If I assess the growth stage of the crop in the field, and it deviates from the Adapt-N estimates, is this a problem?

No. There is some natural variation among cultivars in physiological development, especially during the rapid growth period in the early summer. The Adapt-N tool is generally quite accurate in estimating the growth stage of the corn crop, but it may be off by one or two stages. This has minimal impact on the recommendations.  If discrepancies between the Adapt-N estimates and field observations are greater, we suggest verifying crop input information, especially the date of planting.