Progress 10/01/11 to 09/30/16

Outputs
Target Audience:The audiences that benefited the most from the knowledge gained were rice farmers, extension agents, consultants, fertilizer dealers, students, and scientists working in the field as well as policy makers. The knowledge when placed in their hands improved the efficiency with which the rice crop utilizes the nitrogen fertilizer applied and in turn made rice production more profitable, sustainable, and environmentally sound. The methane emissions research will be used to understand what influences methane emissions from rice fields and ways to minimize this greenhouse gas. Knowledge gained has and will be added to curricula used in classroom and extension training workshops and extension publications and shared with the rice industry and EPA. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?This research has allowed the scientists and our graduate students to attend scientific meetings, workshops, county meetings, and field days so we could better interact with other scientists and the public. It has allowed our county agents and verification coordinators to better understand the science and work with farmers, fertilizer dealers, and consultants to implement the results. How have the results been disseminated to communities of interest?The information has been disseminated to communities of interest via the classroom, seminars, extension training workshops, county meetings, field days,scientific meeting, popular press, as well as extension and scientific publications. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? The N-STaR (Nitrogen Soil Test for Rice) provides field-specific nitrogen (N) recommendations for Arkansas rice production on both silt loam and clay soils to predict the fertilizer N rate required to achieve 95 and 100% relative grain yield. The N fertilizer calibration curves developed for silt loam soils sampled to a 45 cm depth and clay soils sampled to a 30 cm depth had high correlation coefficients of 0.89 and 0.88, respectively. Validation studies conducted in Rice Research Verification Program fields and other commercial farmer fields showed predicted N rates using N-STaR achieved the highest yields and in many cases achieved these yields with less N fertilizer compared to the standard recommended N rate or the farmer's N rate. Proper sampling is imperative in the use of N-STaR and obtaining an accurate N rate. Sampling silt loam soils has presented no problems when the N-STaR sample bucket and ship auger were used, but proper sampling of clay soils is more difficult and more care should be taken when sampling clays. Hybrid rice hectarage has increased substantially in the southern United States and necessitated research into the N nutrition of this new type of rice and how it compares to the traditional pure-line rice. The hybrid had greater soil N uptake compared with the pure-line cultivar and the hybrid had a higher fertilizer N uptake efficiency (FNUE) compared with the pure-line at the location with the lesser native soil N and a similar FNUE at the location with the greater soil N. Also, the pure-line had a higher FNUE and greater N fertilizer response at the location with the greater soil N while the hybrid had a similar FNUE at both locations. The greater total N uptake by the hybrid compared with the pure-line was due to greater soil N uptake at both locations and fertilizer N uptake at the location with the lower native soil N. Midseason N studies were conducted at three locations for three years to examine the influence of midseason N application and its timing on the grain yield of conventional, pure-line rice varieties from Louisiana and Arkansas. In general, a single preflood N application produced a similar to greater yield than the two-way split application. Application of midseason N usually increased grain yield, especially when an inadequate rate of N was applied preflood, and the midseason N application window appeared to be 2 to 3 weeks wide with a tendency at times for grain yield to increase as the midseason N was delayed. This tendency could be due in some years to the short time span (i.e., 2 weeks) between when the preflood N was applied and the rice reached beginning internode elongation (BIE) causing the BIE and BIE+7 days midseason N applications to be applied when the rice was still possibly taking up the preflood N. This might have reduced the uptake and/or the impact on grain yield of these early midseason N applications. In drill-seeded, delayed-flood rice, urea applied preflood is susceptible to ammonia volatilization loss if the flood is not established in a timely manner. Research studies investigated the effectiveness of two sources of the urease inhibitor NBPT, Agrotain-Ultra and Arborite AG-NT, with different adhering technologies. The two sources were compared to untreated urea, applied at an optimal and sub-optimal rate 10, 5, and 1 days prior to flooding (DPF). Sources were compared to untreated urea in terms of grain yield and N loss via NH3 volatilization. Averaged across N sources and rates, rice fertilized 10 DPF produced a lower yield than urea applied 5 DPF or 1 DPF, which did not differ. Yield from the two NBPT sources, did not differ and produced a greater yield than untreated urea. Results indicated that NH3 volatilization from urea (15%) was greater than loss from urea treated with either NBPT source (3%). In rice production, grain yield and milling quality are important factors, which determine the economic return from a given production area. No known research has investigated rice yield and quality of current cultivars produced in the midsouthern United States as affected by nitrogen rate and harvest moisture. Thus, the objective of this study was to investigate current long-grain rice cultivars (standard stature, semi-dwarf, and hybrid) with multiple N rates (0, 45, 90, 135, 180 kg ha-1) harvested at three moisture contents (low, medium, and high). Rough rice grain yield for all three cultivars was maximized at approximately 134 kg N ha-1. In general, little difference was observed in percent milled rice yield across N rates and cultivars. In contrast, nitrogen rate resulted in differences in percent head rice yield, particularly at the low harvest moisture content. In the current study, cultivar, N rate, and harvest moisture had minimal impact on percent milled rice yield, but these factors may result in large differences in percent head rice yield. Over the last few decades, near-infrared reflectance spectroscopy has been researched and developed into a dependable tool for quantifying concentrations of soil organic carbon and closely associated soil organic matter concentrations. The objective of this study was to evaluate the rapid and economical near-infrared reflectance spectroscopy method as a dependable technique for estimating soil organic carbon concentrations. Significant relationships were measured for predicted soil organic carbon concentrations when compared to laboratory-measured soil organic carbon concentrations for all models evaluated, regardless of validation technique. The prediction models using the first derivative of the reflectance values outperformed prediction models using the reflectance values alone. In conclusion, near-infrared reflectance spectroscopy can be utilized as a rapid and accurate technique for measuring soil organic carbon concentrations. Methane emissions from delayed-flood rice as influenced by soil texture, variety, and previous crop were measured in Arkansas. Studies on a silt loam and clay soil following soybean showed that substantially lower emissions occur on clay soils (47 kg CH4/ha-season) as compared to silt-loam soils (200 kg CH4/ha-season). Previous crop (soybean or rice) and variety significantly impacted seasonal emissions. Rice grown following soybean produced 151 kg CH4/ha-season as compared to 218 kg CH4/ha-season when grown in a rice/rice rotation; most of our rice follows soybean. Methane emissions were significantly lower at 133 kg CH4/ha-season with the hybrid CLXL745 compared to the conventional rice varieties, which emitted on average 210 kg CH4/ha. A study comparing the influence of N sources on seasonal CH4 emissions showed seasonal emissions from ammonium sulfate were less than that from either urea or poultry litter which did not differ. This study indicated that ammonium sulfate can potentially reduce CH4 emissions in drill-seeded, delayed-flood rice produced on a silt-loam soil as compared to urea or poultry litter + urea. Thus, it appears variety, previous crop, soil texture, and N source are key subfactors, which could be used to increase the accuracy when estimating statewide methane emissions by EPA.

