Field experiments were conducted in the Guinea savanna ecology of Ghana to evaluate yield response of quality protein maize (Zea mays L.) hybrid to plant density and nitrogen (N) fertilizer. The experiments were conducted at four locations on 16 farmers' fields in 2002 and 2003. Three N rates (0, 90 and 135 kg/ha) were combined with three plant densities (50 000, 62 500 and 71 400 plants/ha) to constitute nine treatments which were tested in a randomized complete block design. Optimal N rate was not affected by plant density. There was no yield response to plant density. However, grain yield had a linear and quadratic response to N at all sites. Grain yield increases as a result of 90 kg N/ha applied over the farmers' practice (0 kg N/ha) at Tumu, Jirapa, Kpongu and Wa were 39%, 85%, 101% and 303% in 2002, respectively. Grain yield increases for the same rate and sites in 2003 were 31%, 83%, 63% and 51%, respectively. Marginal rate of return (MMR) to 90 kg N/ha combined with 62 600 plants/ha was the highest (5564%). Increasing N rate beyond 90 kg/ha did not result in corresponding increase in yield nor net benefit to merit the extra cost that may be incurred. From the study, application of 90 kg N/ha to hybrid maize would give economic yield response and acceptable returns at low risk to farmers, regardless of plant density.

Maize yield was not significantly affected by increased plant density but it increased with nitrogen fertilizer. Economic analyses showed that the marginal rate of return in applying 90 kg N was 113-194%, while 135 kg/ha gave 120-248%.

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