Wheat nitrogen use efficiency in a bed planting system in northwest Mexico

Wheat (Triticum aestivum L.) in the Yaqui Valley, northwest Mexico, is plan ted as a winter crop using a raised-bed, furrow-irrigated system and high f ertilizer N rates, Wheat residues are usually burned before planting maize (Zea mays L,) as a summer crop. The N use of wheat planted following conven tional tillage using a raised-bed system (CTB) incorporating both wheat and maize residues was compared with wheat planted using permanent raised beds (PB) under four residue management treatments: all straw (wheat and maize) left as stubble, straw partly removed (maize residues removed; wheat resid ues retained), all straw removed, and all straw burned. Each wheat plot was split into seven N fertilizer (N-f) treatments: five applied at planting ( 0, 75, 150, 225, and 300 kg ha(-1)) and two at the Ist node stage (150 and 300 kg ha(-1)). Maize received a uniform N-f application of 150 kg ha(-1). The N use efficiency of wheat with 150 kg N-f ha(-1) at the Ist node stage was superior to basal applications at the same rate. Permanent bed-all stra w left as stubble and PB-all straw burned had the highest average wheat gra in yields (5.57 and 5.52 Mg. ha(-1), respectively), N use efficiency (28.2 and 29.1 kg grain kg(-1) of N supply, respectively), and total N uptake (13 3 and 137 kg ha(-1), respectively), Total N uptake for 150 and 300 kg N-f h a(-1) at the 1st node stage was 14 and 8% greater, respectively than at pla nting. In most tillage-straw treatments, 21% of the difference in wheat gra in yields was due to the N supply component at low N rates; at high N rates , 97% was due to N use efficiency.