Publications

• Type: Journal Articles Status: Published Year Published: 2016 Citation: Rogers, C.W., R.J. Norman, T.J. Siebenmorgen, B.C. Griggs, J.T. Hardke, K.R. Brye, and E.E. Gbur. 2016. Rough Rice and Milling Yields as Affected by Nitrogen, Harvest Moisture, and Cultivar. Agron. J. 108:813821. Grigg, B.C., T.J. Siebenmorgen, and R.J. Norman. 2016. Effects of nitrogen rate and harvest moisture content on physicochemical properties and milling yields of rice. Cereal Chem. 93(2):172-181.
• Type: Other Status: Published Year Published: 2016 Citation: Norman, R.J., J.T. Hardke, T.L. Roberts, N.A. Salton, D.L. Frizzell, A.D. Smartt, E. Castaneda-Gonzalez, G.J. Lee. 2016. P.179-194. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2015. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 634.
• Type: Book Chapters Status: Published Year Published: 2016 Citation: Smartt, A.D., K.R. Brye, and R.J. Norman. 2016. Methane emissions from rice production in the United States; A review of controlling factors and summary of research. pp. 179-208. In B.L. Moya and J. Pous (ed.) Greenhouse gases. InTech, Rijeka, Croatia.

Progress 10/01/14 to 09/30/15

Outputs
Target Audience:The audiences that will benefit the most from the knowledge gained are rice farmers, extension agents, consultants, fertilizer dealers, students, and scientists working in the field as well as policy makers. The knowledge when placed in their hands will improve the efficiency with which the rice crop utilizes the N fertilizer applied and in turn make rice production more profitable, sustainable, and environmentally sound. The methane emissions research will be used to understand what influences methane emissions from rice fields and ways to minimize this greenhouse gas. Knowledge gained will be added to curricula used in classroom and extension training workshops and extension publications and shared with the rice industry and EPA. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?This research has allowed the scientists and our graduate students to attend scientific meetings, workshops, county meetings, and field days so we could better interact with other scientists and the public. It has allowed our county agents and verification coordinators to better understand the science and work with farmers, fertilizer dealers, and consultants to implement the results. How have the results been disseminated to communities of interest?The information has been disseminated to communities of interest via the classroom, seminars, extension training workshops, county meetings, field days,scientific meeting, popular press, as well as extension and scientific publications. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? The N-STaR (Nitrogen Soil Test for Rice) provides field-specific nitrogen (N) recommendations for Arkansas rice production on both silt loam and clay soils. We have finished validating the N-STaR calibration curve for silt loam soils in commercial fields in the Rice Research Verification Program. In 2015, we continued our validation of the N-STaR calibration curve for clay soils in commercial fields in the Rice Research Verification Program. A total of seven trials were established in farmer fields across the state on clay soils and included a wide variety of cultivars, both conventional varieties and hybrids. The 2015 results showed N-STaR predicted N rates achieved the highest yields and in many cases achieved these yields with less N fertilizer compared to the standard N rate or the farmer's N rate. In drill-seeded, delayed-flood rice, the large preflood application of N fertilizer is typically urea. Urea is susceptible to ammonia volatilization loss if the flood is not established in a timely manner. Research studies investigated the effectiveness of two sources of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT), Agrotain-Ultra and Arborite AG-NT, with different adhering technologies. The two sources were compared to untreated urea, in terms of preventing rice grain yield reduction attributed to N loss when urea-N was applied at an optimal and sub-optimal rate 10, 5, and 1 days prior to flooding (DPF). Sources were also compared to untreated urea in terms of urea-N loss via NH3 volatilization. Grain yield, averaged across N source and application time, differed among N rates, where the yield from optimally N-fertilized rice was 8208 lb/acre, sub-optimal was 6692 lb/acre, and no N was 4996 lb/acre. Averaged across N sources and rates, rice fertilized 10 DPF produced a lower yield (7227 lb/acre) than urea applied 5 DPF (7673 lb/acre) or 1 DPF (7494 lb/acre), which did not differ. Yield from the two NBPT sources, did not differ (7494 lb/acre and 7584 lb/acre) and produced a greater yield than untreated urea (7316 lb/acre). Results indicated that NH3 volatilization from urea (15%) was greater than loss from urea treated with either NBPT source (3%). Both NBPT sources resulted in greater yields than untreated urea and were similar in effectiveness at inhibiting NH3 volatilization of urea. Over the last few decades, near-infrared reflectance spectroscopy has been researched and developed into a dependable tool for quantifying concentrations of soil organic carbon and closely associated soil organic matter concentrations. Many studies have evaluated near-infrared reflectance spectroscopy along with various multivariate statistical analysis and have reported r2 values for the prediction of soil organic carbon ranging from 0.60 to 0.94, with an average r2 value of 0.79 and for soil organic matter ranging from 0.55 to 0.90 with an average r2 value of 0.74. The objective of this study was to evaluate the rapid and economical near-infrared reflectance spectroscopy method as a dependable technique for estimating soil organic carbon concentrations. Partial least squares regression was used to quantify the soil organic carbon concentration from soil reflectance values or the first derivative of the reflectance values. Two model validation techniques were evaluated. One was a full cross-validation and in the other, 30 percent of the samples were removed from the calibration data set and then tested using the calibrated model. Significant relationships were measured for predicted soil organic carbon concentrations when compared to laboratory-measured soil organic carbon concentrations for all models evaluated, regardless of validation technique. The prediction models using the first derivative of the reflectance values outperformed prediction models using the reflectance values alone. In conclusion, near-infrared reflectance spectroscopy can be utilized as a rapid and accurate technique for measuring soil organic carbon concentrations. Quantifying methane (CH4) emissions from cultivated rice has become more important recently due to climate change and methane's importance as a greenhouse gas. Standard methodology for field assessments of trace gas emissions has been the enclosed-headspace chamber technique. Unfortunately, no direct comparisons among differing chamber sizes have been reported for measured CH4 fluxes and emissions from field-grown rice. Consequently, the objective of our research was to assess the effect of chamber size on CH4 fluxes and season-long emissions from rice grown on a clay soil in Arkansas. Chamber size did not affect CH4 fluxes during the flooded portion of the rice growing season and only slightly affected fluxes after flood release. Cumulative CH4 emissions from this flooded-rice study were not influenced by chamber size in that both the 15.2- and 30.4-cm diameter chambers were similarly adequate for measuring CH4 fluxes and emissions.

