LONG-TERM (35 YEARS) EFFECTS OF FERTILIZATION, ROTATION AND WEATHER ON CORN YIELDS

Long-term effects of fertilization, crop rotation and weather factors [temperature, precipitation, net radiation, maximum (potential) evapotranspiration (ET) and corn heat units (CHU)] on the sustainability of corn grain yields were investigated over 35 yr. Treatments included fertilized and unfertilized continuous corn and rotation corn-oats-alfalfa-alfalfa. The fertilized rotation corn treatment produced the greatest corn grain yields (15% moisture content) with an average of 7.75 t ha(-1) followed by the fertilized continuous corn treatment with 6.02 t ha(-1). Fertilization increased grain yield for continuous corn treatments by 279% and increased grain yields in the rotational corn treatments by 70%. Corn grain yields increased with time with the fertilized rotation treatment, remained relatively constant with the fertilized continuous corn and decreased with the unfertilized treatments. Growing season precipitation was the only weather variable tested which was significantly related to corn grain yield. Precipitation in July was proportional to corn grain yield for all fertilized treatments. Weather variation played little role for unfertilized corn. Continuous corn production was sustained (yields did not decrease with time) when fertilizer was added. There was a considerable yield advantage with fertilized corn when grown in a rotation compared with fertilized continuous corn. Fertilization and crop rotation practices increased and buffered corn yields.