The application of adequate, non-excessive rates of N fertilizer to sugarbeets (Beta vulgaris L.) continues to increase in importance with rising fertilizer costs and industry transitions from yield-based payments to payments weighted toward crop quality. The objectives of this study were to evaluate the use of active sensing for in-season prediction of sugarbeet yield and recoverable sucrose, and to determine if end-of-season sensing could provide a tool for improving N management of the sugarbeet cropping system.

Four replications of six N fertilizer treatments, from 0 to 224 kg N ha^-1, were arranged in a randomized complete block design at four sites in Michigan in 2006 and 2007. Urea N fertilizer was banded at planting at 45 kg N ha^-1 with 28% urea ammonium nitrate solution sidedress applications at the 4-leaf stage comprising remaining N rates. Normalized difference vegetative indices (NDVI) were measured in mid-June, mid-July, mid-August and at harvest with a red-band active sensor. Leaf samples were collected at each sensing event for foliar N analysis. Total leaf biomass and root yield were determined at harvest, and root subsamples were collected for determination of sucrose content, clear juice purity, and amino N concentration.

Results from 2006 indicated in-season NDVI measurements were poorly related to root yield (R² = 0.23, 0.40, and 0.56 for June, July, and August, respectively). However, NDVI in July and August were effective indicators of recoverable white sucrose (R² = 0.79). End-of-season NDVI was also correlated with sugarbeet vegetative top total N (R² = 0.79). Active sensing during the growing season shows promise as a means to estimate recoverable sugar in sugarbeet fields and sensing on day of harvest may improve rotational N management by providing an indication of N return to the cropping system.