Publications

• Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: Davidson, J., T.L. Roberts, J.T. Hardke, N.A. Slaton, R.J. Norman, A. Fulford and C.E. Greub. 2015. Another Step Forward: Validation of N-Star on Rice in Clay Soils in Arkansas. In ASA, CSSA, and SSSA International Annual Meeting Abstracts. Minneapolis, MN. 15-18 Nov. 2015. Madison, WI. Fryer, M.S., N.A. Slaton, T.L. Roberts, R.J. Norman, R.E. DeLong, M.R. Parvej and R.J. Dempsey. 2015. Validation of soil-test based phosphorus and potassium fertilizer recommendations for soybean. In Southern Branch American Society of Agronomy Abstracts. [CD-ROM]. Atlanta, GA. 1-3 Feb. 2015. Madison, WI. Fryer, M.S., N.A. Slaton, T.L. Roberts, R.J. Norman, and R.E. DeLong. 2015. Validation of soil-test based phosphorus and potassium fertilizer recommendations for soybean. In ASA, CSSA, and SSSA International Annual Meeting Abstracts. Minneapolis, MN. 15-18 Nov. 2015. Madison, WI. Richmond, T., N.A. Slaton, J.T. Hardke, T.L. Roberts, and R.J. Norman. Effect of delaying the flood and preflood nitrogen application on rice yield. In Arkansas Crop Management Conference Proceedings. Nov 30-Dec 1, 2015. Fayetteville, AR. Roberts, T.L., R. J. Norman, N.A. Slaton, J.T. Hardke, C.E. Greub, J. Davidson and A.M. Fulford. 2015. Nitrogen management in rice: A case study in best management practices. In ASA, CSSA, and SSSA International Annual Meeting Abstracts. Minneapolis, MN. 15-18 Nov. 2015. Madison, WI. Slaton, N.A., P.O. Martins, T.L. Roberts, and R.J. Norman. 2015. Evaluation of field-moist and dry soil extraction on Mehlich-3 potassium availability on loamy soils from Arkansas. In ASA, CSSA, and SSSA International Annual Meeting Abstracts. Minneapolis, MN. 15-18 Nov. 2015. Madison, WI.
• Type: Journal Articles Status: Published Year Published: 2015 Citation: Bushong, J.T., R.J. Norman, and N.A. Slaton. 2015. Near-infrared reflectance spectroscopy as a method for determining organic carbon concentrations in Soil. Commun. Soil Plant Anal. 46, 14:1791-1801. Rogers, C.W., R.J. Norman, K.R. Brye, N.A. Slaton, and A.D. Smartt. 2015. Comparison of urease inhibitors for use in rice production on a silt-loam soil. Crop For. Turfgr. Man. DOI: 10.2134/cftm2014.0062 Smartt, A.D., K.R. Brye, C.W. Rogers, R.J. Norman, E.E. Gbur, J.T. Hardke, and T.L. Roberts. 2015. Chamber size effects on methane emissions from rice production. Open J. Soil Sci. 5: 227-235.
• Type: Other Status: Published Year Published: 2015 Citation: Davidson, J.T., T.L. Roberts, C.E. Greub, A.M. Fulford, N.A. Slaton, J.T. Hardke, J. Shafer, S. Williamson, and C. Scott. 2015. Validation of the nitrogen soil test for rice (N-STaR) on clay soils in Arkansas. P. 195-201. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Dempsey, R.J., N.A. Slaton, T.L. Roberts, R.J. Norman, R.E. DeLong, M.S. Fryer, and M.R. Parvej. 2015. Ammonia volatilization and rice grain yield as affected by simulated rainfall amount and nitrogen fertilizer amendment. p. 202-207. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Dempsey, R.J., N.A. Slaton, T.L. Roberts, R.J. Norman, R.E. DeLong, M.S. Fryer, and M.R. Parvej. 2015. Rice grain yield as affected by simulated rainfall timing and nitrogen fertilizer amendment. p. 208-213. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Frizzell, D.L., J.T. Hardke, E. Castaneda-Gonzalez, R.J. Norman, and M. Duren. 2015. Influence of nitrogen rate and seeding rate on grain yield of Roy J rice grown at two locations in Arkansas-first year results. p. 222-229. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Norman, R.J., T.L Roberts, J.T. Hardke, N.A. Slaton, K.A.K. Moldenhauer, D.L. Frizzell, M.W. Duren, E. Castaneda-Gonzalez, and A.M. Fulford. 2015. Grain yield response of five new rice varieties to nitrogen fertilization. p. 256-271. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Slaton, N.A., T.L Roberts, R.J. Norman, J.T. Hardke, R.E. DeLong, J.B. Shafer, C.G. Massey, and S.D. Clark. 2015. The effect of delaying preflood-nitrogen fertilization on grain yield of flood-irrigated rice. p. 277-280. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Slaton, N.A., T.L Roberts, R.J. Norman, J.T. Hardke, R.E. DeLong, J.B. Shafer, C.G. Massey, and S.D. Clark. 2015. Rice response to short- and long-term phosphorus and potassium fertilization rate. p. 281-288. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 626. Smartt, A.D., C.W. Rogers, K.R. Brye, R.J. Norman, W.J. Smartt, J.T. Hardke, D.L. Frizzell, and E. Castaneda-Gonzalez. 2015. Growing-season methane fluxes and emissions from a silt loam soil as influenced by rice cultivar. pp. 289-297. In: R.J. Norman, and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2014. Univ. Ark., Agric. Expt. Stn. Res. Ser. 626.
• Type: Books Status: Published Year Published: 2015 Citation: Norman, R.J., and K.A.K. Moldenhauer (eds.). 2015. B.R. Wells Arkansas Rice Research Studies 2014. Univ. of Ark., Agr. Exp. Stn. Res. Ser. 626.

