1.  Materials: Twelve rice cultivars (Starbonnet, Cypress, Wells, Bluebonnet, Lemont, Mars, Carolina Gold, Magnolia, Drew, Guichao, C-GL-13, and Lagrue) were planted in randomized complete block design experiment with 4 replications..  Methods: The growth stage of each plant, using the system of Counce et al (2000), was determined on nine plants per plot. This research was supported by a grant from the Arkansas Rice Research and Promotion Board. We thank Rachel Hillman for her excellent note-taking during this study. Counce, P. A., T. C. Keisling and A. J. Mitchell. 2000. A uniform, adaptive and objective system for expressing rice development. Crop Science 40:436-443. Growth stages of 12 rice cultivars (Oryza sativa L.) expressed in DD50 thermal heat units Nora T. Watson, Paul A. Counce and Terry J. Siebenmorgen I NTRODUCTION S IGNIFICANCE OF FINDINGS A CKNOWLEDGEMENTS R ESULTS From 1996 to 1999, a new developmental staging system was developed for rice (Counce et al, 2000). It would be useful to determine the amount of thermal time necessary to reach each developmental stage so that producers can use the system to determine when to apply pesticides and fertilizers Objectives  To determine the amount of DD50 units required to reach each growth stage of the new Counce et al (2000) rice developmental staging system for 12 rice cultivars M ATERIALS & M ETHODS Fig. 1. Graphic representation of the thermal degree days (DD50) needed to reach the (1) vegetative and (2) reproductive growth stages (Counce et al, 2000) of 12 rice cultivars.  With this staging information, a producer may use the new rice staging system for improving nitrogen and pesticide application timing, and decisions related to water management. The DD50 program and the growth staging system are interchangeable. R2 and VF occur at the same time. R3 is equivalent to heading, R0 is equivalent to panicle initiation and R1 very close to IE=1/2 inch.  Methods (cont.): Weather data from 2002 was obtained from the USDA weather station at Stuttgart, AR, which included average daily high and low temperatures in o F, the two data points necessary to calculate DD50 with the following equation: DD50 = (Daily Max. Temp. ( o F) + Daily Min. Temp. ( o F)) - 50 2 Where Daily Max. Temp. = 94 o F if maximum temperature is >94 o F Daily Min. Temp. = 70 o F if minimum temperature is > 70 o F Table. 1. Average degree days (DD50) required to reach each growth stage for 12 cultivars.  Using the timing intervals, the flexibility of the DD50 program can be greatly increased. If you miss an emergence date for a field, the date for starting DD50 accumulations could be reset at V1, V2, V3 instead. Therefore, instead of just guessing at an emergence date, a grower can find the actual growth stage and time from that date and stage of development. R EFERENCE StageBBNTC-GL-13CGLDCPRSDREWGCHWLGRULMNT MagnoliaMarsSTBNWells V020000000000000 V0330933101226 1126180 V04112125115 112118122127 115125 146 V05238224255242222174254287 233246 211 V06399392397377390278439472 405403 390 V07626633607626620444635659 622613619590 V08724779735771737639772846 765744724716 V0990396890910149277459671042 951915914906 V101140118611251261114890412311279 1215116211601135 V1114931405140314781438108714641492 1542144114971396 V1217641701164116471608136916421666 1764160517561555 V131999 184117291756161118001729 2008178119491674 V142244 2035 18151843 1888 2194186821491784 V152408 2257 1961 2345 2224 V16 2194 R0223521605203217101747199317451772 2187180021851633 R0326431993229319812013227420302114 2488202424641930 R0427092132234720802100233521282187 2533214125162022 R0527992200246721642171241622142252 2614221426292137 R0629742308256422782292251823502361 2776230528042222 R0730622411266924232419266224092505 2977243629882373 R0831752574283225812568285425852788 3079257531432523 R0933162859304329712949301629853024 3330290933172879 1 1 2 2