Nitrogen top dressing period affects wheat grain yields and should occur at the stages when yield potential is established. In order to determine the critical stages related to N needs, field experiments were conducted at Estação Experimental Agronômica/UFRGS, Eldorado do Sul (RS), Brazil, in 1993, 1994 and 1995. Wheat cultivars BR 23 (1993 and 1994) and Embrapa 16 (1995) were sown by the end of June at 300 seeds m-2. Nitrogen treatments (40 kg ha-1) were applied at the following stages: (a) plant emergence, (b) 3rd leaf emission, (c) 5th leaf emission, (d) 7th leaf emission, and (e) booting. In 1994 and 1995, half of the first three treatment plots received another 40 kg ha-1 of N by the 7th leaf emission stage. Grain yield, yield components and harvest index were analized. The critical period for nitrogen supply extended from plant emergence to the 7th leaf emission stage. At the beginning of this period, N is essential to induce the maximum number of spikelets per ear, while at the end it is critical to determine culms per area. Cultivars reacted differently to maximize grains per area. Early N supply to BR 23 cultivar was essential to establish number of grains per ear while for Embrapa 16, N applied late was important to set ears number.
Triticum aestivum; nitrogen; yield components