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

Nitrogen fertilizer management in subsurface drip irrigation (SDI) systems for cotton (Gossypium hirsutum L.) can be very efficient when N is injected with the irrigation water (fertigated) on a daily basis. However, the daily rates and total amounts of N fertigation are uncertain. Normalized difference vegetative index (NDVI), calculated from weekly canopy reflectance measurements can guide N management in SDI cotton. The objective of this 3‐yr study (2007–2009) on an Acuff sandy clay loam (fine‐loamy, mixed, superactive, thermic Aridic Paleustolls) near Lubbock, TX, was to test two canopy reflectance‐based strategies for estimating and adjusting injection rates of urea ammonium nitrate (UAN) fertilizer between first square and early mid‐bloom. We also evaluated three N rates; 50, 100, and 150% of the soil test‐based N recommendation for a 1400 kg lint ha−1 yield goal. In the reflectance‐based N strategy‐1 (RN1), UAN was injected starting at first square at 50% of the soil test N rate. When NDVI in RN1 fell significantly below NDVI of plots with 100% soil test N, the N injection rate was increased to match the injection rate of the 100% soil test plots. The reflectance‐based N strategy‐2 (RN2) had an initial N injection rate equal to that of the 100% soil test N, and was raised to match the 150% soil test N based on NDVI. Nitrogen rates for the RN1 averaged across 3 yr were 22 kg N ha−1 less, or 31% less than the soil test treatment, without hurting lint or seed yields. In 2007, N rates with RN2 were 11 kg N ha−1 higher than the soil test N rate, without any yield benefit. Economic optimum N rates for lint production ranged from 23 kg N ha−1 in 2009 to 75 kg N ha−1 in 2008.

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In subsurface drip irrigation (SDI) systems, water constraints to cotton (Gossypium hirsutum L.) production are greatly minimized and N management becomes the main priority. Injecting N fertilizer into SDI systems should, in theory, be as efficient as the irrigation delivery system itself. The objective of this study was to test duration of N injections, addition of the nitrification inhibitor ammonium thiosulfate (ATS), and using canopy spectral reflectance to guide N injections in a 2‐yr study of SDI cotton in Lubbock, TX. For a 1400 kg ha−1 expected lint yield, the N fertilizer rate was 170 kg N ha−1 minus the 0‐ to 0.6‐m soil NO3–N and an estimate of irrigation water NO3 (20 and 30 kg N ha−1 for 2005 and 2006). We tested injection of urea–NH4NO3 (UAN) between first square and early bloom, and first square and peak bloom. There was no effect of timing or ATS on lint yield. Reflectance‐based N management up to peak bloom resulted in savings of 28 and 17 kg UAN‐N ha−1 for 2005 and 2006, respectively, compared with soil‐test‐based N fertigation. Lint yields were similar between reflectance‐based UAN management (1814 kg ha−1) and UAN injected up to peak bloom based on soil test NO3 (1880 kg ha−1). Nitrogen fertilizer recovery at first open boll was not affected by treatment but was high (62–75%). Flux of N2O + N2 was low (800–2100 g N ha−1) in both years and was not different between fertilized and unfertilized plots. Residual 0‐ to 1.2‐m soil profile NO3 after 2 yr was not affected by N fertigation treatments. Nitrogen fertilizer injections in SDI cotton are utilized very efficiently, and spectral‐reflectance‐based approaches have potential to reduce N fertilizer inputs.

Nitrogen and P fertilizer response for cotton (Gossypium hirsutum L.) and sorghum [Sorghum bicolor (L.) Moench] in a reduced tillage rotation system in the Southern High Plains has not been well studied. During 2000 to 2003, an irrigated study of cotton–sorghum rotation vs. continuous cotton evaluated the crop rotation effects on cotton lint yield and assessed N and P fertilizer and residual fertilizer response for the two systems. Preplant soil samples were collected each spring to determine fertilizer rates. Cotton lint yields and cottonseed N were not affected by rotation with sorghum compared with continuous cotton. Nitrogen fertilizer response was observed in lint yields from 2001 to 2003 in cotton following sorghum, but not in continuous cotton. No P fertilizer or soil residual P response in cotton lint yields was found, regardless of rotation. Grain sorghum yields responded to N fertilizer in 2 yr. No grain sorghum response was observed to P fertilizer, but in 1 yr a yield response to residual P fertilizer relative to zero‐P plots was noted. Seed N uptake was greater in sorghum than in cotton. Nitrogen fertility level increased seed N in sorghum and in cotton following sorghum. Infrequent crop response to P fertilizer was not unexpected, especially when Mehlich‐3 soil P in zero‐P subplots was near the 95% sufficiency level of 20 mg P kg−1. The main finding of this study is that N fertilizer response was more consistent for cotton following sorghum than in a continuous cotton system. In refining N fertilizer recommendations, N debits may be needed for N immobilization in sorghum residue. Nitrogen credit may be appropriate from leaf litter for crops following cotton and for NO3–N in irrigation water.