TILLAGE AND ROTATION EFFECTS ON CORN YIELD RESPONSE TO FERTILIZER NITROGEN ON AQUALF SOILS

Fertilizer N recommendations are often increased for no-till corn (Zea mays L,) relative to conventionally tilled corn, A comparison of rela tive yield responses to fertilizer N is lacking for continuous corn an d corn following soybean [Glycine max (L,) Merr.] when grown with conv entional and no-tillage systems on poorly drained claypan soils, Field studies spanning 5 site-years were conducted on somewhat poorly (Udol lic Ochraqualf) and poorly (Mollic Albaqualf) drained claypan soils in central and northeast Missouri, The experimental design was a split-s plit plot, where whole plots were tillage system (chisel-disk and no-t ill) and subplots were rotation (continuous corn and corn following so ybean), Sub-subplots were N rates of 0, 67, 135, 202, and 269 kg ha(-1 ), applied preplant as either anhydrous NH3 (82-0-0 N-P-K) or knife-in jected urea-ammonium nitrate (UAN) solution (32-0-0), Corn grain yield response to fertilizer N was described by quadratic and quadratic-pla teau models that had similar R(2) values, With continuous corn, Both m odels predicted no-till to require 14 to 17 kg ha(-1) (5-7%) less fert ilizer N for maximum yield and profit than the chisel-disk system. The predicted maximum no-till grain yield was 0.34 Mg ha(-1) less than th e chisel-disk system. No-till corn following soybean required an avera ge additional 17 kg N ha(-1) (8%) for maximum yield (quadratic model) and 45 kg N ha(-1) (35%) for maximum profit (quadratic-plateau) than t he chisel-disk system, The average maximum yield for no-till corn foll owing soybean was 0.20 Mg ha(-1) less than the chisel-disk system, App arent N credit for corn following soybean varied from 0 to 159 kg ha(- 1), With the quadratic model, the average N credit was 54 kg ha(-1) fo r the chisel-disk system and 24 kg ha(-1) for no-till. Decreased no-ti ll corn yields that required equal or greater fertilizer N indicate at least a short-term economic disadvantage for this tillage system on t hese poorly drained soils.