@article{sripada_farrer_weisz_heiniger_white_2007, title={Aerial color infrared photography to optimize in-season nitrogen fertilizer recommendations in winter wheat}, volume={99}, ISSN={["1435-0645"]}, DOI={10.2134/agronj2006.0258}, abstractNote={Remote sensing in the form of aerial color infrared (CIR) photography has been shown to be a useful tool for in‐season N management in winter wheat (Triticum aestivum L.). The objectives of this study were (i) to develop a methodology for predicting in‐season optimum fertilizer N rates for winter wheat at growth stage (GS) 30 directly from aerial CIR photography and (ii) to quantify how the relationships between these optimum N rates and spectral indices respond to different levels of biomass of the wheat crop. Field studies were conducted for three winter wheat growing seasons (2002–2004) over a wide range of soil conditions across North Carolina using a split‐split plot randomized complete block design. Different planting date–seeding rate (PDSR) combinations were applied to create a range of biomass levels at GS 30. Different levels of N were applied at GS 25 (N25) to create a range of N supply and winter wheat radiance, and at GS 30 (N30) to measure grain yield response to N30. Aerial CIR photographs were obtained at each site at GS 30 before N applications. Significant biomass response to PDSR and yield response to N25 and N30 were observed. Optimum N30 ranged from 0 to 124 kg ha−1 with a mean of 55 kg ha−1. Better prediction of optimum N30 rates were obtained with spectral indices calculated relative to high‐N reference strips compared to absolute bands or spectral indices. Biomass measured at GS 30 influenced the strength of the relationship between optimum N30 and spectral indices. When the GS‐30 biomass was >1000 kg ha−1, the best predictor of optimum N30 (R2 = 0.85) was a quadratic model based on measured winter wheat radiance relative to mean radiance in the G band for the high N reference strip (Rel GS).}, number={6}, journal={AGRONOMY JOURNAL}, publisher={American Society of Agronomy}, author={Sripada, Ravi P. and Farrer, Dianne C. and Weisz, Randy and Heiniger, Ronnie W. and White, Jeffrey G.}, year={2007}, pages={1424–1435} }
 @article{weisz_sripada_heiniger_white_farrer_2007, title={In-season tissue testing to optimize soft red winter wheat nitrogen fertilizer rates: Influence of wheat biomass}, volume={99}, ISSN={["0002-1962"]}, DOI={10.2134/agronj2006.0112}, abstractNote={In the southeastern USA, soft red winter wheat (Triticum aestivum L.) N fertilizer recommendations are based on growth stage (GS) 30 tissue testing and models that assume that the relationship between tissue N concentration (Ncon) and optimum N fertilizer rates (MaxN30) is stable across fields differing in GS‐30 biomass. However, previous research has indicated this may not be the case. Consequently, it was critical to re‐evaluate these models. Using a split‐split plot design, six experiments were conducted in North Carolina between 2002 and 2004. Main plots were planting date–seeding rate combinations that produced wheat with different GS‐30 biomass. Subplots and sub‐subplots were five N rates applied at GS‐25 and GS‐30, respectively. Wheat yield was responsive to fertilizer N at all site‐years. The overall relationship between MaxN30 and Ncon was weak (r2 = 0.43). The relationship between MaxN30 and N uptake (Ncon × biomass) was weaker (r2 = 0.27). However, when the data were divided into different biomass classes, the overall model improved (R2 = 0.75). For biomass < 340 kg ha−1, the Ncon at which no additional N fertilizer was required (Ncritical) was 70.0 g N kg−1. As biomass increased, Ncritical decreased to 33.2 g N kg−1. Intermediate classes had slopes of MaxN30 versus Ncon and Ncritical values that were similar to those previously reported. This study indicates that to use tissue testing to determine N fertilizer recommendations across a range of GS‐30 biomass conditions requires information regarding dry matter biomass.}, number={2}, journal={AGRONOMY JOURNAL}, publisher={American Society of Agronomy}, author={Weisz, Randy and Sripada, Ravi P. and Heiniger, Ronnie W. and White, Jeffrey G. and Farrer, Dianne C.}, year={2007}, pages={511–520} }
