Grain yield and kernel crude protein content increases of maize hybrids with late nitrogen side-dressing

Physiological changes incorporated into current maize hybrids suggest the occurrence of modifications in the nitrogen uptake dynamics, improving plant ability to uptake N during grain filling. This may justify late N side-dressing whenever environmental constraints prevent adequate nitrogen supply during crops vegetative development. This study evaluates effects of nitrogen fertilization at booting and silking on grain yield and kernel crude protein contents of commercial maize hybrids. Two experiments were set up in Eldorado do Sul, RS, during the 2001/2002 and 2002/2003 growing seasons. In 2001/2002, treatments were composed of two hybrids (Agroceres 303 and Pioneer 32R21), three nitrogen rates applied during maize vegetative development (30, 80 and 130 kg ha-1) and three nitrogen rates applied at silking (0, 50 and 100 kg ha-1). In 2002/2003, four hybrids (Agroceres 303, Pioneer 32R21, Dekalb 215 and Syngenta Penta) and four nitrogen rates side-dressed at booting (0, 50, 100 and 150 kg.ha-1) were assessed. There were significant increments in grain yield and kernel crude protein content with nitrogen fertilization at booting and silking. Grain yield response to late N side-dressing differed among cultivars. The impact of nitrogen fertilization at silking was higher at the smallest rate of N during the plant vegetative development. Enhancements in grain yield with late N side-dressing resulted from increases in grain weight. Modern hybrids can uptake nitrogen during silking, contradicting the hypothesis that late N side-dressing is not efficient to improve maize grain yield. Alterações morfo-fisiológicas introduzidas nos híbridos modernos de milho sugerem mudanças na dinâmica de absorção do nitrogênio, aumentando a habilidade da planta de absorvê-lo durante o enchimento de grãos. Isto pode justificar a utilização de coberturas nitrogenadas tardias sempre que restrições climáticas impedirem o suprimento adequado de nitrogênio durante o desenvolvimento vegetativo da cultura. Este estudo foi conduzido objetivando avaliar os efeitos da fertilização nitrogenada no emborrachamento e espigamento no rendimento e teor de proteína nos grãos de híbridos de milho. Dois experimentos foram instalados em Eldorado do Sul, RS, nos anos agrícolas de 2001/2002 e 2002/2003. Em 2001/2002, os tratamentos foram compostos por dois híbridos (Agroceres 303 e Pioneer 32R21), três doses de N aplicadas durante o desenvolvimento vegetativo (30, 80 e 130 kg ha-1) e três doses de N apl