North American farmers are planting an average of about 12,350 seeds/ha less than hybrid-specific suggested planting rates derived from small plot research. To evaluate whether this “Planting Rate Gap” is justified, agronomists conducted 353 on-farm planting rate trials throughout the Corn Belt of North America in 2003 and 2004 with the following objectives: 1) to better understand the risks and benefits of planting above and below the farmers' current planting rates, and 2) to test whether small plot plant population research results are applicable to on-farm conditions. Using on-farm strip plots, cooperators planted a hybrid from Pioneer Hi-Bred Int'l. at their “Current Planting Rate” and at 9,880 seeds/ha above and below that level. Planting Rate Gap was defined as the difference between their Current Planting Rate and the optimum plant population for their hybrid as determined by seed company research. On average, growers predicted a 409 kg/ha yield increase when planting 9,880 seeds/ha above and a 553 kg/ha yield decrease when planting 9,880 below their Current Planting Rate. For locations where soil moisture was adequate, the income advantage for planting at the higher rate was consistent with findings from small-plot research. Namely, the larger the Planting Rate Gap, the greater the income advantage for increasing planting rate 9,880 seeds/ha. Under dry soil conditions, there was no advantage for increasing planting rates, even when growers were planting well below suggested levels. Under no circumstances was decreasing the growers' Current Planting Rate profitable. Growers consistently over-estimated the effects of plant population on stalk and root lodging. Small plot and on-farm trials gave comparable results at locations with adequate soil moisture. Some consideration of soil moisture status, particularly during the grain fill period may be needed to more accurately identify optimum planting rates for a given hybrid.