Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/1/2007
Publication Date: 2/18/2008
Citation: Stevens, G., Wrather, A., Rhine, R., Dunn, D., Vories, E.D. 2008. New methods for making midseason nitrogen decisions on rice [abstract]. 32nd Rice Technical Working Group Meetings, February 18-21, 2008, San Diego, CA. 2008 CD-ROM.
Technical Abstract: A simple method is needed to aid farmers with midseason nitrogen (MSN) decisions in dry-seeded, delayed flood rice (Oryza sativa L.). Managing nitrogen fertilization can be a challenge due to potential N losses from urea volatilization before flooding and denitrification after flooding. Extension recommendations for preflood N (PFN) rates in rice are usually based on empirical N field tests and adjustments are made for specific varieties, crop rotation, and soil texture. To help reduce rice N deficiency stress from early season N losses and supply N needs during grain-filling growth stages, midseason N (MSN) by aerial topdressing on rice can be applied near panicle differentiation (R1) growth stage. Measurements such as leaf area index, biomass accumulation, Y-leaf (most recently fully expanded leaf) N concentration, and whole-plant N concentration have been used to estimate midseason plant N sufficiency for determining whether topdressing is likely to increase rice yields. Plant area measurements with a rice gauge have also been used to predict midseason N need. Scientists in Arkansas found that plant area values from a rice gauge were a good estimator of rice dry matter and a more reliable estimator of total N accumulation than Y-leaf N concentrations and SPAD readings. Although use of the rice gauge for predicting rice need for midseason N has been widely promoted by state extension services in the upper Mississippi Delta region, very few rice consultants or farmers use it because of the labor required. This study was conducted to develop thresholds using visual and digital image measurements for predicting rice yield response to MSN. ‘Francis’ and ‘Cheniere’ rice were drill seeded on 19-cm row spacing from 2004 to 2006 on silt loam and clay soils at Glennonville and Portageville, Missouri. Pre-flood N was applied at rates of 0, 39, 78, 118 and 157 kg urea-N ha-1 with and without two MSN applications of 34 kg N ha-1 at panicle differentiation and at R1+ 7d. Plant area observations were made 1-2 d before R1. Three methods of measuring midseason plant area per plot were evaluated 1-2 d before R1 growth stage. For the first method, a yardstick was floated on floodwater between two center drill rows and the numbers visible were counted. Inch digits on the yardstick were approximately 2.0 mm tall. Standing between adjacent rows and leaning over the sampling rows, we counted the inch numbers showing on the yardstick (not hidden by rice leaves) out of 36 numbers possible. When a rice leaf obstructed the view of one digit in a two-digit number to the point that the whole number was not recognized, we did not count that number. For the second method, plant height was measured at the same sample location in each plot. One location per plot was sampled. For the third method, digital images were collected 1 to 3 days before midseason N applications with a camera mounted on 5-ft rod held above the plot in 2005-2007. This method was not used in 2004. The camera was positioned level with the soil surface and recorded a plot area of 32 inches X 45 inch. A computer macro program developed at University of Arkansas was used with Sigma ScanTM Pro 5.0 image software to determine the percentage of green pixels in each photo. We defined green color in Sigma Scan as 52 to 110o hue on the color wheel and a saturation of 35 to 100%. No significant yield increase was produced from MSN when 105 lb N acre-1 was applied preflood with small plot water management. However, in large rice grower fields managing to reduce volatilization and denitrification losses is more difficult. Critical plant area thresholds values for R1 growth stage rice were developed using visual and digital image measurements for predicting rice yield response to MSN. Although plant height is used as an input for estimating rice crop canopy with the rice gau