Progress 10/01/13 to 09/30/14

Outputs
Target Audience: The audiences that will benefit the most from the knowledge gained are rice farmers, extension agents, consultants, fertilizer dealers, students, and scientists working in the field. The knowledge when placed in their hands will improve the efficiency with which the rice crop utilizes the N fertilizer applied and in turn make rice production more profitable, sustainable, and environmentally sound. The methane emissions research will be used to understand what influences methane emissions from rice fields and ways to minimize this greenhouse gas. Knowledge gained will be added to curricula used in classroom and extension training workshops and extension publications and shared with the rice industry and EPA. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? This research has allowed the scientists and our graduate students to attend scientific meetings, workshops, county meetings, and field days so we could better interact with other scientists and the public. It has allowed our county agents and verification coordinators to better understand the science and work with farmers, fertilizer dealers, and consultants to implement the results. How have the results been disseminated to communities of interest? The information has been disseminated to communities of interest via the classroom, seminars, extension training workshops, county meetings, field days,scientific meeting, popular press, as well as extension and scientific publications. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? The N-STaR (Nitrogen Soil Test for Rice) provides field-specific N recommendations for Arkansas rice production on both silt loam and clay soils. Presently, the N-STaR modified bucket and drill are being used to obtain samples for rice produced on silt loams, but an alternative method is being sought for clay soils due to the difficulty of properly sampling this soil texture. The N-STaR method is only as good as the sample taken and for clay soils is highly dependent on the accurate collection of a 0-30 cm soil sample. Because of the apparent difficulty of sampling clay soils five sampling methods, including a Dutch Auger (DA), N-STaR Bucket and Kleen Hole Spade (KHS) with three lubrication methods, were studied across a range of clay soils in Arkansas to determine if there is a viable alternative to the N-STaR Bucket currently used for silt loam soils. Soil samples taken using the KHS probe lubricated with WD-40 were significantly different between samples taken with the DA at one site and at another site, there was a significant difference between the samples taken with the dry KHS and wet KHS probe compared to the DA. The KHS lubricated with water was the most precise method at half of the sites, but not the most accurate. The N-STaR bucket was the most accurate of the methods evaluated at five of the sites when compared to the DA. Overall, the N-STaR sample bucket appears to be the best method for sampling although other sampling methods studied have utility. In rice production, grain yield and milling quality are important factors, which determine the economic return from a given production area. This is due to the fact that the market value of rice is not just determined by total rough rice yield but also by percent head rice yield. No known research has investigated rice yield and quality of current cultivars produced in the midsouthern United States as affected by nitrogen rate and harvest moisture. Thus, the objective of this study was to investigate current long-grain rice cultivars (standard stature, semi-dwarf, and hybrid) with multiple N rates (0, 45, 90, 135, 180 kg ha-1) harvested at three moisture contents (low, medium, and high). Rough rice grain yield for all three cultivars was maximized at approximately 134 kg N ha-1. In general, little difference was observed in percent milled rice yield across N rates and cultivars. In contrast, nitrogen rate resulted in differences in percent head rice yield, particularly at the low harvest moisture content. In the current study, cultivar, N rate, and harvest moisture had minimal impact on percent milled rice yield, but these factors may result in large differences in percent head rice yield. As economic returns are determined by rough rice yield as well as head rice yield, continued research investigating cultivar, N rate, and harvest moisture will help refine harvest, drying, and storage management strategies for rice producers. Research concerning methane (CH4) emissions as affected by previous crop and fertilizer nitrogen (N) source from drill-seeded, delayed-flood rice (Oryza sativa L.) production in Arkansas is limited. Consequently, a study was conducted to investigate the influence of previous crop (soybean or rice) and fertilizer N source (ammonium sulfate, urea, and poultry litter + urea) on the semi-dwarf cultivar Cheniere. Fertilizer N sources were applied at a rate of 168 kg N ha-1, and poultry litter (3.7% N) was applied at a rate of 2242 kg ha-1 with additional urea applied to ensure equal N rates across all treatments. Emissions during the season only differed among N sources. Averaged across previous crop, seasonal emissions from ammonium sulfate (125 kg CH4-C ha-1) were less than that from either urea (151 kg CH4-C ha-1) or poultry litter (170 kg CH4-C ha-1), which did not differ. This study indicated that ammonium sulfate can potentially reduce CH4 emissions in drill-seeded, delayed-flood rice produced on a silt-loam soil as compared to urea or poultry litter + urea. Additionally, the application of poultry litter, at the rate tested during the current study, did not result in significant differences in CH4 emissions compared to urea. Research concerning N source, particularly the addition of poultry litter, will help determine sustainable fertilizer management strategies in relation to the carbon footprint of rice production. A midseason N study was conducted at three locations to examine the influence of midseason N application and its timing on the grain yield of conventional, pure-line rice varieties from Louisiana and Arkansas. A single preflood N application produced a similar or greater yield than the two-way split application. Application of midseason N generally increased grain yield when applied at all of the application times at both preflood N rates at the NEREC and RREC, but not at the PTRS at the higher preflood N rate were there was no response. The midseason N application window appeared to be 2 to 3 weeks wide with a tendency at times for grain yield to increase as the midseason N was delayed. This tendency could be due to the short time span (i.e., 2 weeks) between when the preflood N was applied and the rice reaching BIE causing the BIE and BIE+7 days midseason N applications being applied when the rice was still possibly taking up the preflood N. This might have reduced the uptake and/or the impact on grain yield of these early midseason N applications.

