MAIZE PRODUCTIVITY AS INFLUENCED BY FORM AND AVAILABILITY OF NITROGEN

Although N, especially mixtures of NO3 and NH4, can increase the produ ctivity of maize (Zea mays L.), it is unclear when this N needs to be available for maximum benefit. This study was conducted to determine t he importance of preanthesis N form and the need for available postant hesis N on the productivity of maize. Four single-cross hybrids (B73 x LH51, LH74 x LH51, LH74 x LH82, and LHE136 x LH82) were grown in fiel d-hydroponics and supplied N as either all NO3 or an equal mixture of NO3 and NH4 (mixed N nutrition) until anthesis, whereupon N was withdr awn from one-half of the plants. At physiological maturity, whole shoo ts were harvested, separated into component fractions, weighed, and an alyzed for yield, yield components, and macronutrient composition. Com pared with all NO3, mixed N nutrition increased grain yield of all hyb rids whether the N was available full season (average of 11%) or only during vegetative growth (average of 10%). In contrast, regardless of the N form, N deprivation during grain fill decreased grain yield for three of the four hybrids (range of 6 to 19%) but increased yield (5%) for LHE136 x LH82. Changes in sink-source partitioning, kernel number , and plant N accumulation were closely related to both yield enhancem ent associated with mixed N nutrition, and yield reduction due to N re moval. Therefore, these data show that N form, timing of N availabilit y and genotype are all factors that interact to influence N use and pr oductivity of maize.