Drought Sensitivity Indices for a Sorghum Crop

Abstract

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. The objective of this study was to derive a sorghum drought index (SDI) to express the temporal impacts of climate on grain production. Irrigation (I) and no irrigation (N) treatments were imposed on sorghum during vegetative (GSI), inflorescence (GS2), and grain fill (GS3) growth stages in a randomized, split-factorial block design during two growing seasons in Mead, NE. The soil is Sharpsburg silly clay loam (fine, smectitic, mesic Typic Argiudolls). The irrigation strategy produced eight water treatments ranging from no irrigation in any stage (NNN) to irrigation in each stage (III). Soil water, measured by a neutron meter, was used in plot water balance simulations to estimate evapotranspiration (ET). Ratios of total actual ET (ET a ) and transpiration (T a ) to total potential evapotranspiration (ET p ) were determined for each growth stage. These ratios varied from 0.317 to 0.922 across all stages and treatments. Sensitivity to water availability was depicted by λ exponents in a relative yield model (SDI = Y/Yp = II(ΣETai/ΣETpi) λi , where II 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 (λ = 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 difference in magnitude between stages is as much as a factor of five, indicating that sorghum is sensitive to the timing of rainfall and irrigation. Validation with independent data for another variety and year was satisfactory (r 2 = 0.78 and d-index of agreement = 0.89). We conclude that the effect of short and long term drought on sorghum production for this variety can be monitored through the use of the newly defined SDI. Perhaps the fact that SDI is normalized with respect to the potential yield, a genetic characteristic, will allow it to be more representative of sorghum, irrespective of variety. This possibility seems to merit further investigation.