Prolificacy and nitrogen internal efficiency in maize crops

In maize (Zea mays L.) crop, “defensive practices” such as low plant densities and limited nitrogen (N) fertilization have begun to be progressively adopted in limited-production regions of the world. At low plant density, the expression of prolificacy (i.e., more than one fertile ear per plant) can stabilize maize production. Considering N as the main limiting resource, yield can be defined as a function of i) total N uptake and the efficiency to produce yield from total N uptake, i.e., N internal efficiency (NIE) or ii) total N uptake, N harvest index (NHI) and grain N concentration. The objective of this work was to describe changes in NIE and their related components regulated by prolificacy at both canopy- and plant- scales. Field studies were carried out in Buenos Aires, Argentina, during 2015–2016 and 2016–2017 seasons. Treatments were combinations of two N supplies (N-: 0 and N+: 200 kg N ha⁻¹), two plant densities (4 and 8 plants m⁻²) and five maize hybrids released during the last four decades. High N supply positively impacted yield per unit area (ca. 45 %) by increasing kernel number (ca. 33 %), kernel weight (ca. 18 %) and harvest index (ca. 12 %). At low plant density, high N supply expressed the highest prolificacy (mean 1.65 ears plant⁻¹). The older hybrids displayed the highest prolificacy (mean 1.30 ears plant⁻¹) and the lowest yield per unit area (mean 792 g m⁻²), but showed the greatest grain N concentration (mean 15.1 g kg⁻¹). However, recent hybrids resulted in medium prolificacy (mean 1.11 ears plant⁻¹) with the highest yield per unit area (mean 910 g m⁻²), but with the lowest grain N concentration (mean 13.6 g kg⁻¹). The different pattern of grain N concentration among hybrids coupled with similar NHI (mean 0.80) led to the lowest NIE of the most prolific hybrids (mean 55 g yield g N⁻¹) relative to the less prolific ones (mean 60 g yield g N⁻¹). Variations in NIE among genotypes and plant densities were negatively influenced by grain N concentration under N- and under N+ only when prolificacy = 1. Under N+ with prolificacy > 1, NIE variations were positively determined by NHI. For all genotypes, higher yield per plant (mean 30 %) and total N uptake (mean 25 %) were recorded for the prolific plants, reflected in the greater absolute NIE (mean 5 %). For the oldest hybrid differences between prolific and non-prolific plants in NIE (mean 46.8 vs 41.8 g yield g N⁻¹) were greater than those recorded for the other genotypes due to the higher impact of prolificacy on yield per plant (mean 51 %) than on total N uptake (mean 26 %). Over time, improvements in NIE primarily occurred from greater yield of the apical ear. Overall, the expression of prolificacy appears to be an adequate strategy to stabilize maize production in response to improvements of environmental conditions, specially addressed by low plant density and high N supply.