Publications

• Type: Books Status: Published Year Published: 2014 Citation: Norman, R.J., and Moldenhauer, K.A. K. (eds.). 2014. B.R. Wells Arkansas Rice Research Studies 2013. Univ. of Ark., Agr. Exp. Stn. Res. Ser. 617
• Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Scott, C.L., Roberts, T.L., Williamson, S.M., Grueb, C.E., Norman, R.J., Slaton, N.A., and Fluor, A.M. 2014. Seasonal fluctuation of N-STaR soil test values taken at various soil depths. In Rice Technical Working Group Abstracts. 35th Meeting, New Orleans, LA. Feb. 18-21, 2014.
• Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Grueb, C.E., Roberts, T.L., Norman, R.J., Fulford, A.M., and Slaton, N.A. 2014. Small plot validation of N-STaR for rice produced on clayey soils in Arkansas. In Rice Technical Working Group Abstracts. 35th Meeting, New Orleans, LA. Feb. 18-21, 2014.
• Type: Other Status: Published Year Published: 2014 Citation: Roberts, T.L., Norman, R.J., Slaton, N.A., Hardke, J.T., Greub, C.E., Fulford, A.M., Williamson, S.M., Shafer, J.B., Frizzell, D.L., and Duren, M.W. 2014. Field validation of the nitrogen soil test for rice (N-STaR) for rice produced on clay soils. p. 311-316. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2013. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 617.
• Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Williamson, S.M., Roberts, T.L., Scott, C.L., Norman, R.J., Slaton, N.A., Fulford, A.M., and Grueb, C.E. 2014. Summary of N-STaR rate recommendations in Arkansas during 2013. In Rice Technical Working Group Abstracts. 35th Meeting, New Orleans, LA. Feb. 18-21, 2014.
• Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Davidson, Jarom, Trenton L. Roberts, Richard J. Norman, Nathan A. Slaton, Chester Eugene Greub and Jarrod T Hardke. 2014. Comparison of soil sampling methods for site-specific N rates on clay soils using N-Star. In Abstracts 2014 ASA, CSSA, and SSSA Joint Annual Meetings. Nov. 2-5. Long Beach, CA.
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Brye, K.R., Rogers, C.W., Smartt, A.D., and Norman, R.J. 2014. Soil texture effects on methane emissions from direct-seeded, delayed-flood rice production in Arkansas. Soil Sci. 178: 10: 519-529.
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Rogers, C.W., Brye, K.R., Smartt, A.D., Norman, R.J., Gbur, E.E., and Evans-White, M.A. 2014. Cultivar and previous crop effects on methane emissions from drill-seeded, delayed-flood rice production on a silt-loam soil. Soil Sci. 179: 1: 28-36.
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Gordon, B.L., Slaton, N.A., Norman, R.J., and Roberts, T.L. 2014. Nitrogen-fertilizer equivalence of poultry litter for winter wheat production. Soil Sci. Soc. Am. J. 78: 1674-1685.
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Skinner, J.V, Jr., Brye, K.R., Norman, R.J., and Slaton, N.A. 2014. Spatially Applied Soil Amendments Decrease Soil Property and Crop Yield Variability on a Small-Plot Research Field. Crop Management 2014 13: 1-11. doi:10.2134/CM-2014-0001-RS.
• Type: Other Status: Published Year Published: 2014 Citation: Norman, R.J., Roberts, T.L., Hardke, J.T., Slaton, N.A., Moldenhauer, K.A.K., Frizzell, D.L., Duren, M.W., and Castaneda-Gonzalez, E. 2014. Grain yield response of eight new rice cultivars to nitrogen fertilization. p. 293-302. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2013. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 617.
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Nalley, L.L., Dixon, B.L., Brye, K.R., Rogers, C.W., Myrteza, H., and Norman, R.J. 2014. Estimating cultivar effects on water usage and greenhouse gas emissions in rice production. Agron. J. 106: 6: 1981-1992.
• Type: Other Status: Published Year Published: 2014 Citation: Norman, R.J., Hardke, J.T., Roberts, T.L., Slaton, N.A., Frizzell, D.L., Duren, M.W., and Castaneda-Gonzalez, E. 2014. Response of two rice varieties to midseason nitrogen fertilizer application timing. p. 303-310. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2013. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 617.
• Type: Other Status: Published Year Published: 2014 Citation: Dempsey, R.J., Slaton, N.A., Roberts, T.L., Norman, R.J., DeLong, R.E., and Massey, C.G. 2014. Ammonia volatilization and rice grain yield as affected by simulated rainfall amount and nitrogen fertilizer amendment. p. 239-246. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Arkansas Rice Research Studies 2013. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 617.
• Type: Theses/Dissertations Status: Published Year Published: 2014 Citation: Alkaline Hydrolyzable-Nitrogen, Seeding Date, and Clay-Fixed Ammonium as Potential Indicators of Rice Response to Nitrogen Fertilization in Arkansas
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Reiter, M.S., Daniel, T.C., Slaton, N.A., and Norman, R.J. 2014. Nitrogen availability from granulated fortified poultry litter fertilizers. Soil Sci. Soc. Am. J. 78: 861-867.
• Type: Theses/Dissertations Status: Published Year Published: 2014 Citation: Methane Emissions and Ammonia Volatilization from Drill-Seeded, Delayed-Flood Rice on a Silt-Loam Soil in Arkansas

