Agronomic parameters as indicators of inbred maize nitrogen status

Inbred maize (Zea mays L.) lines are grown for seed in Puerto Rico. In efforts to increase crop yields, some producers may apply nitrogen (N) fertilizer in excess of crop nutrient requirements. The use of cover crops (CC) in rotation can serve as an alternative to continuous cultivation of maize. This experiment was conducted in field plots of Güamaní (fine-loamy over sandy or sandy-skeletal, mixed, superactive, isohyperthermic Torrifluventic Haplustepts) soil in Guayama, Puerto Rico. The cropping systems evaluated were a maize-cowpea [Vigna unguiculata (L.) Walp.] cover crop rotation versus a typical practice of maize fallow (FA), each with five N fertilizer levels. The maize planting sequence was an initial spring 2014 planting, followed by a winter 2014-2015 planting and a final winter 2015-2016 planting. The cowpea cover crop was planted in the summers of 2014, 2015 and 2016. The inbred maize lines were female lines used for commercial hybrid maize production. Mean maize plant densities ranged from 59,391 to 69,182 plants per hectare for the cropping seasons. Indicators of crop N status were the Soil Plant Analysis Development (SPAD-502) Chlorophyll Meter®measuring leaf greenness; the Normalized Difference Vegetation Index (NDVI, measured with GreenSeeker®) and plant height measurements. The application of N fertilizer significantly influenced (p<0.05) leaf greenness (in five out of 13 occasions) and plant height (in seven of 12 occasions) but not NDVI. The use of cover crop significantly influenced leaf greenness (in four out of nine occasions) and plant height (in three out of eight occasions). The agronomic parameters tested did not predict yield or crop N status, thus are not recommended in the decision-making process of N fertilizer management of inbred maize.