Fertilizer-Nitrogen Management to Improve Yields of Inbred Maize Lines.

Inbred-maize pure lines are used for hybrid seed production and have reduced vigor, biomass, leaf area, harvest index, root volume, and seed yields than hybrids. Seed yield increase due to fertilizer-nitrogen (N) is lower than in hybrids but some producers apply higher fertilizer-N rates in efforts to improve yields.  Determination of the optimum inbred maize pure line seed-yield response to fertilizer-N application is a goal to improve crop performance and N-use efficiency.  We report on a series of field experiments conducted in Fluventic Haplustolls of the semiarid southern coast of Puerto Rico. The first (Trial 1) evaluated the effects of antecedent cover crop (cowpea, Vigna unguilata or fallow) and fertilizer N (60, 110, and 160 kg N/ha) on maize (Zea mays L.) inbred seed production.  The rotation was cover crop (summer 2011), maize (winter 2011-2012), cover crop (summer 2012), maize (winter 2012-2013).  Fertilizer-N rates did not affect maize seed yields in both years.  Maize seed yield and biomass (stover plus seed) with antecedent cover crop was about 34 and 19% greater, respectively, only in 2011-2012.  Overall, seed yields and biomass were three-times greater in 2012-2013 than 2011-2012.  Fertilizer-N rate of 60 kg N/ha was about double of seed N extraction for 2011-2012 and in balance for 2012-2013.  The lack of crop N response to fertilizer-N can be partly explained by soil N (0-30 cm) in the range of 43 to 77 kg N/ha for both years and 80 to 134 kg N/ha to a depth of 0 to 100 cm.  The second trial (Trial 2), which was conducted during winter 2012-2013, evaluated the effects of four fertilizer-N levels (0, 90, 135, and 180 kg N/ha) on seed yields of five inbred lines.   In Trial 2, fertilizer-N did not affect any of the agronomic parameters measured. Instead, the greatest influence on seed yield, agronomic crop performance, and crop N extraction was the inbred line.  Seed yields (15.5% moisture) ranged from 693 to 2989 kg/ha with harvest indices ranging from 0.1 to 0.29. Fertilizer-N management rates of inbred maize lines under the conditions studied should approach expected seed N extraction levels.