By Randy Buhler

Colorado State University Cooperative Extension

Logan County Agent (Agronomy)

Many people recognize the squeeze that input costs place on crop production for next year. Nitrogen at $485 per ton represents 17 percent of the value of a bushel of corn priced at $1.80 per bushel. With these squeeze effects and likely corn prices below $2.50 for the year 2006, some growers are considering severe reductions of nitrogen fertilizer. Others are considering not growing corn at all.

The relationship for yield and nitrogen application is not a strong correlation. Fields vary in their responsiveness to added nutrient inputs, especially nitrogen. Hybrids can vary in their response to nitrogen rates. Crops other than corn also vary in response to available nitrogen.

The price example assumed that a pound of nitrogen input equaled a pound of nitrogen removed. This would represent 100 percent nitrogen use efficiency. We all know that situation does not happen. Recent research is changing our thinking about recommended nitrogen rates.

Our common recommendation has been to multiply your yield goal by 105 percent. Then fertilize at the corresponding yield-calibrated nitrogen rate. Recommendations included credit for carryover nitrogen, other nitrogen sources such as water supply, legume crops prior to planting corn, and past manure applications.

Research in Nebraska looked into fertilizing at 80 percent or 90 to 95 percent of yield potential. The 80 percent rate was called the recommended nitrogen rate. The 90 to 95 percent rate was called the intensive management rate. The outcomes demonstrate what happens with a short-term outlook versus a long-term outlook.

For the recommended plan, over a four-year period, the average yield was 223 bushels per acre on irrigated ground. The intensive rate yield was 252 bushels per acre.

The average N-rate for the recommended plan was 174 pounds per acre or a total application of 697 pounds N. The average N-rate for the intensive management was 272 pounds per acre or a total application of 1,090 pounds N.

Nitrogen removal in the grain was 598 pounds from the recommended plan and 705 pounds from the intensive management. Change in soil nitrogen was a loss of 205 pounds for the recommended plan. The intensive plan gained 196 pounds.

The nitrogen use efficiency was 1.29 bushels of grain per pound of N applied on the recommended plan. The intensive management plan efficiency was 0.93 bushels per pound of N applied.

The interesting data point was that 0.5 ton of soil carbon was lost with the recommended plan and 2 tons soil carbon gained with the intensive management plan. N is necessary to get stable carbon storage. There was slightly less carbon dioxide lost from soil and plant respiration under the intensive management plan.

The overall nitrogen use efficiency was reversed from the grain efficiency results when all nitrogen was considered. The recommended plan efficiency was 0.56 pounds N per pound of N applied. The intensive management plan efficiency was 0.83 pounds N per pound of N applied. The difference was the N stored in the organic complex with soil carbon.

The lesson for us is that for a very short time, we can cut our using 105 percent of yield goal to 80 percent of potential yield. Potential yield is based on prior crop results averaged over a period of years. Yield goal can sometimes be many bushels more than the potential yield. Use your records to make a sound decision for your yield goal. In either case, the recommended and intensive management rates represent less N than what we have traditionally been calculating for our nitrogen rates in Colorado.

Another nitrogen rate method is called the Maximum Return to N. This plan uses yield response and crop and nitrogen prices to calculate the best economic return to the cost of nitrogen applied. We do not have sufficient correlation research in enough places to adopt this plan with any great confidence. You may be able to use the method if you have kept careful records of your field-by-field nitrogen application rates and yields. Researchers in Iowa that have developed this method emphasize that corn in rotation uses a different rate of N than corn after corn. For more information, go to the Iowa State University website at http://extension.agron.iastate.edu/soilfertility/nrate.aspx. Once you are on that page, scroll down and click on the "more info" highlight.