Assessing Nitrogen Requirements In Corn Using the Quadratic Plateau Model

The purpose of the project was to develop a method that will allow a farmer to assess the profitability of current rates of nitrogen (N) application. The project was carried out by the St Clair District Soil and Crop Improvement Association from 2000 to 2003. The treatments consisted of several rates of nitrogen - 3 or 4 nitrogen rates below the normal rate of nitrogen, and one rate above. The normal rate of nitrogen was put between each rate. A statistical model (the Quadratic Plateau model) was used to predict the maximum economic rate of nitrogen (MERN) for each field.

The weather over the four years of the project was variable, from wet to very dry. There were several excellent cooperators from each of the three counties that participated each year. The plots were set up well and plot information was received in a timely manner (most times). A lot more could have been learned if the weather had cooperated. The 2003 season provided almost as much useful information as we had collected the previous 3 years.

A few of the cooperators were able to identify the opportunity to reduce nitrogen rates without sacrificing yield. Some cooperators confirmed the rates they are using were correct.

The Allear plots (Graphs 1 and 2) on different fields were similar in that they showed the opportunity for about a 55 lb. N/ac reduction from the normal nitrogen rate in 2002 and a 35 lb reduction in 2003. Two other plots had the potential for 60 and 90 pound per acre reductions in nitrogen rates. Two of the cooperators in 2003 were putting on the correct rate for the field. One plot was in the same area in 2002 and 2003, and the recommendation from 2002 was the right rate for 2003.

Three of the plots showed no response to additional nitrogen. The first plot had 10 tons per acre of turkey manure applied the previous fall, so no response would be expected. The second plot had regular applications of hog manure so no response would be expected there either. The third plot did not have manure or red clover so the lack of response to nitrogen is hard to explain. Another plot was in a good rotation with red clover and had regular applications of cattle manure, so no response to nitrogen would be expected there either. They applied about 60-70 lbs of N per year, which would compensate for uneven manure application or areas of reduced stands of red clover.

Funding for this project was from an O.S.C.I.A Regional Partner Grant and a grant from the Southwest SCIA (Southwest Agricultural Conference). Thanks to Ivan O’Halloran Ridgetown College, University of Guelph for calculating the MERN’s and generating the graphs for this project. Thanks to Agri-Food Laboratories for assistance with the nitrate sample analysis.