Progress 01/01/13 to 09/30/13

Outputs
Target Audience: The audiences that will benefit the most from the knowledge gained are rice farmers, extension agents, consultants, fertilizer dealers,students, and scientists working in the field. The knowledge when placed in their hands will improve the efficiency with which the rice crop utilizes the N fertilizer applied and in turn make rice production more profitable, sustainable, and environmentally sound. The methane emissions research will be used to understand what influences methane emissions from rice fields and ways to minimize this greenhouse gas. Knowledge gained will be added to curricula used in classroom and extension training workshops and extension publications. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? This research has allowed the scientists and our graduate students to attend scientific meetings, workshops, county meetings, and field days so we could better interact with other scientists and the public. It has allowed our county agents and verification coordinators to better understand the science and work with farmers, fertilizer dealers, and consultants to implement the results. How have the results been disseminated to communities of interest? The information has been disseminated to communities of interest via the classroom, seminars, extension training workshops, county meetings, field days,scientific meeting, popular press, as well as extension and scientific publications. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? The nitrogen (N) fertilizer rate calibration curve for our new Nitrogen-Soil Test for Rice, N-STaR, has been completed for clay soils. Sixteen site-years of correlation and fertilizer calibration data have been collected for rice grown on clay soils. The grain yield correlation and N fertilizer rate calibrations were developed using a 15 cm sampling interval to a 45 cm depth. The coefficient of determination (r2) was maximized for N-STaR (r2 = 0.83) when clayey soils were sampled to a 30 cm depth. The N-STaR soil test values obtained from the 0- to 30-cm sampling depth also explained 83% of the observed variability in the N fertilizer rate required to achieve 95% RGY. The results indicate N-STaR is capable of accurately predicting the N fertilizer responsiveness of rice grown on clayey soils sampled to the proper depth in Arkansas. To facilitate the incorporation into crop management and the understanding of the N-STaR, field validation studies were established in producer fields and at experiment stations on silt loam soils across the state of Arkansas. Nitrogen fertilizer rates predicted from the three calibration curves ranged from 22 to 252 kg N ha-1. Rice grain yields fertilized with N rates from the N-STaR 90% RGY curve were equal to or higher than the standard recommendation in 9 of the 14 sites, for rice receiving the N-STaR 95% RGY N rates were equal to or greater than the standard recommendation for 13 of the 14 sites, and rice receiving the N-STaR 100% RGY curve were equal to or greater than the standard N rate at all of the study sites. These results indicate the importance of field-scale demonstration trials of the N-STaR technology to educate producers, consultants and extension personnel of this soil-based N test for rice grown on silt loam soils. Hybrid rice hectarage has increased substantially in the southern United States and necessitated research into the N nutrition of this new type of rice and how it compares to the traditional pure-line rice. Consequently, a study was conducted utilizing 15N-labeled urea applied at a range of N rates on two silt loam soils differing in native soil N to evaluate and compare the N fertilizer uptake efficiency, the native soil N uptake, and total N uptake of a RiceTec hybrid and a pure-line rice cultivar. The hybrid (60.5 kg N ha-1) had greater soil N uptake compared with the pure-line cultivar (51.7 kg N ha-1). The hybrid had a higher fertilizer N uptake efficiency (FNUE) compared with the pure-line at the location with the lesser native soil N (62.2% vs 56.2%, respectively), but had a similar FNUE at the location with the greater soil N (63.8% vs 60.0%, respectively). Also, the pure-line had a higher FNUE and greater N fertilizer response at the location with the greater soil N while the hybrid had a similar FNUE at both locations. The greater total N uptake by the hybrid compared with the pure-line was due to greater soil N uptake at both locations and fertilizer N uptake at the location with the lower native soil N. The results suggest that if the native soil N is below a critical level a pure-line rice cultivar might benefit from a higher rate of N fertilization to maximize FNUE. Rice production has been identified as a significant agricultural contributor of methane (CH4) in the United States. The Environmental Protection Agency (EPA) uses a single CH4 emissions factor of 160 kg CH4-C ha-1 for a primary rice crop. However, this is based on studies that do not adequately represent current management practices in Arkansas. Another concern is preflood fertilizer N applications as urea is prone to loss via ammonia (NH3) volatilization. Thus, the objective of this research was to investigate trace gas emissions from rice on a silt-loam soil, including CH4 emissions as influenced by nitrogen (N) fertilization (i.e., optimal N and no N) in 2011, and previous crop (i.e., soybean or rice) and cultivar (i.e., semi-dwarf, standard-stature, and hybrid) in 2012, and to assess the impact of NH3 volatilization from preflood urea application with and without the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) on grain yield. In 2011, no differences were measured in CH4 emissions due to N fertilization, where mean annual emissions were 195 kg CH4-C ha-1. Methane emissions in 2012 were greater when rice was the previous crop (184 kg CH4-C ha-1) compared to when soybean was the previous crop (127 kg CH4-C ha-1), and emissions were greater from the semi-dwarf (169 kg CH4-C ha-1) and standard-stature pure-line cultivars (186 kg CH4-C ha-1), which did not differ, compared to the hybrid (111 kg CH4-C ha-1). Ammonia volatilization was reduced when using NBPT compared to untreated urea, but its use had no impact on grain yield. Methane results suggest the EPA’s general emissions factor is likely an overestimate of actual CH4 emissions in Arkansas, as 70% of rice production follows soybean and 50% of production is from hybrid rice. Results indicate NH3 volatilization from urea can be reduced by the use of NBPT. Understanding trace gas emissions from rice will improve the long-term sustainability of rice production.

