1. Rationale and Objectives  Summer fallow is a common practice in the western portion of the Central Great Plains.  Summer fallow is inefficient at storing precipitation in the soil and it often results in soil degradation, but it does reduce the risk of crop failure.  There are years when precipitation is sufficient to replace summer fallow with a short-season spring-planted crop (Flexible Fallow).  The objective of this study was to compare two fixed crop rotations [winter wheat-corn-fallow (W-C-F) and winter wheat-corn-triticale (W-C-T)] with a Flexible Fallow rotation using crop simulation modeling (AquaCrop). Materials and Methods  A field study was conducted at the University of Nebraska-Lincoln High Plains Ag Lab located near Sidney, NE, and at the USDA-ARS Central Great Plains Research Station located near Akron, CO from 2006 through 2011.  Each phase of each fixed rotation was present every year.  Four of the 8 replications were irrigated as needed from March through October to bring total precipitation plus irrigation up to 30-yr avg. precipitation for each 2-wk period. The other 4 replications were not irrigated.  Field data were collected and used to calibrate and validate AquaCrop.  Twenty years of historic weather data were used to make a long-term simulation comparing the two fixed rotations and a Flexible Fallow rotation using three soil water thresholds (350, 375, and 400 mm) to decide to fallow or plant triticale. Conclusions  The calibration (Akron, CO) and validation (Sidney. NE) of AquaCrop was acceptable for yield, biomass, and ET in corn and winter wheat.  The biomass calibration for triticale at Akron, CO was acceptable.  The lower values for the validation at Sidney can be attributed to the water content in the top 60 cm, which were lower than at Akron, and AquaCrop is very sensitive to initial soil water content (Garcia-Villa et al., 2009).  At both Akron and Sidney, the most productive scenario for winter wheat was W-C-F (2790 and 2490 kg ha -1, respectively), followed by Flexible Fallow with a 400 mm threshold (2550 and 2400 kg ha -1, respectively). The least productive was W-C-T (2240 and 2290 kg ha -1, respectively).  At Sidney, there was no statistical difference (p-value 0.093) in wheat yield between Flexible Fallow using 400 mm as the soil water threshold for planting triticale and W-C-F.  Triticale was planted in 4 out of 23 years at Sidney with Flexible Fallow using 400 mm as the soil water threshold for planting, with an average forage production for those 4 years of 5580 kg ha -1.  Further analysis, including a simple economic analysis, are needed to better evaluate the potential costs and benefits of Flexible Fallow. Project partially funded by the Anna H. Elliott Fund and The Nebraska Wheat Board Email: jmicelig@gmail.com Comparing Cropping System Productivity Between Fixed Rotations and a Flexible Fallow System Using Modeling and Historical Weather Data in the Semi-Arid Central Great Plains Juan J. Miceli-Garcia, Drew J. Lyon, David C. Nielsen and Timothy J. Arkebauer Figure 1. Triticale (front, left) wheat (back, left) and corn (right) at Sidney, NE. Picture taken 2 Jun 2010. Table 1. Statistics for the comparison between observed and simulated values for seed yield, final biomass, and crop evapotranspiration (ET) for winter wheat and corn, and final biomass and crop ET for triticale, from the calibration of AquaCrop for Akron, CO. and validation at Sidney, NE. Figure 2. Field study just before wheat harvest. Picture taken 13 Jul 2010. Akron, COSidney, NE Mean Variable name nObservedSimulatedMRERMSE d statistic Variable name nObservedSimulatedMRERMSE d statistic Winter wheat Winter wheat Yield (kg ha -1) 202180200012.63990.921 Yield (kg ha -1) 123390253023.613300.993 Biomass (kg ha -1) 207340782011.910500.938 Biomass (kg ha -1) 1211430949016.828800.481 ET (mm) 203803948.6735.10.890 ET (mm) 1253440922.61351 Spring triticale Biomass (kg ha -1) 84370449027.911500.861 Biomass (kg ha -1) 6448055039.316200.421 ET (mm) 815918619.936.60.713 ET (mm) 618631837.81531 Corn Yield (kg ha -1) 123970401020.27540.961 Yield (kg ha -1) 8699073704.395000.999 Biomass (kg ha -1) 12110001252024.224600.764 Biomass (kg ha -1) 8168201577012.929100.457 ET (mm) 124294358.5241.90.886 ET (mm) 84724994.0630.21 Water thresholdEstimateStd ErrorDFt-valuePf > |t| 12-36.839.352-0.940.353 13-75.441.352-1.830.073 14-15043.5522.420.019 15-19746.352-4.25<.0001 23-38.643.452-0.890.337 24-68.345.652-1.50.140 25-16049.552-3.230.002 34-29.645.252-0.660.514 35-12151.052-2.370.021 45-91.553.552-1.710.093 Table 2. Statistical comparison of wheat yield predicted by the simulation for each water threshold at Sidney, NE. Always triticale (1), 350 mm (2), 375 mm (3), 400 mm (4), and always fallow (5). Table 3. Statistical comparison of wheat yield predicted by the simulation for each water threshold at Akron, CO. Always triticale (1), 350 mm (2), 375 mm (3), 400 mm (4), and always fallow (5). Water thresholdEstimateStd ErrorDFt-valuePf > |t| 12-76.962.352-1.240.222 13-15068.652-2.190.033 14-31370.452-4.44<.0001 15-54574.952-7.27<.0001 23-73.170.852-1.030.307 24-23673.552-3.210.002 25-46879.352-5.9<.0001 34-16375.352-2.160.035 35-39583.152-4.75<.0001 45-23282.952-2.790.007