Corn (Zea mays, L) is an important world crop used as livestock feed, human consumption and ethanol production. Early in-season loss of nitrogen (N) continues to be a problem in corn. Ground-based active optical sensors (GBAO) have shown very promising results in predicting crop yield. In these experiments, two GBAO sensors GS and CC were used within forty-six established corn N-rate trials in North Dakota at the six (V6) and twelve (V12) leaf growth stages in 2011, 2012, and 2013. Corn height at V6 and V12 was recorded manually at each site in all three years. At V6, the GS relationship to yield and the INSEY (INSEY = in-season estimate of yield = sensor NDVI / growing degree days from planting date) value was often improved when the sensor NDVI was multiplied times corn height. Segregating the data sets into sites with eastern high clay conventional-till sites surface soil textures (clay more than 30%) and sites with more medium textures improved all INSEY relationships compared to pooling all sites. Eastern high clay conventional-till sites and eatstern medium textured converntional-till sites were further divided into those higher in productivity (yields greater than 10 Mg ha-1) and those lower in productivity (yields less than 10 Mg ha-1). The data categories differed in their sensor relationships to yield. Within all categories, the sensor relationships at V6 were weaker than those at V12. In the lower yielding eastern high clay conventional-till sites, lower yielding eastern medium-textured conventional-till sites, and the eastern no-till sites, no significant relationship was found at V6. At V12, a relatively weak relationship was only found in the low yielding eastern medium-textured coventional-till sites. The GS and CC were found to identify S deficiency at two sites in 2013. Both sensors detected that as N rate increased, the sensor readings generally decreased. This concept could be used by practitioners to screen sites with early season S deficiency, using an N rich strip in the field.