Publications

• Type: Journal Articles Status: Published Year Published: 2013 Citation: Norman, R.J., Roberts, T.L., Slaton, N.A., and Fulford, A.M. 2013. Nitrogen uptake efficiency of a hybrid compared with a conventional, pure-line rice cultivar. Soil Sci. Soc. Am. J. 77:1235-1240. Roberts, T.L., Norman, R.J., Fulford, A.M., and Slaton, N.A. 2013. Field validation of N-STaR for rice produced on silt loam soils in Arkansas. Soil Sci. Soc. Am. J. 77:539-545. Rogers, C.W., Brye, K.R., Norman, R.J., Gbur, E.E., Mattice, J.D., Parkin, T.B., and Roberts, T.L. 2013. Methane emissions from drill-seeded, delayed-flood rice production on a silt loam in Arkansas. J. Environ. Qual. 42:1059-1069. Stiegler, J.C., Richardson, M.D., Karcher, D.E., Roberts, T.L., and Norman, R.J. 2013. Foliar absorption of various inorganic and organic nitrogen sources by creeping bentgrass. Crop Sci. 53:1148-1152. Slaton, N.A., Roberts, T.L., Golden, B.R., Ross, W.J., and Norman, R.J. 2013. Soybean response to phosphorus and potassium supplied as inorganic fertilizer and poultry litter. Agron. J. 105:812-820.
• Type: Other Status: Published Year Published: 2013 Citation: Norman, R.J., Slaton, N.A., and Roberts, T.L. 2013. Soil fertility. In J.T. Hardke (ed.) Rice Production Handbook. Univ. of Ark., Div. of Agri., Coop. Ext. Ser. MP-192. Norman, R.J., and Moldenhauer, K.A.K. (eds.). 2013. B.R. Wells Rice Research Studies 2012. Univ. of Ark., Agr. Exp. Stn. Res. Ser. 609.

Progress 01/01/12 to 12/31/12

Outputs
OUTPUTS: Laboratory and incubation studies as well as all analytical analyses were conducted at the University of Arkansas Main campus. Field studies were conducted at five experiment stations and six commercial rice fields. The research project involved four graduate students as part of their thesis and dissertation research. The information gained from the research was disseminated to communities of interest as follows: i) the Rice Research Board and industry through annual reports and presentations; with other scientists at regional and national meetings, and in refereed publications; and with extension agents, consultants, dealers and farmers through presentations at county and state meetings, field days, workshops/in-service trainings and in publications by the Arkansas Agricultural Experiment Station and Cooperative Extension Service as well as by the popular press. PARTICIPANTS: The research has been funded primarily by the Arkansas Rice Research Board, and partially funded by Agrotain International, Honeywell, Weyerhaeuser, and RiceTec. Collaborating scientists are University of Arkansas graduate students Anthony Fulford, Chris Rogers, Alden Smart, and Jay Wiggins; University of Arkansas professors Drs. Nathan Slaton, Charles Wilson, Trent Roberts and Kris Brye; and Dr. Tim Walker of Mississippi State University, Dr. Dustin Harrell of Louisiana State University, and Dr. Garry McCauley of Texas A & M University. The collaboration of scientist's from other rice producing states in the south is an effort at developing a soil based N test for the entire southern rice growing region. The test is named the Nitrogen-Soil Test for Rice or N-STaR. A calibration curve for silt loam soils has been completed and was validated in Arkansas in 2010 and 2011 and is now being validated in the other cooperating rice producing states. In 2012 the calibration curve for clay soils was completed in 21 small plot tests located at experiment stations and in commercial fields. Validation of the clay soil calibration curve will begin in 2013 with the help of county extension personnel. These validation studies are also being used to educate the extension agents, consultants, farmers and other rice scientists of the utility of the method. Training meetings are currently being conducted to inform farmers, agents, and consultants on the utility of the method and how to properly use it. Knowledge gained from the studies on the influence of urease inhibitors and N source on NH3 volatilization, total-N uptake, and grain yield of delayed flood rice is being presented at professional training meetings to educate extension agents, consultants, dealers, farmers and others in the fertilizer and rice industry on the viable urease inhibitors and N sources to use in delayed flood rice. The results of the three year study comparing midseason N application times for the new varieties has been presented to the Arkansas Rice Research Board and is being presented at professional training meetings this winter to educate extension agents, consultants and farmers. The results from the methane emissions research has been presented to the Arkansas Rice Research Board. TARGET AUDIENCES: The audiences that will benefit the most from the knowledge gained are rice farmers, extension agents, consultants, fertilizer dealers and students. The knowledge when placed in their hands will improve the efficiency with which the rice crop utilizes the N fertilizer applied and in turn make rice production more profitable, sustainable, and environmentally sound. The methane emissions research will be used to understand what influences methane emissions from rice fields and ways to minimize this greenhouse gas. Knowledge gained will be added to curricula used in classroom and extension training workshops and extension publications. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
The nitrogen (N) fertilizer rate calibration curve for our new Nitrogen-Soil Test for Rice, N-STaR, has been completed and validated for rice grown on silt loam soils and is being recommended in the state of Arkansas. We have been working to develop N-STaR grain yield correlation and N fertilizer rate calibration curves for rice grown on clayey soil. A total of 21 site-years of correlation and fertilizer calibration data have been collected. The grain yield correlation and N fertilizer rate calibrations were developed using a 15 cm sampling interval to a 45 cm depth. This sampling protocol enabled the most appropriate sampling depth to be identified for use in conjunction with the N-STaR method. The strength of the grain yield correlation curve (r2 = 0.79) indicated that the greatest predictive ability occurred when clayey soils were sampled to a 30 cm depth. The N fertilizer calibration curve (r2 = 0.88) developed for Arkansas and Mississippi clayey soils also indicated the 0 to 30 cm depth was most appropriate for determination of the N rate needed to achieve 95% relative grain yield. The next step in the development of N-STaR for use in clayey soil production systems will be to conduct validation trials during the 2013 and 2014 growing seasons in order to compare rice grain yields obtained using N-STaR N rate recommendations to the standard N rate recommendation of 202 kg/ha. A study was initiated in 2010 to examine the influence of midseason N application timing on the grain yield of two conventional pure-line rice varieties from Louisiana and Arkansas. The two conventional rice varieties chosen for the study were the Louisiana long grain, semidwarf Cheniere and the Arkansas long grain, short stature Taggart. The midseason N fertilizer was applied at four weekly intervals from beginning internode elongation (BIE) until BIE + 21 days. The three years of study indicated that the varieties do not always display a significant grain yield response to midseason N even when an inadequate preflood N fertilizer rate has been applied. In addition, when there has been a grain yield response to midseason N the application window was two to three weeks wide from BIE to BIE + 14 to 21 days. Methane emissions from delayed-flood rice as influenced by soil texture, variety, and previous crop are being measured in Arkansas. 2012 results on a DeWitt silt loam and a Sharkey clay following soybean indicated that substantially lower emissions occur on clay soils (47 kg CH4/ha-season) as compared to silt-loam soils (200 kg CH4/ha-season). Previous crop (soybean or rice) and variety significantly impacted seasonal emissions. Rice grown following soybean produced 151 kg CH4/ha-season as compared to 218 kg CH4/ha-season when grown in a rice/rice rotation; most of our rice follows soybean. Methane emissions were significantly lower at 133 kg CH4/ha-season with the hybrid CLXL745 compared to the conventional rice varieties Cheniere or Taggart, which emitted 198 and 222 kg CH4/ha, respectively. Thus, it appears variety and soil texture are key subfactors, which could be used to increase the accuracy when estimating statewide methane emissions by EPA.