 @article{farrer_weisz_heiniger_murphy_white_2006, title={Minimizing protein variability in soft red winter wheat: Impact of nitrogen application timing and rate}, volume={98}, ISSN={["0002-1962"]}, DOI={10.2134/agronj2006.0039}, abstractNote={Grain protein content in soft red winter wheat (Triticum aestivum L.) is highly variable across years and environments in the southeastern USA. This variability makes southeastern wheat undesirable to millers and negatively impacts its value in the export market. The objectives of this study were to determine how different N fertilizer rates and application times would affect grain protein variability and to determine if there were N fertilizer recommendations that would minimize regional protein variation. We conducted experiments in the North Carolina Piedmont, Coastal Plain, and Tidewater in 2001 and 2002. At each site–year, we used a split‐plot design with three or five N fertilizer rates at growth‐stage 25 (GS) (main plots), and an additional five N fertilizer rates applied at GS 30 (subplots). Analysis of variance indicated that environment contributed 68 and 90.5% of the variability in yield and test weight, respectively. Though environment contributed 23.3% of grain protein variability, the majority (51.4%) was attributed to timing and rate of N application. As grain protein levels increased at higher N rates, so did overall protein variability. Additionally, applying the majority of N fertilizer at GS 30 increased grain protein variability compared to application at GS 25. Based on these results, our recommendations to reduce grain protein variability in the southeastern USA are to: (i) reduce the range in N fertilizer rates used across the region, (ii) avoid overapplication of N beyond what is required to optimize yield and economic return, and (iii) apply spring N at GS 25.}, number={4}, journal={AGRONOMY JOURNAL}, publisher={American Society of Agronomy}, author={Farrer, Dianne C. and Weisz, Randy and Heiniger, Ronnie and Murphy, J. Paul and White, Jeffrey G.}, year={2006}, pages={1137–1145} }
 @article{shiffler_jolley_christopherson_webb_farrer_haby_2005, title={Pressurized hot water and DTPA-Sorbitol as viable alternatives for soil boron extraction. I. Boron-treated soil incubation and efficiency of extraction}, volume={36}, ISSN={["0010-3624"]}, DOI={10.1080/00103620500196457}, abstractNote={Abstract Serious challenges associated with hot water extraction, the standard extraction method for water‐soluble boron (B), limit its use in commercial soil‐testing laboratories. Several alternatives to make B testing more practical have been proposed and studied; none of the alternatives have readily replaced the hot water method. Two relatively new, promising B extraction methods are pressurized hot water and DTPA‐Sorbitol. Very little reported work compares B extraction values obtained from the standard hot water extraction method and these two alternative methods. This study was conducted to complete an initial step in validating new procedures—extracting the designated nutrient from fertilized, incubated soils by using standard and alternative extraction methods and comparing the resulting values. The three extraction methods were used to extract B from samples of calcareous sand and silt loam soils and limed, loamy fine sand, all which had been treated with 10 levels of B (0–8 mg kg−1) and incubated for 7 and 28 days. The amount of B extracted increased as the rate of B application increased with all three soil‐extraction methods. High correlations (r 0.977–0.999) were observed between extractable B and rate of B application with all three procedures. Correlations between the amount of extractable B using hot water extraction and the value obtained with an alternative extraction method were similar for both methods (r=0.89). Hot water generally extracted the least and pressurized hot water the most B regardless of soil type, rate of application, or duration of incubation. This study suggests the more easily used methods of pressurized hot water and DTPA‐Sorbitol could be recommended as replacements to the cumbersome hot water extraction.}, number={15-16}, journal={COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS}, author={Shiffler, AK and Jolley, VD and Christopherson, JE and Webb, BL and Farrer, DC and Haby, VA}, year={2005}, pages={2179–2187} }