Rising nitrogen fertilizer and energy prices have prompted questions on the nitrogen replacement value of alfalfa when followed by corn. In other words, what is the first year nitrogen credit for field corn following alfalfa in northern Illinois?

Earlier research in Iowa, Wisconsin, Minnesota, and Pennsylvania involving 61 sites revealed a corn grain yield response to nitrogen fertilizer following alfalfa at seven of those sites. The optimum nitrogen rate at those seven sites ranged from 25 to 42 pounds per acre.

The Illinois Agronomy Handbook indicates that for first year corn following an alfalfa stand of 5 plants or more per square foot, the nitrogen rate can be reduced 100 pounds per acre. If there are 2 to 4 plants per square foot, the nitrogen rate can be reduced 50 pounds per acre.

With improvements in corn hybrids and the increases in fertilizer cost and corn price, nitrogen rate studies for first year corn following alfalfa were conducted in 2007 by University of Illinois Extension and funded by SARE (Sustainable Agriculture Research and Education). The two sites were a farm in Bureau County and the University of Illinois Agronomy Research Center in DeKalb County.

At both locations, nitrogen rates of 0, 40, 80, 120, 160, and 200 pounds per acre were applied to corn postemergence. The alfalfa plant density at both sites was at least 4 to 5 plants per square foot. Averaging data from both locations, corn grain yield did not benefit from a nitrogen rate of more than 40 pounds per acre.

It is the intent to continue the studies in 2008. Further information is available by contacting Jim Morrison, Extension Crops Educator, 815-397-7714, morrison@uiuc.edu