Wednesday, 8 October 2008: 8:30 AM
George R. Brown Convention Center, 360F
Fertilizer nitrogen (N) costs have increased nearly 65% in the last 6 years in the Central Great Plains region (CGPR) of the USA. With that increase in N costs farmers have experienced a decrease in crop yields due to drought. The question that arises (from this, decline in yield with increased input cost) is how does optimum fertilizer N rate change when grain yields are low and fertilizer prices are high? Also, how does the price of wheat, corn or millet affect the optimum N rate? In this study, we evaluate dryland winter wheat, corn and millet yield responses to applied N over a six-year period at the USDA-ARS Central Great Plains Research Station at Akron, Colorado. Crops were fertilized at 0, 27, 54 and 80 kg N ha-1 (0, 30, 60 and 90 lbs of N per acre) on a Weld silt loam soil (fine, smectitic, mesic Aridic Paleustolls). Fertilizer was applied in a preplant broadcast application as ammonium nitrate. Soil samples to 60-cm were collected from each plot at planting time before fertilization and after harvest each year. Form measured grain yields relative grain yield was calculated by normalizing each years yield data on the maximum yield measured in a given experiment/year. A response function was fitted to that data to determine the economically optimum N rate. Yields varied from year to year and were correlated to rainfall and temperature during the growing season. However, after calculating relative yield the response to N was observed to be similar irrespective of maximum yield. Maximum yield was calculated to be at 58 to 75 kg of applied N ha-1 (65-95 lbs of N per acre). However, the economically optimum N rate was found to be less than 18 kg N ha-1(20 lbs of N per acre) at yield potentials of less than 1340 kg ha-1 (25 bushels per acre), but increased to up to 46 kg ha-1 (52 lbs of N per acre) at yields near 3800 kg ha-1 (70 bushel per acre) when grain prices were high.
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