat no-till planted after the grain sorghum that had been fertilized with 60 and 120 pounds N per acre were 41 and 46 bushels per acre, respectively, the researcher said.

On the 60 pounds N per acre plots, however, the follow-up wheat crops fertilized at 0, 40, 80, and 120 pounds N per acre yielded 15, 33, 53 and 65 bushels per acre, respectively. After the grain sorghum fertilized with 120 pounds N per acre, wheat fertilized with the same four N rates yielded 21, 40, 57, and 66 bushels per acre, respectively.

"Wheat yield increases with increasing nitrogen rates were consistent within each rate of nitrogen fertilizer on the grain sorghum. However, at the 0, 40, and 80 pounds N per acre rates, wheat yields were higher following 120 pounds N per acre on grain sorghum than after 60 pounds N per acre on grain sorghum," Claassen said. "And, at 120 pounds N per acre, wheat yields were virtually the same (65 and 66 bushels per acre), regardless of the nitrogen history on grain sorghum."The best explanation for why the yield of wheat planted after sorghum fertilized with 120 pounds N per acre was greater than that for wheat planted after sorghum fertilized with only 60 pounds N per acre, he said, was that there was a little extra residual nitrogen the wheat could use.

"Interestingly, there were no differences in yield among the 60, 90, and 120 pounds per acre seeding rates," Claassen said. "This likely resulted from abundant moisture's favoring good stand establishment in each of the three years. We still recommend a seeding rate of at least 90 or 120 pounds per acre for no-till wheat."

For more information, interested persons can visit their county K- State Research and Extension office and ask for Kansas Fertilizer Research reports from 1999 (SRP 847, pgs.61-63), 2001 (SRP 885, pgs. 44-46), and 2003 (SRP 921, pgs. 69-71). The reports also are available on the Web at http://www.oznet.ksu.edu. (Search, for example, for Kansas Fertilizer Research SRP 847.)

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 in Manhattan.