Drought sensitivity indices for a sorghum crop

Grain yield in sorghum [Sorghum bicolor (L,) Moench, cv, DK-57] depends in general on the amount of rainfall and irrigation, and atmospheric processes affecting water use. Timing may significantly modify the yield response. T he objective of this study was to derive a sorghum drought index (SDI) to e xpress the temporal impacts of climate on grain production, Irrigation (I) and no irrigation (N) treatments were imposed on sorghum during vegetative (GS1), inflorescence (GS2), and grain fill (GS3) growth stages in a randomi zed, split-factorial block design during two growing seasons in Mead, NE. T he soil is Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudo lls). The irrigation strategy produced eight water treatments ranging from no irrigation in any stage (NNN) to irrigation in each stage (III), Soil wa ter, measured by a neutron meter, was used in plot water balance simulation s to estimate evapotranspiration (ET), Ratios of total actual ET (ETa) and transpiration (T-a) to total potential evapotranspiration (ETp) were determ ined for each growth stage. These ratios varied from 0.317 to 0.922 across all stages and treatments. Sensitivity to water availability was depicted b y lambda exponents in a relative yield model (SDI = Y/Yp = II(Sigma ETai/Si gma ETpi)(lambda i) where IT indicates multiplication for each growth stage , i = 1 to 3), Yp is the potential yield, taken as 110% of yield in the III treatment. ETa, not Ta, gave the best fit. The indices (lambda = 0.04 (GS1 ), 0.20 (GS2), and 0.18 (GS3)) are smaller for sorghum than for other grain crops, which indicates sorghum is less sensitive to drought. Even so the d ifference in magnitude between stages is as much as a factor of five, indic ating that sorghum Is sensitive to the timing of rainfall and irrigation. V alidation with independent data for another variety and year was satisfacto ry (r(2) = 0.78 and d-index of agreement = 0.89). We conclude that the effe ct of short and long term drought on sorghum production for this variety ca n be monitored through the use of the newly defined SDI, Perhaps the fact t hat SDI is normalized with respect to the potential yield, a genetic charac teristic, will allow it to be more representative of sorghum, irrespective of variety. This possibility seems to merit further investigation.