MAIZE GRAIN-YIELD RESPONSE TO TILLAGE AND FERTILIZER NITROGEN RATES ON A TARA SILT LOAM

Effects of tillage on the appropriate fertilizer N applications needed to achieve maximal grain yield are poorly defined. The study objectiv e was determination of relative corn grain yield response to N applica tion rate for four tillage practices: no-tillage (NT), ridge tillage ( RT), fall chisel plowing (CP) and fall moldboard plowing (MP). Maize ( Zea mays L.) grain yield and N accumulation were monitored over a 6 ye ar period with the same tillage treatment and the same fertilizer N ra te applied each year to each plot. Two hybrids, differing in relative maturity rating, were planted each year. Fertilizer N rates ranged fro m 10 to 190 kg ha(-1) and consisted of 10 kg ha(-1) of liquid starter N applied at planting with varying amounts of fall applied anhydrous a mmonia. With only starter fertilizer, grain yields increased with till age intensity in the order NT less than or equal to RT less than or eq ual to CP less than or equal to MP, With greater than or equal to 55 k g total applied N ha(-1), 6 year average grain yields were unaffected by tillage. Total N removed in grain annually with only starter fertil izer ranged from 21-85 kg ha(-1). Maximal amounts of N removed, about 145 kg N ha(-1), occurred with 100-145 kg applied N ha(-1) for all til lage treatments under the more favorable climatic conditions. Several interactions affecting grain yield appear climatically sensitive with exception of tillage by fertilizer N interactions. Because of variabil ity in climate, planting dates varied by almost 4 weeks. Relative yiel d loss due to planting delay were Fertilizer N (mean change congruent to - 124- -275 kg ha(-1) day(-1)) > Starter N only and MP (mean congru ent to -259 kg ha(-1) day(-1)) > other tillages in general. Yield loss due to delayed planting ranged from 0.0-275 kg ha(-1) day(-1). Grain yield gains due to early spring soil temperatures were 16.0-21.8 kg ha (-1) index-degree(-1) with MP tillage and averaged 2.7-16.7 kg ha(-1) index-degree(-1) more than those of other tillage-hybrid combinations.