Publications

• Roberts, T.L., Ross, W.J., Norman, R.J., Slaton, N.A., and Wilson, Jr., C.E. 2012. Factors influencing the recovery of glucosamine nitrogen from soils commonly cropped to rice. Soil Sci. Soc. Am. J. 76:252-257.
• Roberts, T.L., Norman, R.J., Ross, W.J., Slaton, N.A., and Wilson, Jr., C.E. 2012. Soil depth coupled with soil nitrogen and carbon can improve fertilization of rice in Arkansas. Soil Sci. Soc. Am. J. 76:268-277.
• Norman, R., Slaton, N., Espinoza, L., and Roberts, T. 2012. Nitrogen fertilizer additives. Univ. of Ark., Ark. Coop. Ext. Ser. FSA2169.
• Fulford, A.M., Roberts, T.L., Norman, R.J., Slaton, N.A., Wilson Jr., C.E., Frizzell, D.L., Branson, J.D., Duren, M. W., and Rogers, C.W. 2012. Development of nitrogen soil test for rice correlation and fertilizer calibration curves for rice grown on clayey soils. p. 195-201. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2011. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600.
• Norman, R.J., Roberts, T.L., Wilson Jr., C.E., Slaton, N.A., Frizzell, D.L., and Branson, J.D. 2011. Response of two rice varieties to midseason nitrogen fertilizer application timing. p. 215-219. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2012. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600
• Norman, R.J., Roberts, T.L., Wilson Jr., C.E., Slaton, N.A., Frizzell, D.L., Branson, J.D, Duren, M.W., Moldenhauer, K.A.K., and Gibbons, J.W. 2012. Grain yield response of twelve new rice cultivars to nitrogen fertilization. p. 220-234. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2011. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600.
• Roberts, T.L., Norman, R.J., Slaton, N.A., Wilson, Jr., C.E, Shafer, J., Williamson, S.M., Frizzell, D.L., and Branson, J.D. 2012. Grain yield response of the Clearfield rice cultivars CL111, CL142AR, CL181AR, and CL261 compared to Taggart at a high native nitrogen site. p.235-239. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2011. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600.
• Rogers, C.W., Brye, K.R., Norman, R.J., Gasnier, T., Frizzell, D.L., and Branson, J.D. 2012. Methane emissions from a silt-loam soil under direct-seeded, delayed flood rice management. p. 240-247. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2011. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600.
• Rogers, C.W., Norman, R.J., Roberts, T.L., Slaton, N.A., Wilson, Jr., C.E, Brye, K.R., Fulford, A.M., Frizzell, D.L., and Branson, J.D. 2012. Evaluation of inhibitors to reduce ammonia volatilization for usage in direct-seeded, delayed-flood rice production. p. 248-252. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2011. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600.
• Rogers, C.W., Norman, R.J., Roberts, T.L., Wilson, Jr., C.E., Slaton, N.A., Frizzell, D.L., and Branson, J.D. 2012. Rice grain yield as influenced by nitrogen source, nitrogen rate, and nitrogen application timing. p. 253-264. In R.J. Norman and K.A.K. Moldenhauer (eds.), B.R. Wells Rice Research Studies 2011. Univ. of Ark., Agr. Exp. Sta. Res. Ser. 600.