Sorghum (Sorghum bicolor L. Moench) has high water use efficiency, and is therefore widely cultivated in the Southern High Plains (SHP). Interest in sorghums for biofuel feedstock has increased recently as ethanol demand expands. Unlike grain sorghum, little data are available on N fertilizer requirements for ethanol production from sweet or forage sorghum production. Our objective was to compare ethanol yields and determine optimal N fertilizer needs for ethanol production from sweet sorghum and photoperiod sensitive (PPS) sorghum with limited irrigation in the SHP. Nitrogen fertilizer rates from 0 to 168 N kg ha−1 were tested on four sorghum cultivars (two sweet and two PPS) on Acuff sandy clay loam near Lubbock, TX in 2008 and 2009. Total dry matter (TDM) yields averaged 13 Mg ha−1 across years, cultivars, and N rates. Nitrogen fertilizer response in TDM was observed only in 2009, but bagasse yields responded to N fertilizer in both years. Cellulosic ethanol yields were greater with PPS sorghums than with sweet sorghums in both years. However, total ethanol yields were greater with sweet sorghums than PPS sorghums. Cellulosic ethanol and total ethanol yields responded to N in 2009 only. High preplant soil NO3 in 2008 apparently precluded TDM and ethanol yield response to N fertilizer. The optimum agronomic N fertilizer rate for ethanol and TDM across all four sorghums was 108 kg ha−1 respectively in 2009. The optimum N fertilizer rate for maximum profit with $ 0.70 kg N−1 and $.50 L−1 ethanol was 101 kg ha−1