MAIZE PRODUCTIVITY AS INFLUENCED BY MIXED NITROGEN SUPPLIED BEFORE OR AFTER ANTHESIS

Maize (Zea mays L.) plants produce higher grain yields when supplied with mixtures of NO3 and NH4 than when supplied only with NO3. This study determined whether mixed N should be available before anthesis, or after it, or continuously to elicit yield increases. During vegetative development, plants of four hybrids were grown with nutrient solutions containing N as either all NO3 or a 50:50 (w/w) mixture of NO3-N and NH4-N. At anthesis, one-half of the plants in each group were switched to the other N solution so that four treatment groups were obtained. Plants were harvested at physiological maturity, separated into components, and analyzed for dry matter, reduced N, P, and K. Compared with continuous NO3, mixed N supplied for at least a portion of the life cycle increased the accumulation of shoot reduced N and P, but not K compared with continuous NO3. Average increases in grain yield obtained when mixed N was available during vegetative development were 9% when plants were switched to NO3 at anthesis and 11% for continuous mixed N. In contrast, mixed N supplied only during the postanthesis period did not significantly increase yield compared with plants receiving continuous NO3. In all cases, mixed-N-induced yield increases were associated with increased kernel number. Regardless of timing of availability, mixed N increased the proportion of dry matter partitioned to reproductive fractions (grain, husk, shank, and cob). These results confirm that mixed N nutrition increases grain yield of maize and suggest that vegetative development is the most crucial time to supply mixed N.