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Increasing plant density and improving N fertilizer rate along with the use of high density-tolerant genotypes would lead to maximizing maize (Zea mays L.) grain productivity per unit land area. The objective of this investigation was to match the functions of optimum plant density and adequate nitrogen fertilizer application to produce the highest possible yields per unit area with the greatest maize genotype efficiency. Six maize inbred lines differing in tolerance to low N and high density (D) [three tolerant (T); L-17, L-18, L-53, and three sensitive (S); L-29, L-54, L-55] were chosen for diallel crosses. Parents and crosses were evaluated in the 2012 and 2013 seasons under three plant densities: low (47,600), medium (71,400), and high (95,200) plants ha −1 and three N fertilization rates: low (no N addition), medium (285 kg N ha −1 ) and high (570 kg N ha −1 ). The T × T crosses were superior to the S × S and T × S crosses under the low N-high D environment in most studied traits across seasons. The relationships between the nine environments and grain yield per hectare (GYPH) showed near-linear regression functions for inbreds L54, L29, and L55 and hybrids L18 × L53 and L18 × L55 with the highest GYPH at a density of 47,600 plants ha −1 and N rate of 570 kg N ha −1 and a curvilinear relationship for inbreds L17, L18, and L53 and the rest of the hybrids with the highest GYPH at a density of 95,200 plants ha −1 combined with an N rate of 570 kg N ha −1 . Cross L17 × L54 gave the highest grain yield in this study under both high N-high-D (19.9 t ha −1 ) and medium N-high-D environments (17.6 t ha −1 ).

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Selection for yield (10% intensity) was carried out in two groups of random oat (Avena saliva L.) lines (group T and group G contained 200 and 480 lines, respectively) under low‐, medium‐, and high‐productivity conditions. Selected lines from each selection condition were tested for mean, response, and stability of yield, in evaluation experiments that encompassed production environments from low to high.Actual gains in mean yield from selection averaged across the three selection environments were 16 and 7% in the T and G groups, respectively. Actual advances were 16, 13, and 18% of the population mean under low, medium, and high productivity, respectively, for group T, and 7, 5, and 8%, respectively, for group G. In each group, advances from selection under high and low productivities were superior to that from medium productivity. There was no superiority for one selection condition over another for either response or stability of yield.Lines selected only under low or medium productivity gave mean yields not significantly greater than the respective group means, whereas those selected only under high productivity gave significantly greater ones. The number of lines common to the samples selected under low, medium, and high productivity were three for group T and six for group G. These lines averaged 20% greater than the group means for yield, and were significantly superior under low, medium, and high productivity.Low‐ and high‐productivity conditions were nearly equal for selecting high‐yielding lines of oats, and both were somewhat superior to medium productivity. However, a detailed analysis showed that high productivity tended to differentiate high‐yielding lines best. The most superior lines for all conditions of production were those chosen in all three selection environments and those chosen in both low and high productivity.

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