Our typical nitrogen (N) recommendation for wheat is 2 pounds N per expected bushel of grain. Long-term data indicates that, on average, this is the correct rate. However, this average is too high a third of the time, too low a third of the time and about right the remaining third of the time. How does one know when the rate is correct? More importantly, how does one know before the N is applied?
Enter the GreenSeeker® sensor technology, developed by Oklahoma State University and now available through all Oklahoma Extension Agents. Using reflected red light, this device optically measures the greenness of plants in a strip in the field with a very high N rate versus plants in the rest of the field. Calculations can then be performed over the Internet to calculate the top-dress N rate that will give the optimum response. The process is fairly simple. First, apply some of the total N for the expected wheat crop in the fall. For this example, assume that 80 pounds of N/acre are normally applied in the fall and 60 pounds are top-dressed in February. Instead, apply less N in the fall, say 40 to 60 pounds N/acre, but also apply a strip across the field with a very high rate of N - perhaps 100 pounds of N/acre in a strip on top of the 40-60 pounds. This gives a N-rich strip where the application rate is high enough to achieve the maximum potential yield. Continue treating the field as you normally would during the winter. The field can be grazed, but the cattle need to be pulled off or fenced out at least two weeks prior to an evaluation in February.
In February, before top-dressing, use a GreenSeeker® sensor to scan the wheat both in the N-rich strip and next to the strip. Using the result of the greenness test, planting date, expected yield, expected price and weather, calculations can be performed to determine the optimal top-dress N rate. If there is a large difference between the N-rich strip, where N is non-limiting, versus the rest of the field, then there will probably be a large response to added N. In this case, 2 pounds N may yield many more than 1 bushel of grain and would, therefore, be very economical. However, if there is little or no difference between the N-rich strip and the rest of the field, then 2 pounds of N may yield much less than 1 bushel of grain and would be less economical.
Although this technology is useful, phosphorus, potassium and lime still need to be applied according to soil test recommendations. This system is not perfect, but it comes closer to recommending the correct rate than shooting for the average and only being right a third of the time. For more information, go to www.nue.okstate.edu.